LT3652
1
3652fc
TYPICAL APPLICATION
DESCRIPTION
Power Tracking 2A Battery
Charger for Solar Power
The LT
®
3652 is a complete monolithic step-down bat-
tery charger that operates over a 4.95V to 32V input
voltage range. The LT3652 provides a constant-current/
constant-voltage charge characteristic, with maximum
charge current externally programmable up to 2A. The
charger employs a 3.3V fl oat voltage feedback reference,
so any desired battery fl oat voltage up to 14.4V can be
programmed with a resistor divider.
The LT3652 employs an input voltage regulation loop, which
reduces charge current if the input voltage falls below a
programmed level, set with a resistor divider. When the
LT3652 is powered by a solar panel, the input regulation
loop is used to maintain the panel at peak output power.
The LT3652 can be confi gured to terminate charging
when charge current falls below 1/10 of the programmed
maximum (C/10). Once charging is terminated, the LT3652
enters a low-current (85μA) standby mode. An auto-re-
charge feature starts a new charging cycle if the battery
voltage falls 2.5% below the programmed fl oat voltage.
The LT3652 also contains a programmable safety timer,
used to terminate charging after a desired time is reached.
This allows top-off charging at currents less than C/10.
2A Solar Panel Power Manager With 7.2V LiFePO4 Battery
and 17V Peak Power Tracking
FEATURES
APPLICATIONS
n Input Supply Voltage Regulation Loop for Peak
Power Tracking in (MPPT) Solar Applications
n Wide Input Voltage Range: 4.95V to 32V (40V Abs Max)
n Programmable Charge Rate Up to 2A
n User Selectable Termination: C/10 or On-Board
Termination Timer
n Resistor Programmable Float Voltage Up to 14.4V
Accommodates Li-Ion/Polymer, LiFePO4, SLA,
NiMH/NiCd Chemistries
n No VIN Blocking Diode Required for Battery
Voltages ≤ 4.2V
n 1MHz Fixed Frequency
n 0.5% Float Voltage Reference Accuracy
n 5% Charge Current Accuracy
n 2.5% C/10 Detection Accuracy
n Binary-Coded Open-Collector Status Pins
n 3mm × 3mm DFN12 or MSOP-12 Packages
n Solar Powered Applications
n Remote Monitoring Stations
n LiFePO4 (Lithium Phosphate) Applications
n Portable Handheld Instruments
n 12V to 24V Automotive Systems
Solar Panel Input Voltage
Regulation, Tracks Max Power
Point to Greater Than 98%
SW
VIN
SOLAR PANEL INPUT
(<40V OC VOLTAGE)
VIN_REG
VFB
BOOST
SENSE
BAT
NTC
TIMER
CMSH3-40MA
SYSTEM LOAD
1μF
542k
459k
3652 TA01a
10k
B = 3380
2-CELL LiFePO4 (2 × 3.6V) BATTERY PACK
+
10μF
CMSH1-40MA
10μF
10μH
0.05
LT3652
530k
100k
SHDN
CHRG
FAULT
CMSH3-40MA
CHARGER OUTPUT CURRENT (A)
0.2
INPUT REGULATION VOLTAGE (V)
10
12
14
20
18
16
0.6 11.2
3652 TA01b
22
0.4 0.8 1.61.4 21.8
100% TO 98% PEAK POWER
98% TO 95% PEAK POWER
TA = 25°C
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
LT3652
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3652fc
PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGS
Voltages:
V
IN ........................................................................40V
V
IN_REG, SHDN, CHRG, FAULT ............ VIN + 0.5V, 40V
SW ........................................................................40V
SW-VIN.................................................................4.5V
BOOST ...................................................SW+10V, 50V
BAT, SENSE ...........................................................15V
(Note 1)
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3652EDD#PBF LT3652EDD#TRPBF LFHT 12-Lead Plastic DFN 3mm × 3mm –40°C to 125°C
LT3652IDD#PBF LT3652IDD#TRPBF LFHT 12-Lead Plastic DFN 3mm × 3mm –40°C to 125°C
LT3652EMSE#PBF LT3652EMSE#TRPBF 3652 12-Lead Plastic MSOP –40°C to 125°C
LT3652IMSE#PBF LT3652IMSE#TRPBF 3652 12-Lead Plastic MSOP –40°C to 125°C
Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container.
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/
TOP VIEW
DD PACKAGE
12-LEAD (3mm × 3mm) PLASTIC DFN
12
11
8
9
10
4
5
3
2
1SW
BOOST
SENSE
BAT
NTC
VFB
VIN
VIN_REG
SHDN
CHRG
FAULT
TIMER 67
13
1
2
3
4
5
6
VIN
VIN_REG
SHDN
CHRG
FAULT
TIMER
12
11
10
9
8
7
SW
BOOST
SENSE
BAT
NTC
VFB
TOP VIEW
13
MSE PACKAGE
12-LEAD PLASTIC MSOP
TJMAX = 125°C, θJA = 43°C/W, θJC = 3°C/W
EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB
TJMAX = 125°C, θJA = 43°C/W, θJC = 3°C/W
EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB
BAT-SENSE ......................................... –0.5V to +0.5V
NTC, TIMER, ........................................................2.5V
V
FB ..........................................................................5V
Operating Junction Temperature Range
(Note 2) ............................................. –40°C to 125°C
Storage Temperature Range ................... –65°C to 150°C
LT3652
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ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VIN VIN Operating Range
VIN Start Voltage
VBAT = 4.2 (Notes 3, 4)
VBAT = 4.2 (Note 4)
l
l
4.95
7.5
32 V
V
VIN(OVLO) OVLO Threshold
OVLO Hysteresis
VIN Rising l32 35
1
40 V
V
VIN(UVLO) UVLO Threshold
UVLO Hysteresis
VIN Rising 4.6
0.2
4.95 V
V
VFB(FLT) Float Voltage Reference (Note 6)
l
3.282
3.26
3.3 3.318
3.34
V
V
ΔVRECHARGE Recharge Reference Threshold Voltage Relative to VFB(FLT) (Note 6) 82.5 mV
VFB(PRE) Reference Precondition Threshold VFB Rising (Note 6) 2.3 V
VFB(PREHYST) Reference Precondition Threshold
Hysteresis
Voltage Relative to VFB(PRE) (Note 6) 70 mV
VIN_REG(TH) Input Regulation Reference VFB = 3V; VSENSE – VBAT = 50mV l2.65 2.7 2.75 V
IIN_REG Input Regulation Reference Bias Current VIN_REG = VIN_REG(TH) l35 100 nA
IVIN Operating Input Supply Current CC/CV Mode, ISW = 0
Standby Mode
Shutdown (SHDN = 0)
l2.5
85
15
3.5 mA
μA
μA
IBOOST BOOST Supply Current Switch On, ISW = 0,
2.5 < V(BOOST – SW) < 8.5
20 mA
IBOOST/ISW BOOST Switch Drive ISW = 2A 30 mA/A
VSW(ON) Switch-On Voltage Drop VIN – VSW
, ISW = 2A 350 mV
ISW(MAX) Switch Current Limit l2.5 3 A
VSENSE(PRE) Precondition Sense Voltage VSENSE – VBAT; VFB = 2V 15 mV
VSENSE(DC) Maximum Sense Voltage VSENSE – VBAT; VFB = 3V (Note 7) l95 100 105 mV
VSENSE(C/10) C/10 Trigger Sense Voltage VSENSE – VBAT, Falling l7.5 10 12.5 mV
IBAT BAT Input Bias Current Charging Terminated 0.1 1 μA
ISENSE SENSE Input Bias Current Charging Terminated 0.1 1 μA
IREVERSE Charger Reverse Current
IBAT + ISENSE + ISW
VIN = 0; VBAT = VSENSE = VSW = 4.2V 1 μA
IVFB VFB Input Bias Current Charging Terminated 65 nA
IVFB VFB Input Bias Current CV Operation (Note 5) 110 nA
VNTC(H) NTC Range Limit (High) VNTC Rising l1.25 1.36 1.45 V
VNTC(L) NTC Range Limit (Low) VNTC Falling l0.27 0.29 0.315 V
VNTC(HYST) NTC Threshold Hysteresis % of threshold 20 %
RNTC(DIS) NTC Disable Impedance Impedance to ground l250 500 kΩ
INTC NTC Bias Current VNTC = 0.8V l47.5 50 52.5 μA
VSHDN Shutdown Threshold Rising l1.15 1.2 1.25 V
VSHDN(HYST) Shutdown Hysteresis 120 mV
ISHDN SHDN Input Bias Current –10 nA
VCHRG, VFAULT Status Low Voltage 10mA Load l0.4 V
ITIMER Charge/Discharge Current 25 μA
VTIMER(DIS) Timer Disable Threshold l0.1 0.25 V
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. VIN = 20V, Boost – SW = 4V, SHDN = 2V, VFB = 3.3V, CTIMER = 0.68μF.
