LTC4081
1
4081fa
For more information www.linear.com/LTC4081
TYPICAL APPLICATION
FEATURES DESCRIPTION
500mA Li-Ion Charger
with NTC Input and
300mA Synchronous Buck
The LT C
®
4081 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 specifically designed to work within USB
power specifications.
The CHRG 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
efficiency Burst Mode operation that can be selected by
the MODE pin.
The LTC4081 is available in a 10-lead, low profile (0.75mm)
3mm × 3mm DFN package.
APPLICATIONS
Battery Charger:
n Constant-Current/Constant-Voltage Operation
with Thermal Feedback to Maximize Charge Rate
withoutRisk of Overheating
n Internal 4.5-Hour Safety Timer for Termination
n Charge Current Programmable Up to 500mA with
5% Accuracy
n NTC Thermistor Input for Temperature Qualified
Charging
n C/10 Charge Current Detection Output
n 5µA Supply Current in Shutdown Mode
Switching Regulator:
n
High Efficiency Synchronous Buck Converter
n 300mA Output Current (Constant-Frequency Mode)
n 2.7V to 4.5V Input Range (Powered from BAT Pin)
n 0.8V to VBAT Output Range
n MODE Pin Selects Fixed (2.25MHz) Constant-Frequency
PWM Mode or Low ICC (23µA) Burst Mode
®
Operation
n 2µA BAT Current in Shutdown Mode
n 10-Lead, Low Profile (0.75 mm) 3mm × 3mm DFN Package
n Wireless Headsets
n Bluetooth Applications
n Portable MP3 Players
n Multifunction Wristwatches
L, LT , LT C , LT M , Linear Technology, the Linear logo and Burst Mode are registered trademarks
and ThinSOT and PowerPath are trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Protected by U.S. Patents, including
6522118.
Li-Ion Battery Charger with 1.8V Buck Regulator
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 Efficiency vs Load Current
(VOUT = 1.8V)
500mA
4.2V
Li-Ion/
POLYMER
BATTERY
4.7μF
806Ω
510Ω
806k 4.7μF
4081 TA01a
10pF 1M
LTC4081
NTC
EN_CHRG
MODE
FB
PROG
VCC CHRG
BAT
100k
100k
VOUT
(1.8V/300mA)
1OμH
GND
4.7μF
VCC
(3.75V
TO 5.5V) +
T
SW
EN_BUCK
LTC4081
2
4081fa
For more information www.linear.com/LTC4081
The l denotes specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C, VCC = 5V, VBAT = 3.8V, VEN_CHRG = 0V, VNTC = 0V, VEN_BUCK = VBAT, VMODE=0V. (Note 2)
PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGS
VCC, t < 1ms and Duty Cycle < 1% ............... –0.3V to 7V
VCC Steady State .......................................... –0.3V to 6V
BAT, CHRG ................................................... –0.3V to 6V
EN_CHRG, PROG, NTC ....................–0.3V to VCC + 0.3V
MODE, EN_BUCK ........................... –0.3V to VBAT + 0.3V
FB ................................................................ –0.3V to 2V
(Note 1)
ORDER INFORMATION
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
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 specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VCC Battery Charger Supply Voltage (Note 4) l3.75 5 5.5 V
VBAT Input Voltage for the Switching Regulator (Note 5) l2.7 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 l110 300 µA
ICC_SD Supply Current in Shutdown (Both Battery
Charger and Switching Regulator Off)
VEN_CHRG = 5V, VEN_BUCK = 0, VCC > VBAT
VEN_CHRG = 4V, VEN_BUCK = 0, VCC (3.5V) < VBAT (4V)
l5
2
10 µA
µA
IBAT_SD Battery Current in Shutdown (Both Battery
Charger and Switching Regulator Off)
VEN_CHRG = 5V, VEN_BUCK = 0, VCC > VBAT
VEN_CHRG = 4V, VEN_BUCK = 0, VCC (3.5V) < VBAT (4V)
l0.6
2
5 µA
µA
ELECTRICAL CHARACTERISTICS
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
LTC4081
3
4081fa
For more information www.linear.com/LTC4081
The l denotes specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C, VCC = 5V, VBAT = 3.8V, VEN_CHRG = 0V, VNTC = 0V, VEN_BUCK = VBAT, VMODE=0V. (Note 2)
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Battery Charger
VFLOAT VBAT Regulated Output Voltage IBAT = 2mA
IBAT = 2mA, 4.3V < VCC < 5.5V
l
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
l
l
90
475
100
500
110
525
mA
mA
VUVLO_CHRG VCC Undervoltage Lockout Voltage VCC Rising
VCC Falling
l
l
3.5
2.8
3.6
3.0
3.7
3.2
V
V
VPROG PROG Pin Servo Voltage 0.8k ≤ RPROG ≤ 4k l0.98 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 l2.75 2.9 3.05 V
