LTC4081
1
4081f
■ Wireless Headsets
■ Bluetooth Applications
■ Portable MP3 Players
■ Multifunction Wristwatches
Li-Ion Battery Charger with 1.8V Buck Regulator
500mA Li-Ion Charger
with NTC Input and
300mA Synchronous Buck
The LTC4081 is a complete constant-current/constant-
voltage linear battery charger for a single-cell 4.2V
lithium-ion/polymer battery with an integrated 300mA
synchronous buck converter. A 3mm × 3mm DFN pack-
age and low external component count make the LTC4081
especially suitable for portable applications. Furthermore,
the LTC4081 is specifi cally designed to work within USB
power specifi cations.
The
⎯
C
⎯
H
⎯
R
⎯
G pin indicates when charge current has
dropped to ten percent of its programmed value (C/10).
An internal 4.5 hour timer terminates the charge cycle.
The full-featured LTC4081 battery charger also includes
trickle charge, automatic recharge, soft-start (to limit inrush
current) and an NTC thermistor input used to monitor
battery temperature.
The LTC4081 integrates a synchronous buck converter
that is powered from the BAT pin. It has an adjustable
output voltage and can deliver up to 300mA of load cur-
rent. The buck converter also features low-current high-
effi ciency Burst Mode operation that can be selected by
the MODE pin.
The LTC4081 is available in a 10-lead, low profi le (0.75
mm) 3mm × 3mm DFN package.
Battery Charger:
■ Constant-Current/Constant-Voltage Operation
with Thermal Feedback to Maximize Charge Rate
Without Risk of Overheating
■ Internal 4.5 Hour Safety Timer for Termination
■ Charge Current Programmable Up to 500mA with
5% Accuracy
■ NTC Thermistor Input for Temperature Qualifi ed
Charging
■ C/10 Charge Current Detection Output
■ 5μA Supply Current in Shutdown Mode
Switching Regulator:
■
High Effi ciency Synchronous Buck Converter
■ 300mA Output Current (Constant-Frequency Mode)
■ 2.7V to 4.5V Input Range (Powered from BAT Pin)
■ 0.8V to VBAT Output Range
■ MODE Pin Selects Fixed (2.25MHz) Constant-Frequency
PWM Mode or Low ICC (23μA) Burst Mode® Operation
■ 2μA BAT Current in Shutdown Mode
■ 10-lead, low profi le (0.75 mm) 3mm × 3mm DFN
package
APPLICATIO S
U
FEATURES DESCRIPTIO
U
TYPICAL APPLICATIO
U
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Protected by U.S. Patents,
including 6522118.
500mA
4.2V
Li-Ion/
POLYMER
BATTERY
4.7μF
806Ω
510Ω
806k 4.7μF
4081 TA01a
10pF 1M
LTC4081
NTC
EN_CHRG
MODE
SW
FB
PROG
VCC
EN_BUCK
CHRG
BAT
100k
100k
VOUT
(1.8V/300mA)
1OμH
GND
4.7μF
VCC
(3.75V
TO 5.5V)
+
T
LOAD CURRENT (mA)
0.01
40
EFFICIENCY (%)
POWER LOSS (mW)
60
80
0.1 10 1001 1000
20
0
100
1
10
100
0.1
0.01
1000
4081 TA01b
VBAT = 3.8V
VOUT = 1.8V
L = 10μH
C = 4.7μF
EFFICIENCY
(Burst)
POWER LOSS
(Burst)
EFFICIENCY
(PWM) POWER
LOSS
(PWM)
Buck Effi ciency vs Load Current
(VOUT = 1.8V)
LTC4081
2
4081f
PIN CONFIGURATION
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC4081EDD#PBF LTC4081EDD#TRPBF LDBX 10-Lead (3mm × 3mm) DFN 0°C to 70°C
Consult LTC Marketing for parts specifi ed with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based fi nish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifi cations, go to: http://www.linear.com/tapeandreel/
The ● denotes specifi cations which apply over the full operating tempera-
ture range, otherwise specifi cations are at TA = 25°C, VCC = 5V, VBAT = 3.8V, V
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G = 0V, VNTC = 0V, VEN_BUCK = VBAT, VMODE = 0V.
(Note 2)
ELECTRICAL CHARACTERISTICS
VCC, t < 1ms and Duty Cycle < 1% .............. – 0.3V to 7V
VCC Steady State ......................................... – 0.3V to 6V
BAT,
⎯
C
⎯
H
⎯
R
⎯
G .................................................. – 0.3V to 6V
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G, PROG, NTC ...................– 0.3V to VCC + 0.3V
MODE, EN_BUCK .......................... – 0.3V to VBAT + 0.3V
FB ............................................................... – 0.3V to 2V
(Note 1)
ABSOLUTE AXI U RATI GS
W
WW
U
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VCC Battery Charger Supply Voltage (Note 4) ●3.75 5 5.5 V
VBAT Input Voltage for the Switching
Regulator
(Note 5) ●2.7 3.8 4.5 V
ICC Quiescent Supply Current (Charger On,
Switching Regulator Off)
VBAT = 4.5V (Forces IBAT and IPROG = 0),
VEN_BUCK = 0
●110 300 μA
ICC_SD Supply Current in Shutdown (Both
Battery Charger and Switching
Regulator Off)
V
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G = 5V, VEN_BUCK = 0, VCC > VBAT
V
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G = 4V, VEN_BUCK = 0, VCC (3.5V) <
VBAT (4V)
●5
2
10 μA
μA
BAT Short-Circuit Duration ............................ Continuous
BAT Pin Current ...................................................800mA
PROG Pin Current ....................................................2mA
Junction Temperature .......................................... .125°C
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Storage Temperature Range .................. – 65°C to 125°C
TOP VIEW
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
10
9
6
7
8
4
5
3
2
1
BAT
VCC
EN_CHRG
PROG
NTC
SW
EN_BUCK
MODE
FB
CHRG
11
TJMAX = 110°C, θJA = 43°C/W (NOTE 3)
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
LTC4081
3
4081f
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
IBAT_SD Battery Current in Shutdown (Both
Battery Charger and Switching
Regulator Off)
V
⎯
E
⎯
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G = 5V, VEN_BUCK = 0, VCC > VBAT
V
⎯
E
⎯
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G = 4V, VEN_BUCK = 0, VCC (3.5V) <
VBAT (4V)
●0.6
2
5μA
μA
Battery Charger
VFLOAT VBAT Regulated Output Voltage IBAT = 2mA
IBAT = 2mA, 4.3V < VCC < 5.5V ●
4.179
4.158
4.2
4.2
4.221
4.242
V
V
IBAT Current Mode Charge Current RPROG = 4k; Current Mode; VEN_BUCK = 0
RPROG = 0.8k; Current Mode; VEN_BUCK = 0
●
●
90
475
100
500
110
525
mA
mA
VUVLO_CHRG VCC Undervoltage Lockout Voltage VCC Rising
VCC Falling
●
●
3.5
2.8
3.6
3.0
3.7
3.2
V
V
VPROG PROG Pin Servo Voltage 0.8k ≤ RPROG ≤ 4k ●0.98 1.0 1.02 V
VASD Automatic Shutdown Threshold Voltage (VCC – VBAT), VCC Low to High
(VCC – VBAT), VCC High to Low
60
15
82
32
100
45
mV
mV
tSS_CHRG Battery Charger Soft-Start Time 180 μs
ITRKL Trickle Charge Current VBAT = 2V, RPROG = 0.8k 35 50 65 mA
VTRKL Trickle Charge Threshold Voltage VBAT Rising ●2.75 2.9 3.05 V
VTRHYS Trickle Charge Threshold Voltage
Hysteresis
100 150 350 mV
ΔVRECHRG Recharge Battery Threshold Voltage VFLOAT – VBAT, 0°C < TA < 85°C 70 100 130 mV
ΔVUVCL1,
ΔVUVCL2
(VCC – VBAT) Undervoltage Current
Limit Threshold Voltage
IBAT = 0.9 ICHG
IBAT = 0.1 ICHG
180
90
300
130
mV
mV
tTIMER Charge Termination Timer ●3 4.5 6 hrs
Recharge Time ●1.5 2.25 3 hrs
Low-Battery Charge Time VBAT = 2.5V ●0.75 1.125 1.5 hrs
IC/10 End of Charge Indication Current Level RPROG = 2k (Note 6) ●0.085 0.1 0.115 mA/mA
TLIM Junction Temperature in Constant-
Temperature Mode
115 °C
RON_CHRG Power FET On-Resistance (Between
VCC and BAT)
IBAT = 350mA, VCC = 4V 700 mΩ
fBADBAT Defective Battery Detection
⎯
C
⎯
H
⎯
R
⎯
G Pulse
Frequency
VBAT = 2V 2 Hz
DBADBAT Defective Battery Detection
⎯
C
⎯
H
⎯
R
⎯
G Pulse
Frequency Duty Ratio
VBAT = 2V 75 %
INTC NTC Pin Current VNTC = 2.5V 1 μA
VCOLD Cold Temperature Fault Threshold
Voltage
Rising Voltage Threshold
Hysteresis
0.76 • VCC
0.015 • VCC
V
V
VHOT Hot Temperature Fault Threshold
Voltage
Falling Voltage Threshold
Hysteresis
0.35 • VCC
0.017 • VCC
V
V
VDIS NTC Disable Threshold Voltage Falling Threshold; VCC = 5V
Hysteresis
82
50
mV
mV
fNTC Fault Temperature
⎯
C
⎯
H
⎯
R
⎯
G Pulse
Frequency
2Hz
DNTC Fault Temperature
⎯
C
⎯
H
⎯
R
⎯
G Pulse
Frequency Duty Ratio
25 %
The ● denotes specifi cations which apply over the full operating tempera-
ture range, otherwise specifi cations are at TA = 25°C, VCC = 5V, VBAT = 3.8V, V
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G = 0V, VNTC = 0V, VEN_BUCK = VBAT, VMODE = 0V.