LT3652
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SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
tTIMER Full Charge Cycle Timeout 3 hr
Precondition Timeout 22.5 min
Timer Accuracy l–10 10 %
fOOperating Frequency 1MHz
DC Duty Cycle Range Continuous Operation l15 90 %
ELECTRICAL CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. VIN = 20V, Boost – SW = 4V, SHDN = 2V, VFB = 3.3V, CTIMER = 0.68μF.
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 LT3652EDD is guaranteed to meet performance specifi cations
from 0°C to 125°C junction temperature. Specifi cations over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization, and correlation with statistical process controls. The
LT3652IDD specifi cations are guaranteed over the full –40°C to 125°C
operating junction temperature range. High junction temperatures degrade
operating lifetimes.
Note 3: VIN minimum voltages below the start threshold are only
supported if (VBOOST-VSW) > 2V.
Note 4: This parameter is valid for programmed output battery fl oat
voltages ≤ 4.2V. VIN operating range minimum is 0.75V above the
programmed output battery fl oat voltage (VBAT(FLT) + 0.75V). VIN Start
Voltage is 3.3V above the programmed output battery fl oat voltage
(VBAT(FLT) + 3.3V).
Note 5: Output battery fl oat voltage (VBAT(FLT)) programming resistor
divider equivalent resistance = 250k compensates for input bias current.
Note 6: All VFB voltages measured through 250k series resistance.
Note 7: VSENSE(DC) is reduced by thermal foldback as junction temperature
approaches 125°C.
LT3652
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TYPICAL PERFORMANCE CHARACTERISTICS
Switch Forward Drop (VIN – VSW)
vs Temperature
CC/CV Charging; SENSE Pin Bias
Current vs VSENSE
C/10 Threshold (VSENSE–VBAT)
vs Temperature
VFB Reference Voltage
vs Temperature
VIN Standby Mode Current
vs Temperature
Switch Drive (ISW/IBOOST)
vs Switch Current
TJ = 25°C, unless otherwise noted.
TEMPERATURE (°C)
–50
VFB (FLT)
3.296
3.298
3.300
3.302
050 75
3652 G01a
3.304
–25 25 100 125
TEMPERATURE (°C)
–50
65
IVIN CURRENT (μA)
70
75
80
100
90
050 75
3652 G02
95
85
–25 25 100
SWITCH CURRENT (A)
0
ISW/IBOOST
18
24
30
36
1.6
12
6
0
21
27
33
15
9
3
0.4 0.8 1.2
0.2 1.8
0.6 1.0 1.4 2.0
3652 G03
TEMPERATURE (°C)
–50
320
VSW(ON) (mV)
340
360
380
480
420
050 75
3652 G04
440
460
400
–25 25 100 125
ISW = 2A
VSENSE (V)
0
–350
ISENSE (μA)
–250
–150
–50
100
122.5
50
–300
–200
–100
0
0.5 1.5
3652 G05
VBAT = VBAT(PRE)
VBAT = VBAT(FLT)
TEMPERATURE (°C)
–50
8
VSENSE(C/10) (mV)
9
10
11
12
050 75
3652 G06
–25 25 100 125
TEMPERATURE (°C)
–50
VIN_REG(TH) (V)
2.680
2.685
2.690
2.715
2.710
2.705
2.700
2.695
050 75
3652 G01
2.720
–25 25 100 125
VIN_REG Threshold
vs Temperature: ICHG at 50%
LT3652
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TYPICAL PERFORMANCE CHARACTERISTICS
Maximum Charge Current
(VSENSE–VBAT) vs Temperature
Thermal Foldback – Maximum
Charge Current (VSENSE–VBAT)
vs Temperature
CC/CV Charging; BAT Pin Bias
Current vs VBAT
TA = 25°C, unless otherwise noted.
TEMPERATURE (°C)
–50
99.0
VSENSE(DC) (mV)
99.2
99.6
99.8
100.0
101.0
100.4
050 75
3652 G07
99.4
100.6
100.8
100.2
–25 25 100 125
VFB = 3V
TEMPERATURE (°C)
0
VSENSE(DC) (mV)
40
80
20
60
100
120
45 65 85 105 125
35 13525 55 75 95 115
3652 G08
VBAT (V)
0
–0.4
IBAT (mA)
0.0
0.4
0.8
2.2
1.6
122.5
2.0
1.2
–0.2
0.2
0.6
1.0
1.8
1.4
0.5 1.5 3
3652 G09
VBAT(FLT)
VIN_REG (V)
2.65
VSENSE(DC) (mV)
0
20
80
60
40
2.67 2.69 2.7
3652 G10
100
2.66 2.68 2.722.71 2.73 2.74 2.75
VBAT(FLT) (V)
02
0
IRFB (μA)
8
6
10
12
16
3652 G11
4
2
64 8 10 12 14
TIME (MINUTES)
0
EFFICIENCY (%)
CHARGE CURRENT (A); POWER LOSS (W)
0
0.5
2.0
2.5
1.5
1.0
40 80 100
3652 G12
3.0
35
45
75
65
55
95
85
20 60 140120 160 180 200
CHARGE
CURRENT
EFFICIENCY
POWER
LOSS
VIN = 20V
Maximum Charge Current
(VSENSE–VBAT) vs VIN_REG Voltage
VFLOAT Programming Resistor
Current vs VFLOAT for 2-Resistor
Network
Charge Current, Effi ciency, and
Power Loss vs Time
(ICHG(MAX) = 2A; VFLOAT = 8.2V)
Charger Effi ciency vs Battery
Voltage (ICHG = 2A)
VBAT (V)
70
EFFICIENCY (%)
76
80
82
84
86
88
74
72
78
90
5791113
414153681012
3652 G13
VIN = 20V WITH INPUT BLOCKING DIODE
LT3652
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PIN FUNCTIONS
VIN (Pin 1): Charger Input Supply. VIN operating range
is 4.95V to 32V. VIN must be 3.3V greater than the pro-
grammed output battery fl oat voltage (VBAT(FLT)) for reli-
able start-up. (VIN – VBAT(FLT)) ≥ 0.75V is the minimum
operating voltage, provided (VBOOST – VSW) ≥ 2V. IVIN ~
85μA after charge termination.
VIN_REG (Pin 2): Input Voltage Regulation Reference.
Maximum charge current is reduced when this pin is below
2.7V. Connecting a resistor divider from VIN to this pin
enables programming of minimum operational VIN voltage.
