LT3651-8.2/LT3651-8.4 Monolithic 4A High Voltage 2-Cell Li-Ion Battery Charger FEATURES

LT3651-8.2/LT3651-8.4 Monolithic 4A High Voltage 2-Cell Li-Ion Battery Charger FEATURES
LT3651-8.2/LT3651-8.4
Monolithic 4A High Voltage
2-Cell Li-Ion Battery Charger
FEATURES
DESCRIPTION
Wide Input Voltage Range: 9V to 32V
(40V Absolute Maximum)
n Programmable Charge Current Up to 4A
n Selectable C/10 or Onboard Timer Termination
n Dynamic Charge Rate Programming/Soft-Start
n Programmable Input Current Limit
n±0.5% Float Voltage Accuracy
n±7.5% Charge Current Accuracy
n±4% C/10 Detection Accuracy
n NTC Resistor Temperature Monitor
n Auto-Recharge at 97.5% Float Voltage
n Auto-Precondition at <70% Float Voltage
n Bad Battery Detection with Auto-Reset
n Average Current Mode, Synchronous Switcher
n User Programmable Frequency
n Low Profile (0.75mm) 5mm × 6mm 36-Lead
QFN Package
The LT®3651-8.2/LT3651-8.4 are 2-cell, 4A Li-Ion/Polymer battery chargers that operate over a 9V to 32V input
voltage range. An efficient monolithic average current
mode synchronous switching regulator provides constant
current, constant voltage charging with programmable
maximum charge current. A charging cycle starts with
battery insertion or when the battery voltage drops 2.5%
below the float voltage. Charger termination is selectable
as either charge current or internal safety timer timeout.
Charge current termination occurs when the charge current falls to one-tenth the programmed maximum current
(C/10). Timer based termination is typically set to three
hours and is user programmable (charging continues
below C/10 until timeout). Once charging is terminated,
the LT3651-8.2/LT3651-8.4 supply current drops to 85µA
into a standby mode.
n
The LT3651-8.2/LT3651-8.4 offer several safety features.
A discharged battery is preconditioned with a small trickle
charge and generates a signal if unresponsive. A thermistor
monitors battery temperature, halting charging if out of
range. Excessive die temperature reduces charge current.
Charge current is also reduced to maintain constant input
current to prevent excessive input loading.
APPLICATIONS
Industrial Handheld Instruments
12V to 24V Automotive and Heavy Equipment
n Desktop Cradle Chargers
n Notebook Computers
n
n
The LT3651-8.2/LT3651-8.4 are available in a 5mm ×
6mm 36-lead QFN package.
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.
TYPICAL APPLICATION
12V to 32V 2-Cell 4A Charger
Si7611DN
100k
10V
CLN
LT3651-8.2/LT3651-8.4
ACPR
FAULT
CHRG
EFFICIENCY
1µF
BOOST
CMPSH1-4
10µH
WÜRTH 74477010
SENSE
RT
301k
22µF
VIN
SW
BAT
NTC
TIMER
ILIM RNG/SS GND
24mΩ
100µF
5.0
VBAT = 7.8V
IBAT = 4A
+
2-CELL
Li-Ion
BATTERY
365188284 TA01a
90
4.5
89
4.0
88
3.5
POWER LOSS (W)
10k
CLP
SHDN
TO
SYSTEM
LOAD
EFFICIENCY (%)
VIN
12V TO 32V
Efficiency, Power Loss vs VIN
91
POWER LOSS
87
10
15
20
VIN (V)
25
30
35
3.0
36518284 TA01b
36518284fa
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1
LT3651-8.2/LT3651-8.4
CLP
CLN
GND
VIN
VIN
VIN
RNG/SS
TOP VIEW
36 35 34 33 32 31 30 29
NTC 1
28 ILIM
27 SHDN
ACPR 2
BAT 3
26 CHRG
37
GND
SENSE 4
25 FAULT
24 TIMER
BOOST 5
23 GND