(Note 2)
ELECTRICAL CHARACTERISTICS
LTC4081
4
4081f
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Buck Converter
VFB FB Servo Voltage ●0.78 0.80 0.82 V
IFB FB Pin Input Current VFB = 0.85V –50 50 nA
fOSC Switching Frequency ●1.8 2.25 2.75 MHz
IBAT_NL_CF No-Load Battery Current (Continuous
Frequency Mode)
No-Load for Regulator, V
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G = 5V,
L = 10μH, C = 4.7μF
1.9 mA
IBAT_NL_BM No-Load Battery Current (Burst Mode
Operation)
No-Load for Regulator, V
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G = 5V,
MODE = VBAT, L = 10μH, C = 4.7μF
23 μA
IBAT_SLP Battery Current in SLEEP Mode V
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G = 5V, MODE = VBAT,
VOUT > Regulation Voltage
●10 15 20 μA
VUVLO_BUCK Buck Undervoltage Lockout Voltage VBAT Rising
VBAT Falling
●
●
2.6
2.4
2.7
2.5
2.8
2.6
V
V
RON_P PMOS Switch On-Resistance 0.95 Ω
RON_N NMOS Switch On-Resistance 0.85 Ω
ILIM_P PMOS Switch Current Limit 375 520 700 mA
ILIM_N NMOS Switch Current Limit 700 mA
IZERO_CF NMOS Zero Current in Normal Mode 15 mA
IPEAK Peak Current in Burst Mode Operation MODE = VBAT 50 100 150 mA
IZERO_BM Zero Current in Burst Mode Operation MODE = VBAT 20 35 50 mA
tSS_BUCK Buck Soft-Start Time From the Rising Edge of EN_BUCK to 90%
of Buck Regulated Output
400 μs
Logic
VIH Input High Voltage
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G, EN_BUCK, MODE Pin Low to High ●1.2 V
VIL Input Low Voltage
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G, EN_BUCK, MODE Pin High to Low ●0.4 V
VOL Output Low Voltage (
⎯
C
⎯
H
⎯
R
⎯
G) ISINK = 5mA ●60 105 mV
IIH Input Current High EN_BUCK, MODE Pins at 5.5V, VBAT = 5V ●–1 1 μA
IIL Input Current Low
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G, EN_BUCK, MODE Pins at GND ●–1 1 μA
R
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G Pin Input Resistance V
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G = 5V 1 1.45 3.3 MΩ
I
⎯
C
⎯
H
⎯
R
⎯
G
⎯⎯
C
⎯
H
⎯
R
⎯
G Pin Leakage Current VBAT = 4.5V, V
⎯
C
⎯
H
⎯
R
⎯
G = 5V ●1μA
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC4081 is guaranteed to meet performance specifi cations
from 0°C to 85°C. Specifi cations over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3: Failure to solder the exposed backside of the package to the PC
board ground plane will result in a thermal resistance much higher than
43°C/W.
The ● denotes specifi cations which apply over the full operating tempera-
ture range, otherwise specifi cations are at TA = 25°C, VCC = 5V, VBAT = 3.8V, V
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G = 0V, VNTC = 0V, VEN_BUCK = VBAT, VMODE = 0V.
(Note 2)
ELECTRICAL CHARACTERISTICS
Note 4: Although the LTC4081 charger functions properly at 3.75V, full
charge current requires an input voltage greater than the desired fi nal
battery voltage per ΔVUVCL1 specifi cation.
Note 5: The 2.8V maximum buck undervoltage lockout (VUVLO_BUCK) exit
threshold must fi rst be exceeded before the minimum VBAT specifi cation
applies.
Note 6: IC/10 is expressed as a fraction of measured full charge current
with indicated PROG resistor.
LTC4081
5
4081f
VCC SUPPLY VOLTAGE (V)
4
FLOAT VOLTAGE (V)
4.20
5.5
4.05
3.95
4.5 5
3.90
3.85
4.25
4.15
4.10
4.00
6
4081 G03
CHARGE CURRENT (mA)
0
4.17
4.18
4.21
4.20
150
4081 G01
4.16
4.15
50 100 200 250
4.14
4.13
4.19
FLOAT VOLTAGE (V)
RPROG = 2k
TEMPERATURE (°C)
–50
FLOAT VOLTAGE (V)
4.195
10
4.180
4.170
–30 –10 30
4.165
4.160
4.210
4.205
4.200
4.190
4.185
4.175
50 70 90
4081 G02
TEMPERATURE (°C)
–50
200
250
100
150
100
0–25 50 125
25 75
50
0
CHARGE CURRENT (mA)
4081 G04
VCC = 6V
VBAT = 3V
RPROG = 2k
THERMAL CONTROL
LOOP IN OPERATION
CHARGE CURRENT (mA)
0
VPROG (V)
0.6
0.8
1.0
175
0.4
0.2
025 75 12550 100 150 200
4081 G05
RPROG = 2k
TEMPERATURE (°C)
–50
RDS(ON) (Ω)
0.7
10
0.4
0.2
–30 –10 30
0.1
0
0.9
0.8
0.6
0.5
0.3
50 70 90
4081 G06
VCC = 4V
IBAT = 350mA
TEMPERATURE (°C)
–50
0.50
THRESHOLD VOLTAGE (V)
0.55
0.65
0.70
0.75
30
0.95
0.60
–10
–30 50 70
10 90
0.80
0.85
0.90
4081 G07
FALLING
RISING
TEMPERATURE (°C)
1.0
1.1
1.3
1.4
1.5
1.7
1.2
1.6
4081 G08
–50
PULLDOWN RESISTANCE (MΩ)
30
–10
–30 50 70
10 90
Battery Regulation (Float) Voltage
vs Charge Current
Battery Regulation (Float) Voltage
vs Temperature
Battery Regulation (Float) Voltage
vs VCC Supply Voltage
Charge Current vs Temperature
with Thermal Regulation
(Constant-Current Mode)
PROG Pin Voltage
vs Charge Current
Charger FET On-Resistance
vs Temperature
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G, EN_BUCK and
MODE Pin Threshold Voltage
vs Temperature
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G Pin Pulldown
Resistance vs Temperature
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise
specifi ed)
LTC4081
6
4081f
20
25
35
15
10
5
0
30
4081 G16
BATTERY VOLTAGE (V)
2.5
BUCK INPUT CURRENT (μA)
4.5
3.0 3.5 4.0
IOUT = 1mA
VOUT = 1.8V
L = 10μH
TEMPERATURE (°C)
0.80
NORMALIZED TIMER PERIOD
0.90
1.05
0.85
1.00
0.95
4081 G10
–50 30
–10
–30 50 70
10 90
BATTERY VOLTAGE (V)
2.5
2.26
2.27
2.28
4.0
4081 G11
2.25
2.24
3.0 3.5 4.5
2.23
2.22
FREQUENCY (MHz)
TEMPERATURE (°C)
–60
1.8
FREQUENCY (MHz)
1.9
2.0
2.1
2.2
–20 20 60 100
2.3
2.4
–40 0 40 80
4081 G12
VBAT = 2.7V
VBAT = 4.5V
VBAT = 3.8V
LOAD CURRENT (mA)
0.01
40
EFFICIENCY (%)
POWER LOSS (mW)
60
80
0.1 10 1001 1000
20
0
100
1
10
100
0.1
0.01
1000
4081 G13
VBAT = 3.8V
VOUT = 1.8V
L = 10μH
C = 4.7μF
EFFICIENCY
(BURST)
POWER LOSS
(BURST)
EFFICIENCY
(PWM) POWER
LOSS
(PWM)
BATTERY VOLTAGE (V)
2.5
1.780
BUCK OUTPUT VOLTAGE (V)
1.785
1.790
1.795
1.800
1.805
1.810
3.0 3.5 4.0 4.5
4081 G14
PWM MODE
IOUT = 1mA
VOUT SET FOR 1.8V Burst Mode
OPERATION
TEMPERATURE (˚C)
–50
BUCK OUTPUT VOLTAGE (V)
1.800
1.805
1.810
10 50
1.795
1.790
–30 –10 30 70 90
1.785
1.780
4081 G15
PWM MODE
Burst Mode
OPERATION
IOUT = 1mA
VOUT SET FOR 1.8V
No-Load Buck Input Current
(Burst Mode Operation)