This is typically used to program the peak power voltage
for a solar panel. The LT3652 servos the maximum charge
current required to maintain the programmed operational
VIN voltage, through maintaining the voltage on VIN_REG
at or above 2.7V. If the voltage regulation feature is not
used, connect the pin to VIN.
SHDN (Pin 3): Precision Threshold Shutdown Pin. The
enable threshold is 1.2V (rising), with 120mV of input
hysteresis. When in shutdown mode, all charging functions
are disabled. The precision threshold allows use of the
SHDN pin to incorporate UVLO functions. If the SHDN pin
is pulled below 0.4V, the IC enters a low current shutdown
mode where VIN current is reduced to 15μA. Typical SHDN
pin input bias current is 10nA. If the shutdown function
is not desired, connect the pin to VIN.
CHRG (Pin 4): Open-Collector Charger Status Output;
typically pulled up through a resistor to a reference volt-
age. This status pin can be pulled up to voltages as high
as VIN when disabled, and can sink currents up to 10mA
when enabled. During a battery charging cycle, if required
charge current is greater than 1/10 of the programmed
maximum current (C/10), CHRG is pulled low. A tem-
perature fault also causes this pin to be pulled low. After
C/10 charge termination or, if the internal timer is used
for termination and charge current is less than C/10, the
CHRG pin remains high-impedance.
FAULT (Pin 5): Open-Collector Charger Status Output;
typically pulled up through a resistor to a reference volt-
age. This status pin can be pulled up to voltages as high
as VIN when disabled, and can sink currents up to 10mA
when enabled. This pin indicates fault conditions during a
battery charging cycle. A temperature fault causes this pin
to be pulled low. If the internal timer is used for termina-
tion, a bad battery fault also causes this pin to be pulled
low. If no fault conditions exist, the FAULT pin remains
high-impedance.
TIMER (Pin 6): End-Of-Cycle Timer Programming Pin.
If a timer-based charge termination is desired, connect
a capacitor from this pin to ground. Full charge end-of-
cycle time (in hours) is programmed with this capacitor
following the equation:
t
EOC = CTIMER • 4.4 • 106
A bad battery fault is generated if the battery does not
achieve the precondition threshold voltage within one-
eighth of tEOC, or:
t
PRE = CTIMER • 5.5 • 105
A 0.68μF capacitor is typically used, which generates a
timer EOC at three hours, and a precondition limit time of
22.5 minutes. If a timer-based termination is not desired,
the timer function is disabled by connecting the TIMER
pin to ground. With the timer function disabled, charging
terminates when the charge current drops below a C/10
threshold, or ICHG(MAX)/10
VFB (Pin 7): Battery Float Voltage Feedback Reference. The
charge function operates to achieve a fi nal fl oat voltage of
3.3V on this pin. Output battery fl oat voltage (VBAT(FLT))
is programmed using a resistor divider. VBAT(FLT) can be
programmed up to 14.4V.
The auto-restart feature initiates a new charging cycle
when the voltage at the VFB pin falls 2.5% below the fl oat
voltage reference.
The VFB pin input bias current is 110nA. Using a resistor
divider with an equivalent input resistance at the VFB pin
of 250k compensates for input bias current error.
Required resistor values to program desired VBAT(FLT)
follow the equations:
R1 = (VBAT(FLT) • 2.5 • 105)/3.3 (Ω)
R2 = (R1 • 2.5 • 105)/(R1 - (2.5 • 105)) (Ω)
R1 is connected from BAT to VFB, and R2 is connected
from VFB to ground.
LT3652
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NTC (Pin 8): Battery Temperature Monitor Pin. This pin is
the input to the NTC (Negative Temperature Coeffi cient)
thermistor temperature monitoring circuit. This function is
enabled by connecting a 10kΩ, B = 3380 NTC thermistor
from the NTC pin to ground. The pin sources 50μA, and
monitors the voltage across the 10kΩ thermistor. When
the voltage on this pin is above 1.36 (T < 0°C) or below
0.29V (T > 40°C), charging is disabled and the CHRG and
FAULT pins are both pulled low. If internal timer termina-
tion is being used, the timer is paused, suspending the
charging cycle. Charging resumes when the voltage on NTC
returns to within the 0.29V to 1.36V active region. There
is approximately 5°C of temperature hysteresis associated
with each of the temperature thresholds. The temperature
monitoring function remains enabled while the thermistor
resistance to ground is less than 250k, so if this function
is not desired, leave the NTC pin unconnected.
BAT (Pin 9): Charger Output Monitor Pin. Connect a
10μF decoupling capacitance (CBAT) to ground. Depend-
ing on application requirements, larger value decoupling
capacitors may be required. The charge function operates
to achieve the programmed output battery fl oat voltage
(VBAT(FLT)) at this pin. This pin is also the reference for
the current sense voltage. Once a charge cycle is termi-
nated, the input bias current of the BAT pin is reduced to
< 0.1μA, to minimize battery discharge while the charger
remains connected.
SENSE (Pin 10): Charge Current Sense Pin. Connect the
inductor sense resistor (RSENSE) from the SENSE pin to the
BAT pin. The voltage across this resistor sets the average
charge current. The maximum charge current (ICHG(MAX))
corresponds to 100mV across the sense resistor. This
resistor can be set to program maximum charge cur-
rent as high as 2A. The sense resistor value follows the
relation:
R
SENSE = 0.1/ICHG(MAX) (Ω)
Once a charge cycle is terminated, the input bias current of
the SENSE pin is reduced to < 0.1μA, to minimize battery
discharge while the charger remains connected.
BOOST (Pin 11): Bootstrapped Supply Rail for Switch
Drive. This pin facilitates saturation of the switch transistor.
Connect a 1μF or greater capacitor from the BOOST pin
to the SW pin. Operating range of this pin is 0V to 8.5V,
referenced to the SW pin. The voltage on the decoupling
capacitor is refreshed through a rectifying diode, with
the anode connected to either the battery output voltage
or an external source, and the cathode connected to the
BOOST pin.
SW (Pin 12): Switch Output Pin. This pin is the output
of the charger switch, and corresponds to the emitter of
the switch transistor. When enabled, the switch shorts
the SW pin to the VIN supply. The drive circuitry for this
switch is bootstrapped above the VIN supply using the
BOOST supply pin, allowing saturation of the switch for
maximum effi ciency. The effective on-resistance of the
boosted switch is 0.175Ω.
SGND (Pin 13): Ground Reference and Backside Exposed
Lead Frame Thermal Connection. Solder the exposed lead
frame to the PCB ground plane.
PIN FUNCTIONS
LT3652
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BLOCK DIAGRAM
3652 BD
+
–
+–
+
–
–
+
+
–
+
–
+
–
+
–
TIMER
10mΩ
35V
0.1V
OFFSET
2.3V
4.6V
RESET
ENABLE
COUNT
RESET
C/10
PRECONDITION
1.36V
0.29V
46μA
VIN_REG
BOOST
VIN
SW
SENSE
BAT
VFB
NTC
VINT
2.7V
1.3V
1V 0.15V
1.2V 3.3V 3.218V
TERMINATE
50μA
0.7V
CONTROL LOGIC
RIPPLE
COUNTER
STATUS
TIMER
OSC.
NTC
0.2V
125°C
COUNT
COUNT
OSC
1MHz
LATCH
RS
RS
C-EA
30mV
x2.25
10 × RS
0.3V
VC
TDIE
ITH
MODE
(TIMER OR C/10)
TERMINATE
R
QS
2.7V
UVLO
OVLO
–
+
FAULT
CHRG
STANDBY
SHDN
V-EA
+
–
+
–
+
–
+–
STANDBY
STANDBY
+
–
+
–
+
–
+
–
LT3652
10
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APPLICATIONS INFORMATION
Overview
LT3652 is a complete monolithic, mid-power, multi-chem-
istry buck battery charger, addressing high input voltage
applications with solutions that require a minimum of exter-
nal components. The IC uses a 1MHz constant frequency,
average-current mode step-down architecture.