GND 6
22 SW
SW 7
38
SW
NC 8
NC 9
21 NC
20 NC
19 NC
NC 10
SW
SW
SW
SW
SW
SW
11 12 13 14 15 16 17 18
SW
VIN ........................................................................... 40V
CLN, CLP, SHDN, CHRG,
FAULT, ACPR ................................ VIN + 0.5V Up to 40V
CLP – CLN..............................................................±0.5V
SW ............................................................................40V
SW – VIN...................................................................4.5V
BOOST ........................................... SW + 10V Up to 50V
SENSE, BAT ............................................................. 10V
SENSE-BAT .............................................. –0.5V to 0.5V
TIMER, RNG/SS, ILIM, NTC, RT ............................... 2.5V
Operating Junction Temperature Range
(Notes 2, 3)................................................. –40 to 125°C
Storage Temperature Range.......................–65 to 150°C
PIN CONFIGURATION
RT
(Note 1)
SW
ABSOLUTE MAXIMUM RATINGS
UHE PACKAGE
36-LEAD (5mm × 6mm) PLASTIC QFN
TJMAX = 125°C, θJA = 43°C/W
EXPOSED PAD (PIN 37) IS GND, MUST BE SOLDERED TO PCB
EXPOSED PAD (PIN 38) IS SW, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3651EUHE-8.2#PBF
LT3651EUHE-8.2#TRPBF
365182
36-Lead (5mm × 6mm) Plastic QFN
–40°C to 125°C
LT3651IUHE-8.2#PBF
LT3651IUHE-8.2#TRPBF
365182
36-Lead (5mm × 6mm) Plastic QFN
–40°C to 125°C
LT3651EUHE-8.4#PBF
LT3651EUHE-8.4#TRPBF
365184
36-Lead (5mm × 6mm) Plastic QFN
–40°C to 125°C
LT3651IUHE-8.4#PBF
LT3651IUHE-8.4#TRPBF
365184
36-Lead (5mm × 6mm) Plastic QFN
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping
container.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/
36518284fa
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LT3651-8.2/LT3651-8.4
ELECTRICAL
CHARACTERISTICS
The l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 20V, SHDN = 2V, SENSE = BAT = VBAT(FLT),
CTIMER = 0.68µF, RT = 50k, CLP = CLN = VIN, BOOST – SW = 4V.
PARAMETER
CONDITIONS
VIN Operating Range
VIN OVLO Threshold
MIN
l
VIN Rising
9.0
32
VIN OVLO Hysteresis
VIN UVLO Threshold
35
MAX
VIN Rising
8.7
l
V
40
V
V
9.0
0.2
LT3651-8.2
UNITS
32
1.1
VIN UVLO Hysteresis
Battery Float Voltage, VBAT(FLT)
TYP
V
V
8.16
8.12
8.2
l
8.24
8.28
V
V
8.36
8.32
8.4
l
8.44
8.48
V
V
LT3651-8.4
Battery Recharge Voltage Hysteresis
Threshold Voltage Relative to VBAT(FLT)
–200
mV
Battery Precondition Threshold Voltage, VBAT(PRE)
LT3651-8.2, VBAT Rising
LT3651-8.4, VBAT Rising
5.65
5.80
V
V
Battery Precondition Threshold Hysteresis
Threshold Voltage Relative to VBAT(PRE)
90
mV
Operating VIN Supply Current
CC/CV Mode, Top Switch On, ISW = 0
Standby Mode
Shutdown (SHDN = 0)
8.6
80
17
mA
µA
µA
Top Switch On Voltage
VIN – VSW , ISW = 4A
480
mV
Bottom Switch On Voltage
VSW , ISW = 4A
–140
mV
BOOST Supply Current
Switch High, ISW = 0, 2.5V < (VBOOST – VSW) < 8.5V
17
mA
BOOST Switch Drive
IBOOST/ISW , ISW = 4A
22
mA/A
Precondition Current Sense Voltage
VSENSE – VBAT , VBAT = 5.0V
Input Current Limit Voltage
VCLP – VCLN, ILIM Open
14
l
70
CLP Input Bias Current
95
mV
115
120
CLN Input Bias Current