vs Battery Voltage
Normalized Charge Termination
Time vs Temperature
Buck Oscillator Frequency
vs Battery Voltage
Buck Oscillator Frequency
vs Temperature
Buck Effi ciency vs Load Current
(VOUT = 1.8V)
Buck Output Voltage
vs Battery Voltage
Buck Output Voltage
vs Temperature
VOLTAGE (mV)
70
40
20
10
0
80
60
50
30
4081 G09
ICHRG = 5mA
TEMPERATURE (°C)
–50 30
–10
–30 50 70
10 90
⎯
C
⎯
H
⎯
R
⎯
G Pin Output
Low Voltage vs Temperature
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise
specifi ed)
Buck Effi ciency vs Load Current
(VOUT = 1.5V)
LOAD CURRENT (mA)
0.01
40
EFFICIENCY (%)
POWER LOSS (mW)
60
80
0.1 10 1001 1000
20
0
100
1
10
100
0.1
0.01
1000
4081 G13a
VBAT = 3.8V
VOUT = 1.5V
L = 10μH
C = 4.7μF
EFFICIENCY
(PWM) POWER
LOSS
(PWM)
EFFICIENCY
(BURST)
POWER LOSS
(BURST)
LTC4081
7
4081f
2.7 4.23 3.63.3 3.9 4.5
4081 G24
BATTERY VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (mA)
40
50
60
30
20
0
10
80
70
L = 10μH
VOUT SET FOR 1.8V
BATTERY VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (mA)
300
200
100
500
400
2.7 4.23 3.6
3.3 3.9 4.5
4081 G23
L = 10μH
VOUT SET FOR 1.8V
20
25
35
15
10
5
0
30
NO LOAD INPUT CURRENT (μA)
L = 10μH
C = 4.7μF
VOUT = 1.8V
TEMPERATURE (˚C)
–50 10 50
–30 –10 30 70 90
4081 G18
VBAT = 4.2V
VBAT = 3.8V
VBAT = 2.7V
No-Load Buck Input Current
(Burst Mode Operation)
vs Temperature
Buck Main Switch (PMOS)
On-Resistance vs Battery Voltage
Buck Main Switch (PMOS)
On-Resistance vs Temperature
Buck Synchronous Switch (NMOS)
On-Resistance vs Battery Voltage
Buck Synchronous Switch (NMOS)
On-Resistance vs Temperature
Maximum Output Current
(PWM Mode) vs Battery Voltage
Maximum Output Current (Burst
Mode Operation) vs Battery Voltage
1.0
1.2
0.8
0.6
0.4
0.2
0
4081 G19
BATTERY VOLTAGE (V)
2.5
ON-RESISTANCE (Ω)
4.5 5.0
3.0 3.5 4.0
TEMPERATURE (°C)
–50 10 50
–30 –10 30 70 90
4081 G20
1.0
1.2
0.8
0.6
0.4
0.2
0
ON-RESISTANCE (Ω)
4081 G21
BATTERY VOLTAGE (V)
2.5
ON-RESISTANCE (Ω)
0.8
1.0
1.2
4.5 5.0
0.6
0.4
03.0 3.5 4.0
0.2
4081 G22
TEMPERATURE (°C)
–50 10 50
–30 –10 30 70 90
ON-RESISTANCE (Ω)
0
0.8
1.0
1.2
0.6
0.4
0.2
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise
specifi ed)
LTC4081
8
4081f
200μs/DIV
VOUT
1V/DIV
VEN_BUCK
5V/DIV
4081 G28
0V
0V
50μs/DIV
VOUT
20mV/DIV
AC COUPLED
ILOAD
250mA/DIV
0mA
4081 G25
Output Voltage Transient
Step Response (Burst Mode
Operation)
Charger VPROG Soft-Start
Output Voltage Transient
Step Response (PWM Mode)
Output Voltage Waveform
when Switching Between Burst
and PWM Mode (ILOAD = 10mA)
Buck VOUT Soft-Start
(ILOAD = 50mA)
50μs/DIV
VOUT
50mV/DIV
AC COUPLED
VMODE
5V/DIV
4081 G27
0V
50μs/DIV
VOUT
20mV/DIV
AC COUPLED
ILOAD
50mA/DIV
4081 G26
0mA
50μs/DIV
VPROG
200mV/DIV
4081 G29
0V
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise
specifi ed)
LTC4081
9
4081f
PI FU CTIO S
UUU
BAT (Pin 1):
Charge Current Output and Buck Regulator
Input. Provides charge current to the battery and regulates
the fi nal fl oat voltage to 4.2V. An internal precision resistor
divider from this pin sets the fl oat voltage and is disconnected
in charger shutdown mode. This pin must be decoupled
with a low ESR capacitor for low-noise buck operation.
VCC (Pin 2): Positive Input Supply Voltage. This pin provides
power to the battery charger. VCC can range from 3.75V
to 5.5V. This pin should be bypassed with at least a 1μF
capacitor. When VCC is less than 32mV above the BAT pin
voltage, the battery charger enters shutdown mode.
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G (Pin 3): Enable Input Pin for the Battery Charger.
Pulling this pin above the manual shutdown threshold
(VIH) puts the LTC4081 charger in shutdown mode, thus
stopping the charge cycle. In battery charger shutdown
mode, the LTC4081 has less than 10μA supply current and
less than 5μA battery drain current provided the regula-
tor is not running. Enable is the default state, but the pin
should be tied to GND if unused.
PROG (Pin 4): Charge Current Program and Charge Cur-
rent Monitor Pin. Connecting a 1% resistor, RPROG, to
ground programs the charge current. When charging in
constant-current mode, this pin servos to 1V. In all modes,
the voltage on this pin can be used to measure the charge
current using the following formula:
IV
R
BAT PROG
PROG
=•400
NTC (Pin 5): Input to the NTC (negative temperature coef-
fi cient) Thermistor Temperature Monitoring Circuit. For
normal operation, connect a thermistor from the NTC pin
to ground and a resistor of equal value from the NTC pin
to VCC. When the voltage at this pin drops below 0.35 •
VCC at hot temperatures or rises above 0.76 • VCC at cold,
charging is suspended, the internal timer is frozen and the
⎯
C
⎯
H
⎯
R
⎯
G pin output will start to pulse at 2Hz. Pulling this
pin below 0.016 • VCC disables the NTC feature. There is
approximately 3°C of temperature hysteresis associated
with each of the input comparator’s thresholds.
⎯
C
⎯
H
⎯
R
⎯
G (Pin 6): Open-Drain Charge Status Output. The
charge status indicator pin has three states: pulldown,
high impedance state, and pulsing at 2Hz. This output can
be used as a logic interface or as an LED driver. When the
battery is being charged, the
⎯
C
⎯
H
⎯
R
⎯
G pin is pulled low by
an internal N-channel MOSFET. When the charge current
drops to 10% of the full-scale current, the
⎯
C
⎯
H
⎯
R
⎯
G pin is
forced to a high impedance state. When the battery volt-
age remains below 2.9V for one quarter of the full charge
time, the battery is considered defective, and the
⎯
C
⎯
H
⎯
R
⎯
G
pin pulses at a frequency of 2Hz with 75% duty cycle.
When the NTC pin voltage rises above 0.76 • VCC or drops
below 0.35 • VCC, the
⎯
C
⎯
H
⎯
R
⎯
G pin pulses at a frequency of
2Hz (25% duty cycle).
FB (Pin 7): Feedback Pin for the Buck Regulator. A resistor
divider from the regulator’s output to the FB pin programs
the output voltage. Servo value for this pin is 0.8V.