The LT3652 incorporates a 2A switch that is driven by
a bootstrapped supply to maximize efficiency during
charging cycles. Wide input range allows operation to full
charge from voltages as high as 32V. A precision threshold
shutdown pin allows incorporation of UVLO functionality
using a simple resistor divider. The IC can also be put into
a low-current shutdown mode, in which the input supply
bias is reduced to only 15μA.
The LT3652 employs an input voltage regulation loop,
which reduces charge current if a monitored input volt-
age falls below a programmed level. When the LT3652 is
powered by a solar panel, the input regulation loop is used
to maintain the panel at peak output power.
The LT3652 automatically enters a battery precondition
mode if the sensed battery voltage is very low. In this mode,
the charge current is reduced to 15% of the programmed
maximum, as set by the inductor sense resistor, RSENSE.
Once the battery voltage reaches 70% of the fully charged
float voltage, the IC automatically increases maximum
charge current to the full programmed value.
The LT3652 can use a charge-current based C/10 termina-
tion scheme, which ends a charge cycle when the battery
charge current falls to one tenth of the programmed
maximum charge current. The LT3652 also contains an
internal charge cycle control timer, for timer-based termina-
tion. When using the internal timer, the IC combines C/10
detection with a programmable time constraint, during
which the charging cycle can continue beyond the C/10
level to top-off a battery. The charge cycle terminates
when a specific time elapses, typically 3 hours. When
the timer-based scheme is used, the IC also supports bad
battery detection, which triggers a system fault if a battery
stays in precondition mode for more than one eighth of
the total charge cycle time.
Once charging is terminated, the LT3652 automatically
enters a low-current standby mode where supply bias
currents are reduced to 85μA. The IC continues to monitor
the battery voltage while in standby, and if that voltage
falls 2.5% from the full-charge float voltage, the LT3652
engages an automatic charge cycle restart. The IC also
automatically restarts a new charge cycle after a bad bat-
tery fault once the failed battery is removed and replaced
with another battery.
The LT3652 contains provisions for a battery temperature
monitoring circuit. This feature monitors battery tempera-
ture using a thermistor during the charging cycle. If the
battery temperature moves outside a safe charging range
of 0°C to 40°C, the IC suspends charging and signals a
fault condition until the temperature returns to the safe
charging range.
The LT3652 contains two digital open-collector outputs,
which provide charger status and signal fault conditions.
These binary-coded pins signal battery charging, standby
or shutdown modes, battery temperature faults, and bad
battery faults.
General Operation (See Block Diagram)
The LT3652 uses average current mode control loop
architecture, such that the IC servos directly to average
charge current. The LT3652 senses charger output voltage
through a resistor divider via the VFB pin. The difference
between the voltage on this pin and an internal 3.3V volt-
age reference is integrated by the voltage error amplifier
(V-EA). This amplifier generates an error voltage on its
output (ITH), which corresponds to the average current
sensed across the inductor current sense resistor, RSENSE,
which is connected between the SENSE and BAT pins.
The ITH voltage is then divided down by a factor of 10,
and imposed on the input of the current error amplifier
(C-EA). The difference between this imposed voltage and
the current sense resistor voltage is integrated, with the
resulting voltage (VC) used as a threshold that is compared
against an internally generated ramp. The output of this
comparison controls the charger’s switch.
LT3652
11
3652fc
APPLICATIONS INFORMATION
The ITH error voltage corresponds linearly to average
current sensed across the inductor current sense resistor,
allowing maximum charge current control by limiting the
effective voltage range of ITH. A clamp limits this voltage
to 1V which, in turn, limits the current sense voltage to
100mV. This sets the maximum charge current, or the
current delivered while the charger is operating in con-
stant-current (CC) mode, which corresponds to 100mV
across RSENSE. The ITH voltage is pulled down to reduce
this maximum charge current should the voltage on the
VIN_REG pin falls below 2.7V (VIN_REG(TH)) or the die tem-
perature approaches 125°C.
If the voltage on the VFB pin is below 2.3V (VFB(PRE)),
the LT3652 engages precondition mode. During the
precondition interval, the charger continues to operate in
constant-current mode, but the maximum charge current
is reduced to 15% of the maximum programmed value
as set by RSENSE.
When the charger output voltage approaches the float volt-
age, or the voltage on the VFB pin approaches 3.3V (VFB(FLT)),
the charger transitions into constant-voltage (CV) mode
and charge current is reduced from the maximum value.
As this occurs, the ITH voltage falls from the limit clamp
and servos to lower voltages. The IC monitors the ITH volt-
age as it is reduced, and detection of C/10 charge current
is achieved when ITH = 0.1V. If the charger is configured
for C/10 termination, this threshold is used to terminate
the charge cycle. Once the charge cycle is terminated,
the CHRG status pin becomes high-impedance and the
charger enters low-current standby mode.
The LT3652 contains an internal charge cycle timer that
terminates a successful charge cycle after a programmed
amount of time. This timer is typically programmed to
achieve end-of-cycle (EOC) in 3 hours, but can be con-
figured for any amount of time by setting an appropriate
timing capacitor value (CTIMER). When timer termination
is used, the charge cycle does not terminate when C/10
is achieved. Because the CHRG status pin responds to
the C/10 current level, the IC will indicate a fully-charged
battery status, but the charger continues to source low
currents into the battery until the programmed EOC time
has elapsed, at which time the charge cycle will terminate.
At EOC when the charging cycle terminates, if the battery
did not achieve at least 97.5% of the full float voltage,
charging is deemed unsuccessful, the LT3652 re-initiates,
and charging continues for another full timer cycle.
Use of the timer function also enables bad-battery detec-
tion. This fault condition is achieved if the battery does
not respond to preconditioning, such that the charger
remains in (or enters) precondition mode after 1/8th of
the programmed charge cycle time. A bad battery fault
halts the charging cycle, the CHRG status pin goes high-
impedance, and the FAULT pin is pulled low.
When the LT3652 terminates a charging cycle, whether
through C/10 detection or by reaching timer EOC, the
average current mode analog loop remains active, but
the internal float voltage reference is reduced by 2.5%.
Because the voltage on a successfully charged battery is
at the full float voltage, the voltage error amp detects an
over-voltage condition and ITH is pulled low. When the
voltage error amp output drops below 0.3V, the IC enters
standby mode, where most of the internal circuitry is dis-
abled, and the VIN bias current is reduced to 85μA. When
the voltage on the VFB pin drops below the reduced float
reference level, the output of the voltage error amp will
climb, at which point the IC comes out of standby mode
and a new charging cycle is initiated.
VIN Input Supply
The LT3652 is biased directly from the charger input supply
through the VIN pin. This supply provides large switched
currents, so a high-quality, low ESR decoupling capacitor
is recommended to minimize voltage glitches on VIN. The
VIN decoupling capacitor (CVIN) absorbs all input switching
LT3652
12
3652fc
ripple current in the charger, so it must have an adequate
ripple current rating. RMS ripple current (ICVIN(RMS)) is:
I
CVIN(RMS) ≅ ICHG(MAX) • (VBAT / VIN)•([VIN / VBAT] – 1)1/2,
where ICHG(MAX) is the maximum average charge current
(100mV/RSENSE). The above relation has a maximum at
VIN = 2 • VBAT
, where:
I
CVIN(RMS) = ICHG(MAX)/2.
The simple worst-case of ½ • ICHG(MAX) is commonly
used for design.