VTRHYS Trickle Charge Threshold Voltage Hysteresis 100 150 350 mV
DVRECHRG Recharge Battery Threshold Voltage VFLOAT – VBAT, 0°C < TA < 85°C 70 100 130 mV
DVUVCL1,
DVUVCL2
(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 l3 4.5 6 hrs
Recharge Time l1.5 2.25 3 hrs
Low-Battery Charge Time VBAT = 2.5V l0.75 1.125 1.5 hrs
IC/10 End of Charge Indication Current Level RPROG = 2k (Note 6) l0.085 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 mW
fBADBAT Defective Battery Detection CHRG
Pulse Frequency
VBAT = 2V 2 Hz
DBADBAT Defective Battery Detection CHRG
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 CHRG Pulse Frequency 2 Hz
DNTC Fault Temperature CHRG Pulse Frequency
Duty Ratio
25 %
LTC4081
4
4081fa
For more information www.linear.com/LTC4081
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 specifications
from 0°C to 85°C. Specifications 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.
Note 4: Although the LTC4081 charger functions properly at 3.75V, full
charge current requires an input voltage greater than the desired final
battery voltage per DVUVCL1 specification.
Note 5: The 2.8V maximum buck undervoltage lockout (VUVLO_BUCK) exit
threshold must first be exceeded before the minimum VBAT specification
applies.
Note 6: IC/10 is expressed as a fraction of measured full charge current
with indicated PROG resistor.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Buck Converter
VFB FB Servo Voltage l0.78 0.80 0.82 V
IFB FB Pin Input Current VFB = 0.85V –50 50 nA
fOSC Switching Frequency l1.8 2.25 2.75 MHz
IBAT_NL_CF No-Load Battery Current (Continuous
Frequency Mode)
No-Load for Regulator, VEN_CHRG = 5V,
L = 10µH, C = 4.7µF
1.9 mA
IBAT_NL_BM No-Load Battery Current (Burst Mode
Operation)
No-Load for Regulator, VEN_CHRG = 5V,
MODE = VBAT, L = 10µH, C = 4.7µF
23 µA
IBAT_SLP Battery Current in SLEEP Mode VEN_CHRG = 5V, MODE = VBAT,
VOUT > Regulation Voltage
l10 15 20 µA
VUVLO_BUCK Buck Undervoltage Lockout Voltage VBAT Rising
VBAT Falling
l
l
2.6
2.4
2.7
2.5
2.8
2.6
V
V
RON_P PMOS Switch On-Resistance 0.95 W
RON_N NMOS Switch On-Resistance 0.85 W
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 EN_CHRG, EN_BUCK, MODE Pin Low to High l1.2 V
VIL Input Low Voltage EN_CHRG, EN_BUCK, MODE Pin High to Low l0.4 V
VOL Output Low Voltage (CHRG) ISINK = 5mA l60 105 mV
IIH Input Current High EN_BUCK, MODE Pins at 5.5V, VBAT = 5V l–1 1 µA
IIL Input Current Low EN_CHRG, EN_BUCK, MODE Pins at GND l–1 1 µA
REN_CHRG EN_CHRG Pin Input Resistance VEN_CHRG = 5V 1 1.45 3.3 MW
ICHRG CHRG Pin Leakage Current VBAT = 4.5V, VEN_CHRG = 5V l1 µA
ELECTRICAL CHARACTERISTICS
The l denotes specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C, VCC = 5V, VBAT = 3.8V, VEN_CHRG = 0V, VNTC = 0V, VEN_BUCK = VBAT, VMODE=0V. (Note 2)
LTC4081
5
4081fa
For more information www.linear.com/LTC4081
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
EN_CHRG, EN_BUCK and
MODE Pin Threshold Voltage
vs Temperature
EN_CHRG Pin Pull-Down
Resistance vs Temperature
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise specified)
LTC4081
6
4081fa
For more information www.linear.com/LTC4081
20
25
35
15
10
5
0
30
4081 G17
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 G15
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 G16
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 Efficiency 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
CHRG Pin Output
Low Voltage vs Temperature
TYPICAL PERFORMANCE CHARACTERISTICS
Buck Efficiency 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 G14
VBAT = 3.8V
VOUT = 1.5V
L = 10μH
C = 4.7μF
EFFICIENCY
(PWM) POWER
LOSS
(PWM)
EFFICIENCY
(BURST)
POWER LOSS
(BURST)
(TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise specified)
LTC4081
7
4081fa
For more information www.linear.com/LTC4081
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 specified)
LTC4081
8
4081fa
For more information www.linear.com/LTC4081
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 G26
0V
50μs/DIV
VOUT
20mV/DIV
AC COUPLED
ILOAD
50mA/DIV
4081 G27
0mA
50μs/DIV
VPROG
200mV/DIV
4081 G29
0V
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise specified)
LTC4081
9
4081fa