MODE (Pin 8): Burst Mode Enable Pin. Tie this pin high
to force the LTC4081 regulator into Burst Mode operation
for all load conditions. Tie this pin low to force constant-
frequency mode operation for all load conditions. Do not
fl oat this pin.
EN_BUCK (Pin 9): Enable Input Pin for the Buck Regulator.
Pull this pin high to enable the regulator, pull low to shut
down. Do not fl oat this pin.
SW (Pin 10): Switch Pin for the Buck Regulator. Minimize
the length of the metal trace connected to this pin. Place
the inductor as close to this pin as possible.
GND (Pin 11): Ground. This pin is the back of the Exposed
Pad package and must be soldered to the PCB for electrical
connection and rated thermal performance.
LTC4081
10
4081f
BLOCK DIAGRA
W
Figure 1. LTC4081 Block Diagram
–+
–+
6MP4
MP3 MP1
X1 X400
VCC
R1
R2
CHARGER
ENABLE
CHRG
PROG
11
GND
4081 BD
1V
0.1V
PROG
C1
0.1V
D3
D2
D1
1.22V
–+
CA
MA
–+
VA
1
10
7
2
BAT
SW
0.8V
L1 VOUT
COUT
R8
CPL
FB
COUNTER
LOGIC
CHARGER
OSCILLATOR
CHARGE
CONTROL
+
–
2.9V
BAT
BADBAT
UVLO
SUSPEND
–
+
C3
C2
EN_CHRG
REN
0.82V
CHARGER
SHUTDOWN
3
–
+
C6
0.82V
ENABLE BUCK
EN_BUCK
MODE
9
–
+
C7
0.82V
8
PWM
CONTROL
AND DRIVE
–
+
2.25MHz
BUCK
OSCILLATOR
ERROR
AMP
LINEAR BATTERY CHARGER
SYNCHRONOUS BUCK CONVERTER
MN1 R7
MP2
RPROG
+
–
115°C
TDIE
TA
PULSE
LOGIC
4
RNOM
RNTC
VCC
VCC
–
+
–
+
–
+
TOO COLD
TOO HOT
NTC_EN
R9
C8
C9
C10
R10
R11
R12
5NTC
–
+
–
+
C4
C5
VBAT + 80mV
VCC
3.6V
T
LTC4081
11
4081f
OPERATIO
U
The LTC4081 is a full-featured linear battery charger with
an integrated synchronous buck converter designed pri-
marily for handheld applications. The battery charger is
capable of charging single-cell 4.2V Li-Ion batteries. The
buck converter is powered from the BAT pin and has a
programmable output voltage providing a maximum load
current of 300mA. The converter and the battery charger
can run simultaneously or independently of each other.
BATTERY CHARGER OPERATION
Featuring an internal P-channel power MOSFET, MP1,
the battery charger uses a constant-current/constant-
voltage charge algorithm with programmable current.
Charge current can be programmed up to 500mA with a
fi nal fl oat voltage of 4.2V ±0.5%. The
⎯
C
⎯
H
⎯
R
⎯
G open-drain
status output indicates when C/10 has been reached. No
blocking diode or external sense resistor is required; thus,
the basic charger circuit requires only two external com-
ponents. An internal charge termination timer adheres to
battery manufacturer safety guidelines. Furthermore, the
LTC4081 battery charger is capable of operating from a
USB power source.
A charge cycle begins when the voltage at the VCC pin
rises above 3.6V and approximately 82mV above the BAT
pin voltage, a 1% program resistor is connected from the
PROG pin to ground, and the
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G pin is pulled below
the shutdown threshold (VIL).
When the BAT pin approaches the fi nal fl oat voltage of
4.2V, the battery charger enters constant-voltage mode
and the charge current begins to decrease. When the
current drops to 10% of the full-scale charge current, an
internal comparator turns off the N-channel MOSFET driving
the
⎯
C
⎯
H
⎯
R
⎯
G pin, and the pin becomes high impedance.
An internal thermal limit reduces the programmed charge
current if the die temperature attempts to rise above a
preset value of approximately 115°C. This feature protects
the LTC4081 from excessive temperature and allows the
user to push the limits of the power handling capability
of a given circuit board without the risk of damaging the
LTC4081 or external components. Another benefi t of the
thermal limit is that charge current can be set
according
to typical, rather than worst-case, ambient temperatures
for a given application with the assurance that the battery
charger will automatically reduce the current in worst-case
conditions.
An internal timer sets the total charge time, tTIMER (typi-
cally 4.5 hours). When this time elapses, the charge cycle
terminates and the
⎯
C
⎯
H
⎯
R
⎯
G pin assumes a high impedance
state even if C/10 has not yet been reached. To restart
the charge cycle, remove the input voltage and reapply
it or momentarily force the
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G pin above VIH. A
new charge cycle will automatically restart if the BAT pin
voltage falls below VRECHRG (typically 4.1V).
Constant-Current / Constant-Voltage /
Constant-Temperature
The LTC4081 battery charger uses a unique architecture
to charge a battery in a constant-current, constant-volt-
age and constant-temperature fashion. Figure 1 shows a
Simplifi ed Block Diagram of the LTC4081. Three of the
amplifi er feedback loops shown control the constant-cur-
rent, CA, constant-voltage, VA, and constant-temperature,
TA modes. A fourth amplifi er feedback loop, MA, is used to
increase the output impedance of the current source pair,
MP1 and MP3 (note that MP1 is the internal P-channel
power MOSFET). It ensures that the drain current of MP1
is exactly 400 times the drain current of MP3.
Amplifi ers CA and VA are used in separate feedback loops
to force the charger into constant-current or constant-
voltage mode, respectively. Diodes D1 and D2 provide
priority to either the constant-current or constant-voltage
loop, whichever is trying to reduce the charge current the
most. The output of the other amplifi er saturates low which
effectively removes its loop from the system. When in
constant-current mode, CA servos the voltage at the PROG
pin to be precisely 1V. VA servos its non-inverting input
to 1.22V when in constant-voltage mode and the internal
resistor divider made up of R1 and R2 ensures that the
battery voltage is maintained at 4.2V. The PROG pin volt-
age gives an indication of the charge current anytime in
the charge cycle, as discussed in “Programming Charge
Current” in the Applications Information section.
LTC4081
12
4081f
If the die temperature starts to creep up above 115°C
due to internal power dissipation, the transconductance
amplifi er, TA, limits the die temperature to approximately
115°C by reducing the charge current. Diode D3 ensures
that TA does not affect the charge current when the die
temperature is below 115°C. In thermal regulation, the
PROG pin voltage continues to give an indication of the
charge current.
In typical operation, the charge cycle begins in constant-
current mode with the current delivered to the battery equal
to 400V/RPROG. If the power dissipation of the LTC4081
results in the junction temperature approaching 115°C, the
amplifi er (TA) will begin decreasing the charge current to
limit the die temperature to approximately 115°C. As the
battery voltage rises, the LTC4081 either returns to full
constant-current mode or enters constant-voltage mode
straight from constant-temperature mode.
Battery Charger Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the VCC
input voltage and keeps the battery charger off
until VCC
rises above 3.6V and approximately 82mV above the BAT
pin voltage. The 3.6V UVLO circuit has a built-in hysteresis
of approximately 0.6V, and the 82mV automatic shutdown
threshold has a built-in hysteresis of approximately 50mV.
During undervoltage lockout conditions, maximum battery
drain current is 5
μ
A and maximum supply current is 10μA.
Undervoltage Charge Current Limiting (UVCL)
The battery charger in the LTC4081 includes undervoltage
charge current limiting that prevents full charge current
until the input supply voltage reaches approximately 300mV
above the battery voltage (ΔVUVCL1). This feature is particu-
larly useful if the LTC4081 is powered from a supply with
long leads (or any relatively high output impedance). See
Applications Information section for further details.
Trickle Charge and Defective Battery Detection
At the beginning of a charge cycle, if the battery volt-
age is below 2.9V, the battery charger goes into trickle
charge mode, reducing the charge current to 10% of the
programmed current. If the low battery voltage persists
for one quarter of the total time (1.125 hr), the battery is
assumed to be defective, the charge cycle terminates and
the
⎯
C
⎯
H
⎯
R
⎯
G pin output pulses at a frequency of 2Hz with
a 75% duty cycle. If, for any reason, the battery voltage
rises above 2.9V, the charge cycle will be restarted. To
restart the charge cycle (i.e., when the dead battery is
replaced with a discharged battery less than 2.9V), the
charger must be reset by removing the input voltage and
reapplying it or temporarily pulling the
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G pin above
the shutdown threshold.
Battery Charger Shutdown Mode
The LTC4081’s battery charger can be disabled by pulling
the
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G pin above the shutdown threshold (VIH).