Bulk capacitance is a function of desired input ripple volt-
age (ΔVIN), and follows the relation:
C
IN(BULK) = ICHG(MAX) • (VBAT/VIN) / ΔVIN (μF)
Input ripple voltages above 0.1V are not recommended.
10μF is typically adequate for most charger applica-
tions.
Charge Current Programming
The LT3652 charger is configurable to charge at average
currents as high as 2A. Maximum charge current is set by
choosing an inductor sense resistor (RSENSE) such that
the desired maximum average current through that sense
resistor creates a 100mV drop, or:
R
SENSE = 0.1 / ICHG(MAX)
where ICHG(MAX) is the maximum average charge current.
A 2A charger, for example, would use a 0.05Ω sense
resistor.
BOOST Supply
The BOOST bootstrapped supply rail drives the internal
switch and facilitates saturation of the switch transistor.
Operating range of the BOOST pin is 0V to 8.5V, as refer-
APPLICATIONS INFORMATION
Figure 1. Programming Maximum Charge
Current Using RSENSE
SW
BOOST
SENSE
BAT
RSENSE
LT3652
3652 F01
enced to the SW pin. Connect a 1μF or greater capacitor
from the BOOST pin to the SW pin.
The voltage on the decoupling capacitor is refreshed
through a diode, with the anode connected to either the
battery output voltage or an external source, and the
cathode connected to the BOOST pin. Rate the diode
average current greater than 0.1A, and reverse voltage
greater than VIN(MAX).
To refresh the decoupling capacitor with a rectifying diode
from the battery with battery float voltages higher than
8.4V, a >100mA Zener diode can be put in series with
the rectifying diode to prevent exceeding the BOOST pin
operating voltage range.
LT3652
13
3652fc
APPLICATIONS INFORMATION
VIN / BOOST Start-Up Requirement
The LT3652 operates with a VIN range of 4.95V to 32V,
however, a start-up voltage requirement exists due to
the nature of the non-synchronous step-down switcher
topology used for the charger. If there is no BOOST supply
available, the internal switch requires (VIN – VSW) ≥ 3.3V
to reliably operate. This requirement does not exist if the
BOOST supply is available and (VBOOST – VSW) > 2V.
When an LT3652 charger is not switching, the SW pin is
at the same potential as the battery, which can be as high
as VBAT(FLT). As such, for reliable start-up, the VIN supply
must be at least 3.3V above VBAT(FLT). Once switching
begins and the BOOST supply capacitor gets charged
such that (VBOOST – VSW) > 2V, the VIN requirement no
longer applies.
In low VIN applications, the BOOST supply can be powered
by an external source for start-up, eliminating the VIN
start-up requirement.
VBAT Output Decoupling
An LT3652 charger output requires bypass capacitance
connected from the BAT pin to ground (CBAT). A 10μF
ceramic capacitor is required for all applications. In systems
where the battery can be disconnected from the charger
output, additional bypass capacitance may be desired for
visual indication for a no-battery condition (see the Status
Pins section).
If it is desired to operate a system load from the LT3652
charger output when the battery is disconnected, additional
bypass capacitance is required. In this type of application,
excessive ripple and/or low amplitude oscillations can oc-
cur without additional output bulk capacitance. For these
applications, place a 100μF low ESR non-ceramic capacitor
(chip tantalum or organic semiconductor capacitors such
as Sanyo OS-CONs or POSCAPs) from BAT to ground,
in parallel with the 10μF ceramic bypass capacitor. This
additional bypass capacitance may also be required in
systems where the battery is connected to the charger
with long wires. The voltage rating of CBAT must meet or
exceed the battery float voltage.
Inductor Selection
The primary criterion for inductor value selection in an
LT3652 charger is the ripple current created in that inductor.
Once the inductance value is determined, an inductor must
also have a saturation current equal to or exceeding the
maximum peak current in the inductor. An inductor value
(L), given the desired amount of ripple current (ΔIMAX)
can be approximated using the relation:
L = (10 RSENSE / ΔIMAX) • VBAT(FLT) •
[1 – (VBAT(FLT) / VIN(MAX))] (μH)
In the above relation, ΔIMAX is the normalized ripple cur-
rent, VIN(MAX) is the maximum operational voltage, and
VF is the forward voltage of the rectifying Schottky diode.
Ripple current is typically set within a range of 25% to
35% of ICHG(MAX), so an inductor value can be determined
by setting 0.25 < ΔIMAX < 0.35.
Figure 2. Zener Diode Reduces Refresh
Voltage for BOOST Pin
SW
BOOST
SENSE
BAT
LT3652
3652 F02
LT3652
14
3652fc
APPLICATIONS INFORMATION
Magnetics vendors typically specify inductors with
maximum RMS and saturation current ratings. Select an
inductor that has a saturation current rating at or above (1+
ΔIMAX/2) • ICHG(MAX), and an RMS rating above ICHG(MAX).
Inductors must also meet a maximum volt-second product
requirement. If this specification is not in the data sheet of
an inductor, consult the vendor to make sure the maximum
volt-second product is not being exceeded by your design.
The minimum required volt-second product is:
V
BAT(FLT) • (1 – VBAT(FLT)/VIN(MAX)) (V • μS)
Rectifier Selection
The rectifier diode from SW to GND, in a LT3652 battery
charger provides a current path for the inductor current
when the main power switch is disabled. The rectifier is
selected based upon forward voltage, reverse voltage, and
maximum current. A Schottky diode is required, as low
Figure 3. 7.2V at 1.5A Switched Inductor Values
MAXIMUM OPERATIONAL VIN VOLTAGE (V)
12 16
4
SWITCHED INDUCTOR VALUE (μH)
12
10
14
16
32
3652 F03
8
6
2420 28
Figure 4. Feedback Resistors from BAT to VFB
Program Float Voltage
BAT +
VFB
RFB2
RFB1
LT3652
3652 F04
forward voltage yields the lowest power loss and highest
efficiency. The rectifier diode must be rated to withstand
reverse voltages greater than the maximum VIN voltage.
The minimum average diode current rating (IDIODE(MAX))
is calculated with maximum output current (ICHG(MAX)),
maximum operational VIN, and output at the precondition
threshold (VBAT(PRE), or 0.7 • VBAT(FLT)):
I
DIODE(MAX) > ICHG(MAX) • (VIN(MAX) – VBAT(PRE)) / VIN(MAX)) (A)
For example, a rectifier diode for a 7.2V, 2A charger with
a 25V maximum input voltage would require:
I
DIODE(MAX) > 2 • (25 – 0.7[7.2]) / 25), or
I
DIODE(MAX) > 1.6A
Battery Float Voltage Programming
The output battery float voltage (VBAT(FLT)) is programmed
by connecting a resistor divider from the BAT pin to VFB.
VBAT(FLT) can be programmed up to 14.4V.
LT3652
15
3652fc
For a three-resistor network, RFB1 and RFB2 follow the
relation:
R
FB2/RFB1 = 3.3/(VBAT(FLT) – 3.3)
Example:
For VBAT(FLT) = 3.6V:
R
FB2/RFB1 = 3.3/(3.6 - 3.3) = 11.
Setting divider current (IRFB) = 10μA yields:
R
FB2 = 3.3/10μA
RFB2 = 330k
Solving for RFB1:
R
FB1 = 330k/11
RFB1 = 30k
The divider equivalent resistance is:
R
FB1||RFB2 = 27.5k
To satisfy the 250k equivalent resistance to the VFB
pin:
R
FB3 = 250k − 27.5k
RFB3 = 223k.