For more information www.linear.com/LTC4081
BAT (Pin 1):
Charge Current Output and Buck Regulator
Input. Provides charge current to the battery and regulates
the final float voltage to 4.2V. An internal precision resistor
divider from this pin sets the float 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.
EN_CHRG (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:
IBAT =
V
PROG
R
PROG
•400
NTC (Pin 5): Input to the NTC (negative temperature coef-
ficient) 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
CHRG 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.
CHRG (Pin 6): Open-Drain Charge Status Output. The
charge status indicator pin has three states: pull-down,
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 CHRG pin is pulled low by
an internal N-channel MOSFET. When the charge current
drops to 10% of the full-scale current, the CHRG 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 CHRG
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 CHRG 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
float 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 float 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.
PIN FUNCTIONS
LTC4081
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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
+
–
115C
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
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OPERATION
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
final float voltage of 4.2V ±0.5%. The CHRG 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
components. 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
EN_CHRG pin is pulled
below the shutdown threshold (VIL).
When the BAT pin approaches the final float 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
CHRG 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 benefit 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
CHRG 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 EN_CHRG pin above VIH. A new
charge cycle will automatically restart if the BAT pin volt-
age 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-voltage
and constant-temperature fashion. Three of the amplifier
feedback loops shown control the constant-current, CA,
constant-voltage, VA, and constant-temperature, TA modes
(see Block Diagram). A fourth amplifier 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 cur-
rent of MP1 is exactly 400 times the drain current of MP3.
Amplifiers 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 amplifier 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 voltage gives an indication of the charge current any-
time in the charge cycle, as discussed in “Programming
Charge Current” in the Applications Information section.
If the die temperature starts to creep up above 115°C
due to internal power dissipation, the transconductance
amplifier, 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
LTC4081
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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
amplifier (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 (DVUVCL1). 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 CHRG 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 EN_CHRG pin above
the shutdown threshold.
Battery Charger Shutdown Mode
The LTC4081’s battery charger can be disabled by pulling
the EN_CHRG 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.
CHRG Status Output Pin
The charge status indicator pin has three states: pull-down,
pulsing at 2Hz (see Trickle Charge and Defective Battery
Detection and Battery Temperature Monitoring) and high
impedance. The pull-down state indicates that the bat-
tery charger is in a charge cycle. A high impedance state
indicates 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 CHRG 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, CHRG will stay in the strong pull-down 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 current
ramps from zero to full-scale current over a period of ap-
proximately 180µs. This has the effect of minimizing the
transient current load on the power supply during start-up.
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.