In shutdown mode, the battery drain current is reduced
to about 2μA and the VCC supply current to about 5μA
provided the regulator is off. When the input voltage is
not present, the battery charger is in shutdown and the
battery drain current is less than 5μA.
⎯
C
⎯
H
⎯
R
⎯
G Status Output Pin
The charge status indicator pin has three states: pulldown,
pulsing at 2Hz (see Trickle Charge and Defective Battery
Detection and Battery Temperature Monitoring) and high
impedance. The pulldown state indicates that the battery
charger is in a charge cycle. A high impedance state indi-
cates that the charge current has dropped below 10% of
the full-scale current or the battery charger is disabled.
When the timer runs out (4.5 hrs), the
⎯
C
⎯
H
⎯
R
⎯
G pin is also
forced to the high impedance state. If the battery charger
is not in constant-voltage mode when the charge current
is forced to drop below 10% of the full-scale current by
UVCL,
⎯
C
⎯
H
⎯
R
⎯
G will stay in the strong pulldown state.
Charge Current Soft-Start
The LTC4081’s battery charger includes a soft-start circuit
to minimize the inrush current at the start of a charge
cycle. When a charge cycle is initiated, the charge cur-
rent ramps from zero to full-scale current over a period
of approximately 180μs. This has the effect of minimizing
the transient current load on the power supply during
start-up.
OPERATIO
U
LTC4081
13
4081f
Timer and Recharge
The LTC4081’s battery charger has an internal charge
termination timer that starts when the input voltage is
greater than the undervoltage lockout threshold and at
least 82mV above BAT, and the battery charger is leaving
shutdown.
At power-up or when exiting shutdown, the charge time
is set to 4.5 hours. Once the charge cycle terminates, the
battery charger continuously monitors the BAT pin voltage
using a comparator with a 2ms fi lter time. When the aver-
age battery voltage falls below 4.1V (which corresponds
to 80%-90% battery capacity), a new charge cycle is initi-
ated and a 2.25 hour timer begins. This ensures that the
battery is kept at, or near, a fully charged condition and
eliminates the need for periodic charge cycle initiations.
The
⎯
C
⎯
H
⎯
R
⎯
G output assumes a strong pulldown state dur-
ing recharge cycles until C/10 is reached or the recharge
cycle terminates.
Battery Temperature Monitoring via NTC
The battery temperature is measured by placing a nega-
tive temperature coeffi cient (NTC) thermistor close to the
battery pack. The NTC circuitry is shown in Figure 2.
To use this feature, connect the NTC thermistor, RNTC, be-
tween the NTC pin and ground and a resistor, RNOM, from
the NTC pin to VCC. RNOM should be a 1% resistor with a
value equal to the value of the chosen NTC thermistor at
25°C (this value is 10k for a Vishay NTHS0603NO1N1002J
thermistor). The LTC4081 goes into hold mode when the
value of the NTC thermistor drops to 0.53 times the value
of RNOM, which corresponds to approximately 40°C, and
when the value of the NTC thermistor increases to 3.26
times the value of RNOM, which corresponds to approxi-
mately 0°C. Hold mode freezes the timer and stops the
charge cycle until the thermistor indicates a return to a
valid temperature. For a Vishay NTHS0603NO1N1002J
thermistor, this value is 32.6k which corresponds to ap-
proximately 0°C. The hot and cold comparators each have
approximately 3°C of hysteresis to prevent oscillation
about the trip point.
When the charger is in Hold mode (battery temperature
is either too hot or too cold) the
⎯
C
⎯
H
⎯
R
⎯
G pin pulses in a
2Hz, 25% duty cycle frequency unless the charge task is
fi nished or the battery is assumed to be defective. If the
NTC pin is grounded, the NTC function will be disabled.
SWITCHING REGULATOR OPERATION:
The switching buck regulator in the LTC4081 can be turned
on by pulling the EN_BUCK pin above VIH. It has two user-
selectable modes of operation: constant-frequency (PWM)
mode and Burst Mode Operation. The constant-frequency
mode operation offers low noise at the expense of effi ciency
whereas the Burst Mode operation offers higher effi ciency
at light loads at the cost of increased noise, higher output
voltage ripple, and less output current. A detailed descrip-
tion of different operating modes and different aspects of
operation follow. Operations can best be understood by
referring to the Block Diagram.
OPERATIO
U
4081 F02
RNOM
RNTC
VCC
–
+
–
+
–
+
TOO COLD
TOO HOT
NTC_ENABLE
0.76 • VCC
0.35 • VCC
0.016 • VCC
6NTC
T
Figure 2. NTC Circuit Information
LTC4081
14
4081f
OPERATIO
U
Constant-Frequency (PWM) Mode Operation
The switching regulator operates in constant-frequency
(PWM) mode when the MODE pin is pulled below VIL. In
this mode, it uses a current mode architecture including
an oscillator, an error amplifi er, and a PWM comparator
for excellent line and load regulation. The main switch
MP2 (P-channel MOSFET) turns on to charge the inductor
at the beginning of each clock cycle if the FB pin voltage
is less than the 0.8V reference voltage. The current into
the inductor (and the load) increases until it reaches the
peak current demanded by the error amp. At this point,
the main switch turns off and the synchronous switch
MN1 (N-channel MOSFET) turns on allowing the inductor
current to fl ow from ground to the load until either the
next clock cycle begins or the current reduces to the zero
current (IZERO) level.
Oscillator: In constant-frequency mode, the switching
regulator uses a dedicated oscillator which runs at a fi xed
frequency of 2.25MHz. This frequency is chosen to mini-
mize possible interference with the AM radio band.
Error Amplifi er: The error amplifi er is an internally com-
pensated transconductance (gm) amplifi er with a gm
of 65
μ
mhos. The internal 0.8V reference voltage is
compared to the voltage at the FB pin to generate a
current signal at the output of the error amplifi er. This cur-
rent signal represents the peak inductor current required
to achieve regulation.
PWM Comparator: Lossless current sensing converts
the PMOS switch current signal to a voltage which is
summed with the internal slope compensation signal.
The PWM comparator compares this summed signal to
determine when to turn off the main switch. The switch
current sensing is blanked for ~12ns at the beginning of
each clock cycle to prevent false switch turn-off.
Burst Mode Operation
Burst Mode operation can be selected by pulling the
MODE pin above VIH. In this mode, the internal oscil-
lator is disabled, the error amplifi er is converted into a
comparator monitoring the FB voltage, and the inductor
current swings between a fi xed IPEAK (~100mA) and IZERO
(35mA) irrespective of the load current as long as the FB
pin voltage is less than or equal to the reference voltage
of 0.8V. Once VFB is greater than 0.8V, the control logic
shuts off both switches along with most of the circuitry
and the regulator is said to enter into SLEEP mode. In
SLEEP mode, the regulator only draws about 20
μ
A from
the BAT pin provided that the battery charger is turned
off. When the output voltage droops about 1% from its
nominal value, the regulator wakes up and the inductor
current resumes swinging between IPEAK and IZERO. The
output capacitor recharges and causes the regulator to
re-enter the SLEEP state if the output load remains light
enough. The frequency of this intermittent burst operation
depends on the load current. That is, as the load current
drops further, the regulator turns on less frequently. Thus
Burst Mode operation increases the effi ciency at light loads
by minimizing the switching and quiescent losses. However,
the output voltage ripple increases to about 2%.
To minimize ripple in the output voltage, the current limits
for both switches in Burst Mode operation are reduced
to about 20% of their values in the constant-frequency
mode. Also the zero current of the synchronous switch
is changed to about 35mA thereby preventing reverse
conduction through the inductor. Consequently, the regu-
lator can only deliver approximately 67mA of load current
while in Burst Mode operation. Any attempt to draw more
load cur
rent will cause the output voltage to drop out of
regulation.
Current Limit
To prevent inductor current runaway, there are absolute
current limits (ILIM) on both the PMOS main switch and
the NMOS synchronous switch. These limits are internally
set at 520mA and 700mA respectively for PWM mode. If
the peak inductor current demanded by the error amplifi er
ever exceeds the PMOS ILIM, the error amplifi er will be
ignored and the inductor current will be limited to PMOS
ILIM. In Burst Mode operation, the PMOS current limit is
reduced to 100mA to minimize output voltage ripple.
LTC4081
15
4081f
Zero Current Comparator
The zero or reverse current comparator monitors the induc-
tor current to the output and shuts off the synchronous
rectifi er when this current reduces to a predetermined
value (IZERO). In fi xed frequency mode, this is set to nega-
tive 15mA meaning that the regulator allows the inductor
current to fl ow in the reverse direction (from the output to
ground through the synchronous rectifi er) to a maximum
value of 15mA. This is done to ensure that the regulator
is able to regulate at very light loads without skipping any
cycles thereby keeping output voltage ripple and noise low
at the cost of effi ciency.