Because the VFB pin is a relatively high impedance node,
stray capacitances at this pin must be minimized. Special
attention should be given to any stray capacitances that
can couple external signals onto the pin, which can pro-
duce undesirable output transients or ripple. Effects of
parasitic capacitance can typically be reduced by adding
a small-value (20pF to 50pF) feedforward capacitor from
the BAT pin to the VFB pin.
Extra care should be taken during board assembly. Small
amounts of board contamination can lead to significant
shifts in output voltage. Appropriate post-assembly board
APPLICATIONS INFORMATION
Using a resistor divider with an equivalent input resistance
at the VFB pin of 250k compensates for input bias current
error. Required resistor values to program desired VBAT(FLT)
follow the equations:
R
FB1 = (VBAT(FLT) • 2.5 • 105) / 3.3 (Ω)
R
FB2 = (R1 • (2.5 • 105)) / (R1- (2.5 • 105)) (Ω)
The charge function operates to achieve the final float
voltage of 3.3V on the VFB pin. The auto-restart feature
initiates a new charging cycle when the voltage at the VFB
pin falls 2.5% below that float voltage.
Because the battery voltage is across the VBAT(FLT) pro-
gramming resistor divider, this divider will draw a small
amount of current from the battery (IRFB) at a rate of:
IRFB = 3.3 / RFB2
Precision resistors in high values may be hard to ob-
tain, so for some lower VBAT(FLT) applications, it may be
desirable to use smaller-value feedback resistors with an
additional resistor (RFB3) to achieve the required 250k
equivalent resistance. The resulting 3-resistor network,
as shown in Figure 5, can ease component selection
and/or increase output voltage precision, at the expense of
additional current through the feedback divider.
BAT +
VFB
RFB2
RFB3
RFB1
LT3652
3652 F05
Figure 5. A Three-Resistor Feedback Network Can
Ease Component Selection
LT3652
16
3652fc
cleaning measures should be implemented to prevent
board contamination, and low-leakage solder flux is
recommended.
Input Supply Voltage Regulation
The LT3652 contains a voltage monitor pin that enables
programming a minimum operational voltage. Connect-
ing a resistor divider from VIN to the VIN_REG pin enables
programming of minimum input supply voltage, typically
used to program the peak power voltage for a solar panel.
Maximum charge current is reduced when the VIN_REG pin
is below the regulation threshold of 2.7V.
If an input supply cannot provide enough power to satisfy
the requirements of an LT3652 charger, the supply voltage
will collapse. A minimum operating supply voltage can
thus be programmed by monitoring the supply through
a resistor divider, such that the desired minimum volt-
age corresponds to 2.7V at the VIN_REG pin. The LT3652
servos the maximum output charge current to maintain
the voltage on VIN_REG at or above 2.7V.
Programming of the desired minimum voltage is ac-
complished by connecting a resistor divider as shown in
Figure 6. The ratio of RIN1/RIN2 for a desired minimum
voltage (VIN(MIN)) is:
R
IN1/RIN2 = (VIN(MIN)/2.7) – 1
If the voltage regulation feature is not used, connect the
VIN_REG pin to VIN.
Figure 6. Resistor Divider Sets Minimum VIN
VIN
VIN_REG
RIN2
RIN1 LT3652
INPUT
SUPPLY
3652 F06
APPLICATIONS INFORMATION
Figure 7. Temperature Characteristics for Solar Panel
Output Voltage
TEMPERATURE (°C)
5
PANEL VOLTAGE (V)
25 45 55
15 35
3652 F07
VOC(25°C)
VMP(25°C)
VMP
VOC
VOC TEMP CO.
VOC – VMP
MPPT Temperature Compensation
A typical solar panel is comprised of a number of series-con-
nected cells, each cell being a forward-biased p-n junction.
As such, the open-circuit voltage (VOC) of a solar cell has
a temperature coefficient that is similar to a common p-n
diode, or about –2mV/°C. The peak power point voltage
(VMP) for a crystalline solar panel can be approximated as
a fixed voltage below VOC, so the temperature coefficient
for the peak power point is similar to that of VOC.
Panel manufacturers typically specify the 25°C values for
VOC, VMP
, and the temperature coefficient for VOC, making
determination of the temperature coefficient for VMP of a
typical panel straight forward.
The LT3652 employs a feedback network to program the
VIN input regulation voltage. Manipulation of the network
makes for efficient implementation of various temperature
compensation schemes for a maximum peak power track-
ing (MPPT) application. As the temperature characteristic
for a typical solar panel VMP voltage is highly linear, a
LT3652
17
3652fc
APPLICATIONS INFORMATION
simple solution for tracking that characteristic can be
implemented using an LM234 3-terminal temperature
sensor. This creates an easily programmable, linear
temperature dependent characteristic.
In the circuit shown in figure 8,
RIN1 = –RSET • (TC • 4405), and
RIN2 = RIN1/({[VMP(25°C) + RIN1 • (0.0674/RSET)]/VIN_REG} – 1)
Where: TC = temperature coefficient (in V/°C), and
VMP(25°C) = maximum power voltage at 25°C
For example, given a common 36-cell solar panel that has
the following specified characteristics:
Open Circuit Voltage (VOC) = 21.7V
Maximum Power Voltage (VMP) = 17.6V
Open-Circuit Voltage Temperature Coefficient (VOC) =
–78mV/°C
Figure 8. MPPT Temperature Compensation Network
VIN_REG
LT3652
VIN
VIN LM234
3658 F08
V+
V–
R
RSET
RIN1
RIN2
As the temperature coefficient for VMP is similar to that
of VOC, the specified temperature coefficient for VOC
(TC) of –78mV/°C and the specified peak power voltage
(VMP(25°C)) of 17.6V can be inserted into the equations to
calculate the appropriate resistor values for the tempera-
ture compensation network in Figure 8. With RSET equal
to 1000Ω, then:
R
SET = 1k
R
IN1 = –1k • (–0.078 • 4405 ) = 344k
R
IN2 = 344k/({[17.6 + 344k • (0.0674/1k)]/2.7} – 1)
= 24.4k
Battery Voltage Temperature Compensation
Some battery chemistries have charge voltage require-
ments that vary with temperature. Lead-acid batteries in
particular experience a significant change in charge volt-
age requirements as temperature changes. For example,
manufacturers of large lead-acid batteries recommend
a float charge of 2.25V/cell at 25°C. This battery float
voltage, however, has a temperature coefficient which is
typically specified at –3.3mV/°C per cell.
In a manner similar to the MPPT temperature correction
outlined previously, implementation of linear battery
charge voltage temperature compensation can be ac-
complished by incorporating an LM234 into the output
feedback network.
For example, a 6-cell lead acid battery has a float charge
voltage that is commonly specified at 2.25V/cell at 25°C,
or 13.5V, and a –3.3mV/°C per cell temperature coefficient,
LT3652
18
3652fc
APPLICATIONS INFORMATION
or –19.8mV/°C. Using the feedback network shown in
Figure 9, with the desired temperature coefficient (TC)
and 25°C float voltage (VFLOAT(25°C)) specified, and using
a convenient value of 2.4k for RSET, necessary resistor
values follow the relations:
RFB1 = –RSET • (TC • 4405)
= –2.4k • (–0.0198 • 4405) = 210k
RFB2 = RFB1 / ({[VFLOAT(25°C) + RFB1 • (0.0674/
RSET)] / VFB} – 1)
= 210k/({[13.5 + 210k • (0.0674/2.4k)]/3.3} – 1)
= 43k
RFB3 = 250k - RFB1||RFB2
= 250k – 210k||43k = 215k (see the Battery Float
Voltage Programming section)
While the circuit in Figure 9 creates a linear temperature
characteristic that follows a typical –3.3mV/°C per cell
lead-acid specification, the theoretical float charge voltage
characteristic is slightly nonlinear. This nonlinear charac-
teristic follows the relation VFLOAT(1-CELL) = 4 × 10–5 (T2)
– 6 × 10–3(T) + 2.375 (with a 2.18V minimum), where
T = temperature in °C. A thermistor-based network can
be used to approximate the nonlinear ideal temperature
characteristic across a reasonable operating range, as
shown in Figure 10.