OPERATION
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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 filter time. When the aver-
age battery voltage falls below 4.1V (which corresponds
to 80% – 90% battery capacity), a new charge cycle is
initiated 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 CHRG output assumes a strong pull-down 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 coefficient (NTC) thermistor close to
the battery pack. The NTC circuitry is shown in Figure 1.
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
approximately 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 CHRG pin pulses in a
2Hz, 25% duty cycle frequency unless the charge task is
finished 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 efficiency
whereas the Burst Mode operation offers higher efficiency
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.
4081 F01
RNOM
RNTC
VCC
–
+
–
+
–
+
TOO COLD
TOO HOT
NTC_ENABLE
0.76 • VCC
0.35 • VCC
0.016 • VCC
6NTC
T
Figure 1. NTC Circuit Information
OPERATION
LTC4081
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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 amplifier, 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 flow 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
fixed frequency of 2.25MHz. This frequency is chosen to
minimize possible interference with the AM radio band.
Error Amplifier: The error amplifier is an internally com-
pensated transconductance (gm) amplifier 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 amplifier. This current 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 amplifier is converted into a
comparator monitoring the FB voltage, and the inductor
current swings between a fixed 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 efficiency 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 regulator 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 amplifier
ever exceeds the PMOS ILIM, the error amplifier 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.
Zero Current Comparator
The zero or reverse current comparator monitors the induc-
tor current to the output and shuts off the synchronous
rectifier when this current reduces to a predetermined
value (IZERO). In fixed frequency mode, this is set to
OPERATION
LTC4081
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negative 15mA meaning that the regulator allows the
inductor current to flow in the reverse direction (from the
output to ground through the synchronous rectifier) 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 efficiency.
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
efficiency 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
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
OPERATION
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
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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:
RPROG =400 •
1V
I
BAT
, IBAT =400 •
1V
R
PROG
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:
IBAT =
V
PROG
RPROG
•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 0.2W to 1W
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:
RPROG ≤
2π•100kHz •C
Figure 2. Isolating Capacitive Load
on PROG Pin and Filtering
4081 F02
CFILTER
CHARGE
CURRENT
MONITOR
CIRCUITRY
RPROG
LTC4081
PROG
GND
10k
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
filter can be used on the PROG pin to measure the average
battery current as shown in Figure 2. A 10k resistor has
been added between the PROG pin and the filter 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 un-
less 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, DVUVCL), undervoltage charge
current limiting will begin to reduce the charge current in
an attempt to maintain DVUVCL 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.
Operation from Current Limited Wall Adapter
By using a current limited wall adapter as the input sup-
ply, the LTC4081 can dissipate significantly less power
when programmed for a current higher than the limit of
the wall adapter.
APPLICATIONS INFORMATION
LTC4081
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VCC
MP1
MN1
1k
4k
1k
1ICHG
2
D1
4Li-Ion
BATTERY
SYSTEM
LOAD
4081 F03
LTC4081
BAT
USB
POWER
(100mA)
5V WALL
ADAPTER
(500mA)
PROG +
Figure 3. Combining Wall Adapter and USB Power
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 sup-
ply 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 = 1.33kW)
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 0.7W with a 5V supply.
The actual on-resistance will be slightly higher due to the
fact that the input supply will have collapsed 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 3 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
pull-down resistor.
Typically a wall adapter can supply significantly 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:
P
D=VCC −VBAT
( )
•IBAT +P
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:
P
D_BUCK =VOUT •IOUT
1
η−1
Where VOUT is the regulated output of the switching
regulator, IOUT is the regulator load and
η
is the regulator
efficiency 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:
TA = 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:
TA = 115°C – (6V – 3V) • (250mA) • 43°C/W
TA = 115°C – 0.75W • 43°C/W = 115°C – 32.25°C
TA = 82.75°C
APPLICATIONS INFORMATION
LTC4081
18
4081fa
For more information www.linear.com/LTC4081
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:
IBAT =
115°C−T
A
V
CC
−V
BAT
( )
•θ
JA
Using the previous example with an ambient temperature of
85°C, the charge current will be reduced to approximately:
I
BAT =
115°C−85°C
6V−3V
( )
• 43°C/W =
30°C
129°C/A =232.6mA
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
conditions, such as connecting the battery charger input to
a live power source. Adding a 1
W
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).