However, in Burst Mode operation, IZERO is set to positive
35mA meaning that the synchronous switch is turned off
as soon as the current through the inductor to the output
decreases to 35mA in the discharge cycle. This preserves
the charge on the output capacitor and increases the overall
effi ciency at light loads.
Soft-Start
The LTC4081 switching regulator provides soft-start in
both modes of operation by slowly charging an internal
capacitor. The voltage on this capacitor, in turn, slowly
ramps the current limits of both switches from a low value
to their respective maximum values over a period of about
400
μ
s. The soft-start capacitor is discharged completely
whenever the regulator is disabled.
Short-Circuit Protection
In the event of a short circuit at the output or during
start-up, VOUT will be near zero volts. Since the downward
slope of the inductor current is ~VOUT/L, the inductor
current may not get a chance to discharge enough to
avoid a runaway situation. Because the current sensing
OPERATIO
U
is blanked for ~12ns at the beginning of each clock cycle,
inductor current can build up to a dangerously high level
over a number of cycles even if there is a hard current
limit on the main PMOS switch. This is why the switching
regulator in the LTC4081 also monitors current through
the synchronous NMOS switch and imposes a hard limit
on it. If the inductor current through the NMOS switch at
the end of a discharge cycle is not below this limit, the
regulator skips the next charging cycle thereby preventing
inductor current runaway.
Switching Regulator Undervoltage Lockout
Whenever VBAT is less than 2.7V, an undervoltage lock-
out circuit keeps the regulator off, preventing unreliable
operation. However, if the regulator is already running
and the battery voltage is dropping, the undervoltage
comparator does not shut down the regulator until VBAT
drops below 2.5V.
Dropout Operation
When the BAT pin voltage approaches VOUT, the duty cycle
of the switching regulator approaches 100%. When VBAT
is approximately equal to VOUT, the regulator is said to be
in dropout. In dropout, the main switch (MP2) stays on
continuously with the output voltage being equal to the
battery voltage minus the voltage drops across the main
switch and the inductor.
Global Thermal Shutdown
The LTC4081 includes a global thermal shutdown which
shuts off the entire device (battery charger and switch-
ing regulator) if the
die temperature exceeds 160°C. The
LTC4081 resumes normal
operation once the temperature
drops approximately 14°C.
LTC4081
16
4081f
APPLICATIO S I FOR ATIO
WUUU
BATTERY CHARGER
Programming Charge Current
The battery charge current is programmed using a single
resistor from the PROG pin to ground. The charge current
is 400 times the current out of the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
RV
IIV
R
PROG BAT BAT PROG
==400 1400 1
•, •
The charge current out of the BAT pin can be determined
at any time by monitoring the PROG pin voltage and using
the following equation:
IV
R
BAT PROG
PROG
=•400
Stability Considerations
The LTC4081 battery charger contains two control loops:
constant-voltage and constant-current. The constant-
voltage loop is stable without any compensation when a
battery is connected with low impedance leads. Excessive
lead length, however, may add enough series inductance
to require a bypass capacitor of at least 1
μ
F from BAT to
GND. Furthermore, a 4.7μF capacitor with a 0.2Ω to 1Ω
series resistor from BAT to GND is required to keep ripple
voltage low when the battery is disconnected.
In constant-current mode, the PROG pin voltage is in
the feedback loop, not the battery voltage. Because of
the additional pole created by PROG pin capacitance,
capacitance on this pin must be kept to a minimum. With
no additional capacitance on the PROG pin, the battery
charger is stable with program resistor values as high
as 25k. However, additional capacitance on this node
reduces the maximum allowed program resistor. The pole
frequency at the PROG pin should be kept above 100kHz.
Therefore, if the PROG pin is loaded with a capacitance,
CPROG, the following equation should be used to calculate
the maximum resistance value for RPROG:
RkHz C
PROG PROG
≤π
1
2 100••
Average, rather than instantaneous, battery current may be
of interest to the user. For example, when the switching
regulator operating in low-current mode is connected in
parallel with the battery, the average current being pulled
out of the BAT pin is typically of more interest than the
instantaneous current pulses. In such a case, a simple RC
fi lter can be used on the PROG pin to measure the average
battery current as shown in Figure 3. A 10k resistor has
been added between the PROG pin and the fi lter capacitor
to ensure stability.
Undervoltage Charge Current Limiting (UVCL)
USB powered systems tend to have highly variable source
impedances (due primarily to cable quality and length). A
transient load combined with such impedance can easily trip
the UVLO threshold and turn the battery charger off unless
undervoltage charge current limiting is implemented.
Consider a situation where the LTC4081 is operating under
normal conditions and the input supply voltage begins to
sag (e.g. an external load drags the input supply down).
If the input voltage reaches VUVCL (approximately 300mV
above the battery voltage, ΔVUVCL), undervoltage charge
current limiting will begin to reduce the charge current in
an attempt to maintain ΔVUVCL between VCC and BAT. The
LTC4081 will continue to operate at the reduced charge
current until the input supply voltage is increased or volt-
age mode reduces the charge current further.
Figure 3. Isolating Capacitive Load
on PROG Pin and Filtering
4081 F03
CFILTER
CHARGE
CURRENT
MONITOR
CIRCUITRY
RPROG
LTC4081
PROG
GND
10k
LTC4081
17
4081f
VCC
MP1
MN1
1k 4k
1k
1ICHG
2
D1
4Li-Ion
BATTERY
SYSTEM
LOAD
4081 F04
LTC4081
BAT
USB
POWER
(100mA)
5V WALL
ADAPTER
(500mA)
PROG +
Figure 4. Combining Wall Adapter and USB Power
APPLICATIO S I FOR ATIO
WUUU
Operation from Current Limited Wall Adapter
By using a current limited wall adapter as the input sup-
ply, the LTC4081 can dissipate signifi cantly less power
when programmed for a current higher than the limit of
the wall adapter.
Consider a situation where an application requires a 200mA
charge current for a discharged 800mAh Li-Ion battery.
If a typical 5V (non-current limited) input supply is avail-
able then the peak power dissipation inside the part can
exceed 300mW.
Now consider the same scenario, but with a 5V input
supply with a 200mA current limit. To take advantage
of the supply, it is necessary to program the LTC4081
to charge at a current greater than 200mA. Assume that
the LTC4081 charger is programmed for 300mA (i.e.,
RPROG = 1.33kΩ) to ensure that part tolerances maintain
a programmed current higher than 200mA. Since the
battery charger will demand a charge current higher than
the current limit of the input supply, the supply voltage
will collapse to the battery voltage plus 200mA times the
on-resistance of the internal PFET. The on-resistance of
the battery charger power device is approximately 0.7Ω
with a 5V supply. The actual on-resistance will be slightly
higher due to the fact that the input supply will have col-
lapsed to less than 5V. The power dissipated during this
phase of charging is approximately 30mW. That is a ten
times improvement over the non-current limited supply
power dissipation.
USB and Wall Adapter Power
Although the LTC4081 allows charging from a USB port,
a wall adapter can also be used to charge Li-Ion batter-
ies. Figure 4 shows an example of how to combine wall
adapter and USB power inputs. A P-channel MOSFET,
MP1, is used to prevent back conducting into the USB
port when a wall adapter is present and Schottky diode,
D1, is used to prevent USB power loss through the 1k
pulldown resistor.
Typically a wall adapter can supply signifi cantly more
current than the current-limited USB port. Therefore, an
N-channel MOSFET, MN1, and an extra program resistor
can be used to increase the charge current when the wall
adapter is present.
Power Dissipation
The conditions that cause the LTC4081 battery charger to
reduce charge current through thermal feedback can be
approximated by considering the total power dissipated
in the IC. For high charge currents, the LTC4081 power
dissipation is approximately:
PVV I P
D CC BAT BAT D BUCK
=−
()
+•_
Where PD is the total power dissipated within the IC, VCC
is the input supply voltage, VBAT is the battery voltage, IBAT
is the charge current and PD_BUCK is the power dissipation
due to the regulator. PD_BUCK can be calculated as:
PVI
D BUCK OUT OUT_•=−
⎛
⎝
⎜⎞
⎠
⎟
11
η
LTC4081
18
4081f
APPLICATIO S I FOR ATIO
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Where VOUT is the regulated output of the switching
regulator, IOUT is the regulator load and η is the regulator
effi ciency at that particular load.