LT3652 RFB3
215k RFB2
43k
RSET
2.4k
RFB1
210k
6-CELL
LEAD-ACID
BATTERY
LM234
3652 F09a
V+
V–
R
BAT
VFB
+
Figure 9. Lead-Acid 6-Cell Float Charge Voltage vs
Temperature Has –19.8mV/°C Characteristic Using LM234 with
Feedback Network
TEMPERATURE (°C)
–10
VFLOAT (V)
10 5040 60
02030
3652 F09b
12.6
12.8
13.0
13.2
13.4
13.6
13.8
14.0
14.2
14.3
–19.8mV/°C
LT3652
196k
198k
6-CELL
LEAD-ACID
BATTERY
22k
B = 3380
3652 F10a
BAT
69k
69k
VFB
+
TEMPERATURE (°C)
–10
VFLOAT (V)
10 5040 60
02030
3652 F10b
12.8
13.0
13.2
13.4
13.6
13.8
14.0
14.6
14.4
14.2
14.8
THEORETICAL VFLOAT
PROGRAMMED VBAT(FLOAT)
Figure 10. Thermistor-Based Temperature Compensation
Network Programs VFLOAT to Closely Match Ideal Lead-Acid
Float Charge Voltage for 6-Cell Charger
LT3652
19
3652fc
Status Pins
The LT3652 reports charger status through two open
collector outputs, the CHRG and FAULT pins. These pins
can accept voltages as high as VIN, and can sink up to
10mA when enabled.
The CHRG pin indicates that the charger is delivering
current at greater that a C/10 rate, or 1/10th of the pro-
grammed maximum charge current. The FAULT pin signals
bad battery and NTC faults. These pins are binary coded,
and signal following the table below, where ON indicates
pin pulled low, and OFF indicates pin high-impedance:
STATUS PINS STATE
CHARGER STATUS
CHRG FAULT
OFF OFF Not Charging — Standby or Shutdown Mode
OFF ON Bad Battery Fault (Precondition Timeout / EOC
Failure)
ON OFF Normal Charging at C/10 or Greater
ON ON NTC Fault (Pause)
If the battery is removed from an LT3652 charger that is
configured for C/10 termination, a sawtooth waveform
of approximately 100mV appears at the charger output,
due to cycling between termination and recharge events,
This cycling results in pulsing at the CHRG output. An
LED connected to this pin will exhibit a blinking pattern,
indicating to the user that a battery is not present. The
frequency of this blinking pattern is dependent on the
output capacitance.
C/10 Termination
The LT3652 supports a low-current based termination
scheme, where a battery charge cycle terminates when the
current output from the charger falls to below one-tenth
of the maximum current, as programmed with RSENSE.
The C/10 threshold current corresponds to 10mV across
RSENSE. This termination mode is engaged by shorting
the TIMER pin to ground.
APPLICATIONS INFORMATION
When C/10 termination is used, a LT3652 charger will
source battery charge current as long as the average
current level remains above the C/10 threshold. As the
full-charge float voltage is achieved, the charge current
falls until the C/10 threshold is reached, at which time the
charger terminates and the LT3652 enters standby mode.
The CHRG status pin follows the charger cycle, and is high
impedance when the charger is not actively charging.
When VBAT drops below 97.5% of the full-charged float
voltage, whether by battery loading or replacement of the
battery, the charger automatically re-engages and starts
charging.
There is no provision for bad battery detection if C/10
termination is used.
Timer Termination
The LT3652 supports a timer based termination scheme, in
which a battery charge cycle is terminated after a specific
amount of time elapses. Timer termination is engaged
when a capacitor (CTIMER) is connected from the TIMER
pin to ground. The timer cycle EOC (TEOC) occurs based
on CTIMER following the relation:
C
TIMER = TEOC • 2.27 x 10–7 (Hours)
Timer EOC is typically set to 3 hours, which requires a
0.68μF capacitor.
The CHRG status pin continues to signal charging at a C/10
rate, regardless of what termination scheme is used. When
timer termination is used, the CHRG status pin is pulled
low during a charging cycle until the charger output cur-
rent falls below the C/10 threshold. The charger continues
to top-off the battery until timer EOC, when the LT3652
terminates the charging cycle and enters standby mode.
Termination at the end of the timer cycle only occurs if
the charging cycle was successful. A successful charge
cycle is when the battery is charged to within 2.5% of the
LT3652
20
3652fc
APPLICATIONS INFORMATION
full-charge float voltage. If a charge cycle is not successful
at EOC, the timer cycle resets and charging continues for
another full timer cycle.
When VBAT drops below 97.5% of the full-charge float
voltage, whether by battery loading or replacement of the
battery, the charger automatically reengages and starts
charging.
Preconditioning and Bad Battery Fault
A LT3652 has a precondition mode, where charge current
is limited to 15% of the programmed ICHG(MAX), as set by
RSENSE. The precondition current corresponds to 15mV
across RSENSE.
Precondition mode is engaged while the voltage on the
VFB pin is below the precondition threshold (2.3V, or
0.7 • VBAT(FLT)). Once the VFB voltage rises above the
precondition threshold, normal full-current charging can
commence. The LT3652 incorporates 70mV of threshold
hysteresis to prevent mode glitching.
When the internal timer is used for termination, bad battery
detection is engaged. There is no provision for bad battery
detection if C/10 termination is used. A bad battery fault
is triggered when the voltage on VFB remains below the
precondition threshold for greater than 1/8 of a full timer
cycle (1/8 EOC). A bad battery fault is also triggered if a
normally charging battery re-enters precondition mode
after 1/8 EOC.
When a bad battery fault is triggered, the charging cycle
is suspended, so the CHRG status pin becomes high-
impedance. The FAULT pin is pulled low to signal a fault
detection.
Cycling the charger’s power or SHDN function initiates a
new charging cycle, but a LT3652 charger does not re-
quire a reset. Once a bad battery fault is detected, a new
timer charging cycle initiates when the VFB pin exceeds
the precondition threshold voltage. During a bad battery
fault, 0.5mA is sourced from the charger, so removing
the failed battery allows the charger output voltage to rise
and initiate a charge cycle reset. As such, removing a bad
battery resets the LT3652, so a new charge cycle is started
by connecting another battery to the charger output.
Battery Temperature Monitor and Fault
The LT3652 can accommodate battery temperature moni-
toring by using an NTC (negative temperature co-efficient)
thermistor close to the battery pack. The temperature
monitoring function is enabled by connecting a 10kΩ,
B = 3380 NTC thermistor from the NTC pin to ground. If the
NTC function is not desired, leave the pin unconnected.
The NTC pin sources 50μA, and monitors the voltage
dropped across the 10kΩ thermistor. When the voltage
on this pin is above 1.36V (0°C) or below 0.29V (40°C),
the battery temperature is out of range, and the LT3652
triggers an NTC fault. The NTC fault condition remains until
the voltage on the NTC pin corresponds to a temperature
within the 0°C to 40°C range. Both hot and cold thresholds
incorporate hysteresis that correspond to 5°C.
If higher operational charging temperatures are desired,
the temperature range can be expanded by adding series
resistance to the 10k NTC resistor. Adding a 0.91k resistor
will increase the effective hot temperature to 45°C.
During an NTC fault, charging is halted and both status
pins are pulled low. If timer termination is enabled, the
timer count is suspended and held until the fault condi-
tion is relieved.