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:
RNOM =
R
COLD
3.266
• RNTC at 25°C
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:
RNOM =
R
HOT
0.5325
• RNTC at 25°C
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:
R
NOM =
R
COLD
3.266 • RNTC at 25°C
= 2.816
3.266
• 10k =8.62k
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
APPLICATIONS INFORMATION
LTC4081
19
4081fa
For more information www.linear.com/LTC4081
4081 F04
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 4. NTC Circuits
between the cold and hot trip points, a resistor, R1, can be
added in series with RNTC (see Figure 4). The values of the
resistors are calculated as follows:
RNOM =
R
COLD
−R
HOT
3.266−0.5325
R1 = 0.5325
3.266−0.5325
• RCOLD −RHOT
( )
−RHOT
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:
R
NOM =RCOLD −RHOT
3.266−0.5325 = 10k • 2.816−0.4086
( )
3.266−0.5325
= 8.8k, 8.87k is the nearest 1% value.
R
1 = 10k • 0.5325
3.266−0.5325
• 2.816−0.4086
( )
−0.4086
= 604W, 604 is the nearest 1% value.
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 looking up the temperature coefficient of the thermistor
at 40°C, the tolerance error can be calculated in degrees
centigrade. Consider the Vishay NTHS0603N01N1003J
thermistor, which has a temperature coefficient of –4%/°C at
40°C. Dividing the tolerance by the temperature coefficient,
±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 coefficient 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)=
1+TOL
6.14 • RCOLD
RHOT
−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)=
1+0.05
6.14 • 6.13−1
• 100
−5
= −0.95°C, 1.05°C
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 amplifier. A voltage divider from
APPLICATIONS INFORMATION
LTC4081
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4081fa
For more information www.linear.com/LTC4081
VOUT to ground (as shown in the Block Diagram) programs
the output voltage via FB using the formula:
VOUT =0.8V • 1+R7
R8
Keeping the current low (<5µA) in these resistors maxi-
mizes efficiency, 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
DIL decreases with higher inductance and increases with
higher VIN or VOUT:
DIL=VOUT
f
OSC
•L • 1−VOUT
V
IN
Accepting larger values of DIL 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
DIL = 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 specified maximum, the inductor value should
be chosen according to the following equation:
L≥VOUT
f
0
•DI
L
• 1−VOUT
V
IN MAX
( )
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 efficiency,
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 efficiency. Also choose an induc-
tor 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 operation. 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.
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.
APPLICATIONS INFORMATION
LTC4081
21
4081fa
For more information www.linear.com/LTC4081
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 specified 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.
APPLICATIONS INFORMATION
LTC4081
22
4081fa
For more information www.linear.com/LTC4081
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
0.40 ±0.10
BOTTOM VIEW—EXPOSED PAD
1.65 ±0.10
(2 SIDES)
0.75 ±0.05
R = 0.125
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 REV C 0310
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
0.70 ±0.05
3.55 ±0.05
PACKAGE
OUTLINE
0.25 ±0.05
0.50 BSC
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699 Rev C)
PIN 1 NOTCH
R = 0.20 OR
0.35 × 45°
CHAMFER
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
LTC4081
23
4081fa
For more information www.linear.com/LTC4081
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 07/15 Modified Typical Application diagrams 1, 24
LTC4081
24
4081fa
For more information www.linear.com/LTC4081
LINEAR TECHNOLOGY CORPORATION 2007
LT 0715 REV A • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC4081
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(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)
CBAT
4.7μF
RPROG
806Ω
D1
R2
806k COUT
4.7μF
4081 TA02a
CPL
10pF R1
715k
RNOM
100k
RNTC
100k
VOUT
(1.5V/300mA)
L1
1OμH
CIN
4.7μF
500mA
R3
510Ω
LTC4081
NTC
EN_CHRG
MODE
FB
PROG
VCC CHRG
BAT
GND
VCC
(3.75V
TO 5.5V) +
T
SW
EN_BUCK
4.2V
Li-Ion/
POLYMER
BATTERY