It is not necessary to perform worst-case power dissipa-
tion scenarios because the LTC4081 will automatically
reduce the charge current to maintain the die temperature
at approximately 115°C. However, the approximate ambi-
ent temperature at which the thermal feedback begins to
protect the IC is:
T
A = 115°C – PDθJA
TA = 115°C – (VCC – VBAT) • IBAT • θJA if the regulator
is off.
Example: Consider the extreme case when an LTC4081 is
operating from a 6V supply providing 250mA to a 3V Li-Ion
battery and the regulator is off. The ambient temperature
above which the LTC4081 will begin to reduce the 250mA
charge current is approximately:
T
A = 115°C – (6V – 3V) • (250mA) • 43°C/W
TA = 115°C – 0.75W • 43°C/W = 115°C – 32.25°C
T
A = 82.75°C
If there is more power dissipation due to the regulator,
the thermal regulation will begin at a somewhat lower
temperature. In the above circumstances, the LTC4081
can be used above 82.75°C, but the charge current will be
reduced from 250mA. The approximate current at a given
ambient temperature can be calculated:
ICT
VV
BAT A
CC BAT JA
=°−
−
()
115
•θ
Using the previous example with an ambient temperature
of 85°C, the charge current will be reduced to approxi-
mately:
ICC
VV CW
C
CA
BAT =°− °
−
()
°=°
°=
115 85
63 43
30
129 2
•/ /332 6.mA
Furthermore, the voltage at the PROG pin will change
proportionally with the charge current as discussed in the
Programming Charge Current section.
VCC Bypass Capacitor
Many types of capacitors can be used for input bypassing;
however, caution must be exercised when using multi-layer
ceramic capacitors. Because of the self-resonant and high
Q characteristics of some types of ceramic capacitors, high
voltage transients can be generated under some start-up con-
ditions, such as connecting the battery charger input to a live
power source. Adding a 1
Ω
series resistor in series with an
X5R ceramic capacitor will minimize start-up voltage transients.
For more information, refer to Application Note 88.
Thermistors
The LTC4081 NTC trip points are designed to work with therm-
istors whose resistance-temperature characteristics follow
Vishay Dale’s “R-T Curve 1.” The Vishay NTHS0603NO1N1002J
is an example of such a thermistor. However, Vishay Dale
has many thermistor products that follow the “R-T Curve 1”
characteristic in a variety of sizes. Furthermore, any thermis-
tor whose ratio of RCOLD to RHOT is about 5 will also work
(Vishay Dale R-T Curve 1 shows a ratio of RCOLD to RHOT of
3.266/0.5325 = 6.13).
LTC4081
19
4081f
APPLICATIO S I FOR ATIO
WUUU
Power conscious designs may want to use thermistors whose
room temperature value is greater than 10k. Vishay Dale has a
number of values of thermistor from 10k to 100k that follow
the “R-T Curve 1.” Using different R-T curves, such as Vishay
Dale “R-T Curve 2”, is also possible. This curve, combined with
LTC4081 internal thresholds, gives temperature trip points of
approximately 0°C (falling) and 40°C (rising), a delta of 40°C.
This delta in temperature can be moved in either direction by
changing the value of RNOM with respect to RNTC. Increasing
RNOM will move both trip points to higher temperatures. To
calculate RNOM for a shift to lower temperature for example,
use the following equation:
RRRatC
NOM COLD NTC
=°
3 266 25
.•
where RCOLD is the resistance ratio of RNTC at the desired cold
temperature trip point. If you want to shift the trip points to
higher temperatures use the following equation:
RRRatC
NOM HOT NTC
=°
0 5325 25
.•
where RHOT is the resistance ratio of RNTC at the desired hot
temperature trip point.
Here is an example using a 100k R-T Curve 2 thermistor
from Vishay Dale. The difference between the trip points is
40°C, from before, and we want the cold trip point to be 0°C,
which would put the hot trip point at 40°C. The RNOM needed
is calculated as follows:
RRRatC
NOM COLD NTC
=°
=
3 266 25
2 816
3
.•
.
.. •.
266 10 8 62kk=
The nearest 1% value for RNOM is 8.66k. This is the value used
to bias the NTC thermistor to get cold and hot trip points of
approximately 0°C and 40°C respectively. To extend the delta
between the cold and hot trip points, a resistor, R1, can be
added in series with RNTC (see Figure 5). The values of the
resistors are calculated as follows:
RRR
R
NOM COLD HOT
=−
−
=
3 266 0 5325
0 5325
3 266
1
..
.
.−−
⎛
⎝
⎜⎞
⎠
⎟−
()
−
0 5325.•R R R
COLD HOT HOT
where RNOM is the value of the bias resistor and RHOT and
RCOLD are the values of RNTC at the desired temperature trip
points. Continuing the example from before with a desired
trip point of 50°C:
RRR k
NOM COLD HOT
=−
−=−
3 266 0 5325
10 2 816 0
..
•. .
44086
3 266 0 5325
88 887
()
−
=
..
.,.kkis %.
•.
.
the nearest value
Rk
1
10 0 5325
326
1=66 0 5325 2 816 0 4086 0 4086
−
⎛
⎝
⎜⎞
⎠
⎟−
()
−
.•. . .
,%.=604 604 1Ωis the nearest value
4081 F05
RNOM
8.87k
RNTC
10k
VCC
–
+
–
+
–
+
TOO COLD
TOO HOT
NTC_ENABLE
R1
604Ω
0.76 • VCC
0.35 • VCC
0.016 • VCC
6NTC
T
Figure 5. NTC Circuits
LTC4081
20
4081f
NTC Trip Point Error
When a 1% resistor is used for RHOT, the major error in the 40°C
trip point is determined by the tolerance of the NTC thermistor.
A typical 100k NTC thermistor has ±10% tolerance. By look-
ing up the temperature coeffi cient of the thermistor at 40°C,
the tolerance error can be calculated in degrees centigrade.
Consider the Vishay NTHS0603N01N1003J thermistor, which
has a temperature coeffi cient of –4%/°C at 40°C. Dividing
the tolerance by the temperature coeffi cient, ±5%/(4%/°C) =
±1.25°C, gives the temperature error of the hot trip point.
The cold trip point error depends on the tolerance of the NTC
thermistor and the degree to which the ratio of its value at
0°C and its value at 40°C varies from 6.14 to 1. Therefore,
the cold trip point error can be calculated using the tolerance,
TOL, the temperature coeffi cient of the thermistor at 0°C, TC
(in %/°C), the value of the thermistor at 0°C, RCOLD, and the
value of the thermistor at 40°C, RHOT. The formula is:
Temperature Error C
TOL R
R
COLD
HOT
() .•
°=
+1
614 −−
⎛
⎝
⎜⎞
⎠
⎟
1 100•
TC
For example, the Vishay NTHS0603N01N1003J thermistor
with a tolerance of ±5%, TC of –5%/°C and RCOLD/RHOT of
6.13, has a cold trip point error of:
Temperature Error C()
.
.•.
°=
+−
⎛
⎝
1005
614 613 1
⎜⎜ ⎞
⎠
⎟
−
• 100
5
.,.=− ° °095 105CC
SWITCHING REGULATOR
Setting the Buck Converter Output Voltage
The LTC4081 regulator compares the FB pin voltage with
an internal 0.8V reference to generate an error signal at the
output of the error amplifi er. A voltage divider from VOUT
to ground (as shown in the Block Diagram) programs the
output voltage via FB using the formula:
VV
R
R
OUT =+
⎡
⎣
⎢⎤
⎦
⎥
08 1 7
8
.•
Keeping the current low (<5μA) in these resistors maxi-
mizes effi ciency, but making them too low may allow stray
capacitance to cause noise problems and reduce the phase
margin of the error amp loop. To improve the frequency
response, a phase-lead capacitor (CPL) of approximately
10pF can be used. Great care should be taken to route the
FB line away from noise sources, such as the inductor or
the SW line.
Inductor Selection
The value of the inductor primarily determines the cur-
rent ripple in the inductor. The inductor ripple
current ΔIL decreases with higher inductance and
increases with higher VIN or VOUT:
ΔIV
fL
V
V
LOUT
OSC
OUT
IN
=−
⎛
⎝
⎜⎞
⎠
⎟
••1
APPLICATIO S I FOR ATIO
WUUU
LTC4081
21
4081f
Accepting larger values of ΔIL allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability. A
reasonable starting point for setting ripple current is ΔIL
=0.3 • ILIM, where ILIM is the peak switch current limit.