Thermal Foldback
The LT3652 contains a thermal foldback protection feature
that reduces maximum charger output current if the IC
junction temperature approaches 125°C. In most cases,
on-chip temperatures servo such that any excessive tem-
perature conditions are relieved with only slight reductions
in maximum charger current.
LT3652
21
3652fc
In some cases, the thermal foldback protection feature
can reduce charger currents below the C/10 threshold. In
applications that use C/10 termination (TIMER=0V), the
LT3652 will suspend charging and enter standby mode
until the excessive temperature condition is relieved.
Layout Considerations
The LT3652 switch node has rise and fall times that are
typically less than 10nS to maximize conversion efficiency.
The switch node (Pin SW) trace should be kept as short
as possible to minimize high frequency noise. The input
capacitor (CIN) should be placed close to the IC to minimize
this switching noise. Short, wide traces on these nodes
also help to avoid voltage stress from inductive ringing.
The BOOST decoupling capacitor should also be in close
proximity to the IC to minimize inductive ringing. The
SENSE and BAT traces should be routed together, and
these and the VFB trace should be kept as short as pos-
sible. Shielding these signals from switching noise with
a ground plane is recommended.
High current paths and transients should be kept iso-
lated from battery ground, to assure an accurate output
APPLICATIONS INFORMATION
voltage reference. Effective grounding can be achieved
by considering switched current in the ground plane,
and careful component placement and orientation can
effectively steer these high currents such that the battery
reference does not get corrupted. Figure 11 illustrates an
effective grounding scheme using component placement
to control ground currents. When the switch is enabled
(loop #1), current flows from the input bypass capacitor
(CIN) through the switch and inductor to the battery posi-
tive terminal. When the switch is disabled (loop #2), the
current to the battery positive terminal is provided from
ground through the freewheeling Schottky diode (DF). In
both cases, these switch currents return to ground via the
output bypass capacitor (CBAT).
The LT3652 packaging has been designed to efficiently
remove heat from the IC via the Exposed Pad on the
backside of the package, which is soldered to a copper
footprint on the PCB. This footprint should be made as
large as possible to reduce the thermal resistance of the
IC case to ambient air.
SW
VIN
RSENSE
SENSE
BAT
VFB
+
LT3652
DF
CIN CBAT VBAT
2
3652 F11
1
Figure 11. Component Orientation Isolates High Current Paths
from Sensitive Nodes
LT3652
22
3652fc
TYPICAL APPLICATIONS
2-Cell Li-Ion Charger (8.3V at 2A) With 3 Hour Timer Termination Powered by
Inexpensive 12V at 1A Unregulated Wall Adapter; VIN_REG Loop Servos Maximum Charge
Current to Prevent AC Adapter Output from Drooping Lower than 12V
Basic 2A 1-Cell LiFePO4 Charger (3.6V Float) With C/10 Termination
SW
VIN
AC ADAPTER INPUT
12V AT 1A
SH-DC121000
VIN_REG
VFB
BOOST
SENSE
BAT
NTC
TIMER
MBRS340 D3
8.2μH
VISHAY
1HLP-2525CZ8R2M11
MBRS340
0.68μF
10k 626k
1μF 1N914
412k
SYSTEM
LOAD
R1 10k
B = 3380
REMOVABLE 2-CELL Li-Ion PACK
(8.3V FLOAT)
+
1μF
10μF
0.05
100μF
LT3652
10μF
SHDN
CHRG
FAULT
10k
51k
330k
47k
+
3652 TA02a
SW
VIN
VIN
5V TO 32V (40V MAX)
VIN_REG
VFB
BOOST
SENSE
BAT
NTC
TIMER
CMDSH2-4L
SYSTEM
LOAD
1μF
30k
223k
3652 TA03
+
10μF
C3
10μF
0.05
5.6μH
LT3652
SHDN
CHRG
FAULT
CMSH3-40MA
330k
LiFePO4 CELL
OUTPUT CURRENT (A)
0
2
OUTPUT VOLTAGE (V)
6
10
14
20
0.4 0.8 1.2 1.4 1.6 1.8
1
18
4
8
12
16
0.2 0.6
3652 TA02b
SH-DC121000
AC Adapter V vs I Characteristics
LT3652
23
3652fc
PACKAGE DESCRIPTION
DD Package
12-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1725 Rev A)
3.00 ±0.10
(4 SIDES)
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
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 AND TIE BARS 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
0.75 ±0.05
R = 0.115
TYP
16
127
PIN 1
TOP MARK
(SEE NOTE 6)
0.200 REF
0.00 – 0.05
(DD12) DFN 0106 REV A
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.23 ± 0.05
0.25 ± 0.05
2.25 REF
2.38 ±0.05
1.65 ±0.05
2.10 ±0.05
0.70 ±0.05
3.50 ±0.05
PACKAGE
OUTLINE
PIN 1 NOTCH
R = 0.20 OR
0.25 × 45°
CHAMFER
2.38 ±0.10
2.25 REF
0.45 BSC
0.45 BSC
LT3652
24
3652fc
MSE Package
12-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1666 Rev B)
PACKAGE DESCRIPTION
MSOP (MSE12) 0608 REV B
0.53 p 0.152
(.021 p .006)
SEATING
PLANE
0.18
(.007)
1.10
(.043)
MAX
0.22 –0.38
(.009 – .015)
TYP
0.86
(.034)
REF
0.650
(.0256)
BSC
12
12 11 10 9 8 7
7
DETAIL “B”
16
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.254
(.010) 0o – 6o TYP
DETAIL “A”
DETAIL “A”
GAUGE PLANE
RECOMMENDED SOLDER PAD LAYOUT
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 p 0.102
(.112 p .004)
2.845 p 0.102
(.112 p .004)
4.039 p 0.102
(.159 p .004)
(NOTE 3)
1.651 p 0.102
(.065 p .004)
0.1016 p 0.0508
(.004 p .002)
123456
3.00 p 0.102
(.118 p .004)
(NOTE 4)
0.406 p 0.076
(.016 p .003)
REF
4.90 p 0.152
(.193 p .006)
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
0.12 REF
0.35
REF
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
0.889 p 0.127
(.035 p .005)
0.42 p 0.038
(.0165 p .0015)
TYP
0.65
(.0256)
BSC
LT3652
25
3652fc
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibilit y is assumed for its use. Linear Technology Corporation makes no representa-
t i o n t h a t t h e i n t e r c o n n e c t i o n o f i t s c i r c u i t s a s d e s c r i b e d h e r e i n w i l l n o t i n f r i n g e o n e x i s t i n g p a t e n t r i g h t s .
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
B 2/10 Add MSOP-12 Package 1, 2, 24
C5/10Corrected SHDN Pin Labels 3, 4
(Revision history begins at Rev B)
LT3652
26
3652fc
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2010
LT 0510 REV C • PRINTED IN USA
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Characteristics Up to 14.4V; When Charge Current Falls to 0.1A Charger Switches to 13.5V Float Charge Mode;
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Battery Voltage is Below 10V; 0°C to 45°C Battery Temperature Charging Range
SW
VIN
SOLAR PANEL INPUT
<40V OC VOLTAGE
16V PEAK POWER VOLTAGE
VIN_REG
VFB
BOOST
SENSE
BAT
NTC
TIMER
22μH MBRS340
309k
100k
1μF 1N914 BZX84C6V2L
910
174k
1N41481M
SYSTEM
LOAD
WURTH
7447779122
12V LEAD
ACID BATTERY
10k
B = 3380
muRata
NCP18XH103
+
10μF
0.1
100μF
LT3652
10μF
4.7μF
SHDN
CHRG
FAULT
499k
100k +
3652 TA04
MBRS140