The largest ripple current occurs at the maximum input
voltage. To guarantee that the ripple current stays below a
specifi ed maximum, the inductor value should be chosen
according to the following equation:
LV
fI
V
V
OUT
L
OUT
IN MAX
≥−
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
()
0
1
••
Δ
For applications with VOUT = 1.8V, the above equation
suggests that an inductor of at least 6.8μH should be used
for proper operation.
Many different sizes and shapes of inductors are
available from numerous manufacturers. To maximize
effi ciency, choose an inductor with a low DC resistance.
Keep in mind that most inductors that are very thin or
have a very small volume typically have much higher core
and DCR losses and will not give the best effi ciency. Also
choose an inductor with a DC current rating at least 1.5
times larger than the peak inductor current limit to ensure
that the inductor does not saturate during normal opera-
tion. To minimize radiated noise use a toroid or shielded
pot core inductor in ferrite or permalloy materials. Table
1 shows a list of several inductor manufacturers.
APPLICATIO S I FOR ATIO
WUUU
Table 1. Recommended Surface Mount Inductor Manufacturers
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Murata www.murata.com
Toko www.tokoam.com
Input and Output Capacitor Selection
Since the input current waveform to a buck converter is a
square wave, it contains very high frequency components.
It is strongly recommended that a low equivalent series
resistance (ESR) multilayer ceramic capacitor be used to
bypass the BAT pin which is the input for the converter.
Tantalum and aluminum capacitors are not recommended
because of their high ESR. The value of the capacitor on
BAT directly controls the amount of input voltage ripple for
a given load current. Increasing the size of this capacitor
will reduce the input ripple.
To prevent large VOUT voltage steps during transient
load conditions, it is also recommended that a ceramic
capacitor be used to bypass VOUT. A typical value for this
capacitor is 4.7μF.
Multilayer Ceramic Chip Capacitors (MLCC) typically have
exceptional ESR performance. MLCCs combined with a
carefully laid out board with an unbroken ground plane will
yield very good performance and low EMI emissions.
LTC4081
22
4081f
There are several types of ceramic capacitors with consider-
ably different characteristics. Y5V ceramic capacitors have
apparently higher packing density but poor performance
over their rated voltage or temperature ranges. Under
given voltage and temperature conditions, X5R and X7R
ceramic capacitors should be compared directly by case
size rather than specifi ed value for a desired minimum
capacitance. Some manufacturers provide excellent data on
their websites about achievable capacitance. Table 2 shows
a list of several ceramic capacitor manufacturers.
Table 2. Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden www.t-yuden.com
AVX www.avxcorp.com
Murata www.murata.com
TDK www.tdk.com
Board Layout Considerations
To be able to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on the
backside of the LTC4081’s package has a good thermal
contact to the PC board ground. Correctly soldered to a
2500mm2 double-sided 1 oz. copper board, the LTC4081
has a thermal resistance of approximately 43°C/W. Failure
to make thermal contact between the exposed pad on the
backside of the package and the copper board will result
in thermal resistance far greater than 43°C/W.
Furthermore due to its high frequency switching circuitry,
it is imperative that the input capacitor, BAT pin capaci-
tor, inductor, and the output capacitor be as close to the
LTC4081 as possible and that there is an unbroken ground
plane under the LTC4081 and all of its high frequency
components.
APPLICATIO S I FOR ATIO
WUUU
LTC4081
23
4081f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However,
no responsibility is assumed for its use. Linear Technology Corporation makes no representation that
the interconnection of its circuits as described herein will not infringe on existing patent rights.
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699)
3.00 ±0.10
(4 SIDES)
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
0.38 ± 0.10
BOTTOM VIEW—EXPOSED PAD
1.65 ± 0.10
(2 SIDES)
0.75 ±0.05
R = 0.115
TYP
2.38 ±0.10
(2 SIDES)
15
106
PIN 1
TOP MARK
(SEE NOTE 6)
0.200 REF
0.00 – 0.05
(DD) DFN 1103
0.25 ± 0.05
2.38 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
1.65 ±0.05
(2 SIDES)2.15 ±0.05
0.50
BSC
0.675 ±0.05
3.50 ±0.05
PACKAGE
OUTLINE
0.25 ± 0.05
0.50 BSC
PACKAGE DESCRIPTIO
U
LTC4081
24
4081f
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
LT 0707 • PRINTED IN USA
PART NUMBER DESCRIPTION COMMENTS
Battery Chargers
LTC3550
Dual Input USB/AC Adapter Li-Ion Battery Charger
with Adjustable Output 600mA Buck Converter
Synchronous Buck Converter, Effi ciency: 93%, Adjustable Output: 600mA,
Charge Current: 950mA Programmable, USB Compatible, Automatic Input Power
Detection and Selection
LTC3550-1
Dual Input USB/AC Adapter Li-Ion Battery Charger
with 600mA Buck Converter
Synchronous Buck Converter, Effi ciency: 93%, Output: 1.875V at 600mA,
Charge Current: 950mA Programmable, USB Compatible, Automatic Input Power
Detection and Selection
LTC4054 Standalone Linear Li-Ion Battery Charger with
Integrated Pass Transistor in ThinSOTTM
Thermal Regulation Prevents Overheating, C/10 Termination
LTC4061 Standalone Li-Ion Charger with Thermistor
Interface
4.2V, ±0.35% Float Voltage, Up to 1A Charge Current, 3mm × 3mm
DFN Package
LTC4061-4.4 Standalone Li-Ion Charger with Thermistor
Interface
4.4V (Max), ±0.4% Float Voltage, Up to 1A Charge Current, 3mm × 3mm
DFN Package
LTC4062 Standalone Linear Li-Ion Battery Charger with
Micropower Comparator
Up to 1A Charge Current, Charges from USB Port, Thermal Regulation
3mm × 3mm DFN Package
LTC4063 Li-Ion Charger with Linear Regulator Up to 1A Charge Current, 100mA, 125mV LDO, 3mm × 3mm DFN Package
LTC4080 Standalone 500mA Charger with 300mA
Synchronous Buck
For 1-Cell Li-Ion/Polymer Batteries; Trickle Charge; Timer Termination +C/10;
Thermal Regulation, Buck Output: 0.8V to VBAT, Buck Input: 2.7V to 5.5V, 3mm ×
3mm DFN-10 Package
Power Management
LTC3405/LTC3405A 300mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter
95% Effi ciency, VIN: 2.7V to 6V, VOUT = 0.8V, IQ = 20μA, ISD < 1μA,
ThinSOT Package
LTC3406/LTC3406A 600mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter
95% Effi ciency, VIN: 2.5V to 5.5V, VOUT = 0.6V, IQ = 20μA, ISD < 1μA,
ThinSOT Package
LTC3411 1.25A (IOUT), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Effi ciency, VIN: 2.5V to 5.5V, VOUT = 0.8V, IQ = 60μA, ISD < 1μA,
MS Package
LTC3440 600mA (IOUT), 2MHz, Synchronous Buck-Boost
DC/DC Converter
95% Effi ciency, VIN: 2.5V to 5.5V, VOUT = 2.5V, IQ = 25μA, ISD < 1μA,
MS Package
LTC4411/LTC4412 Low Loss PowerPathTM Controller in ThinSOT Automatic Switching Between DC Sources, Load Sharing, Replaces ORing Diodes
LTC4413 Dual Ideal Diode in DFN 2-Channel Ideal Diode ORing, Low Forward On-Resistance, Low Regulated
Forward Voltage, 2.5V ≤ VIN ≤ 5.5V
ThinSOT and PowerPath are trademarks of Linear Technology Corporation.
RELATED PARTS
TYPICAL APPLICATIO
U
Li-Ion Battery Charger with 1.5V Buck Regulator
500mA
4.2V
Li-Ion/
POLYMER
BATTERY
CBAT
4.7μF
RPROG
806Ω
R3
510Ω
D1
R2
806k
COUT
4.7μF
4081 TA02a
CPL
10pF
R1
715k
LTC4081
NTC
EN_CHRG
MODE
SW
FB
PROG
VCC
EN_BUCK
CHRG
BAT
RNOM
100k
RNTC
100k
VOUT
(1.5V/300mA)
L1
1OμH
GND
CIN
4.7μF
VCC
(3.75V
TO 5.5V)
+
T
Buck Effi ciency vs Load Current
(VOUT = 1.5V)
LOAD CURRENT (mA)
0.01
40
EFFICIENCY (%)
POWER LOSS (mW)
60
80
0.1 10 1001 1000
20
0
100
1
10
100
0.1
0.01
1000
4081 TA02b
VBAT = 3.8V
VOUT = 1.5V
L = 10μH
C = 4.7μF
EFFICIENCY
(Burst)
POWER LOSS
(Burst)
EFFICIENCY
(PWM) POWER
LOSS
(PWM)