nA
36
ILIM Bias Current
l
43
50
mV
µA
57
11.5
µA
System Current Limit Programming Gain
VILIM/(VCLP – VCLN), VILIM = 0.5V
Maximum Charge Current Sense Voltage
VSENSE – VBAT , VBAT = 7.5V, VRNG/SS > 1.1V
l
88
95
103
V/V
mV
C/10 Trigger Sense Voltage
VSENSE – VBAT
l
4.5
8.6
12.3
mV
BAT Input Bias Current
Charging Terminated
0.1
1
µA
SENSE Input Bias Current
Charging Terminated
0.1
1
µA
l
44
50
56
µA
Charge Current Limit Programming Gain
VRNG/SS/(VSENSE – VBAT), VRNG/SS = 0.5V
l
8.5
10.8
12.5
V/V
NTC Range Limit (High)
VNTC Rising
l
1.25
1.36
1.45
V
NTC Range Limit (Low)
VNTC Falling
l
0.27
0.29
0.31
NTC Threshold Hysteresis
% of Threshold
NTC Disable Impedance
Minimum External Impedance to GND
l
150
NTC Bias Current
VNTC = 0.75V
l
46.5
50
53.5
µA
Shutdown Threshold
VSHDN Rising
l
1.15
1.20
1.23
V
RNG/SS Bias Current
V
10
%
470
kΩ
Shutdown Hysteresis
95
mV
SHDN Input Bias Current
–10
nA
Status Low Voltage
VCHRG, VFAULT , VACPR, Load = 10mA
l
0.45
V
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LT3651-8.2/LT3651-8.4
ELECTRICAL
CHARACTERISTICS
The l denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 20V, SHDN = 2V, SENSE = BAT = VBAT(FLT),
CTIMER = 0.68µF, RT = 50k, CLP = CLN = VIN, BOOST – SW = 4V.
PARAMETER
CONDITIONS
MIN
TYP
25
µA
0.1
0.25
V
TIMER Charge/Discharge Current
TIMER Disable Threshold
l
Full Charge Cycle Time-Out
3
Precondition Timeout
l
RT = 50kΩ
RT = 250kΩ
Minimum SW On-Time, tON(MIN)
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 LT3651-8.2/LT3651-8.4 are tested under pulse loaded
conditions such that TJ = TA. The LT3651-8.2E/LT3651-8.4E are
guaranteed to meet performance specifications from 0°C to 85°C junction
temperature. Specifications over the –40°C to 125°C operating junction
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3651-8.2I/LT3651-8.4I are
guaranteed over the full –40°C to 125°C operating junction temperature
range. The junction temperature (TJ in °C) is calculated from the ambient
temperature (TA in °C) and power dissipation (PD in Watts) according to
the formula:
TJ = TA + PD • θJA
where θJA (in °C/W) is the package thermal impedance.
–13
UNITS
Hour
22.5
Timer Accuracy
Switcher Operating Frequency, fO
MAX
Minute
13
%
1.1
250
MHz
kHz
150
ns
Note 3: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. The maximum
rated junction temperature will be exceeded when this protection is active.
Continuous operation above the specified absolute maximum operating
junction temperature may impair device reliability or permanently damage
the device.
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LT3651-8.2/LT3651-8.4
TYPICAL PERFORMANCE CHARACTERISTICS
Battery Float Voltage
vs Temperature
IVIN (µA)
∆VBAT(FLT) (%)
0.5
0
–0.5
–25
25
50
75
0
TEMPERATURE (°C)
100
100
150
95
100
90
50
85
0
CURRENT (µA)
1.0
–1.0
–50
SENSE and BAT Pin Currents
vs BAT Voltage (VSENSE = VBAT)
VIN Standby Mode Current
vs Temperature
80
75
70
–150
60
–250
55
–300
25
50
0
TEMPERATURE (°C)
75
36518284 G01
–350
100
IBAT
–50
–200
–25
ISENSE
–100
65
50
–50
125
LT3651-8.4
0
1
2
3
5
4
VBAT (V)
6
Maximum Charge Current
vs VRNG/SS as a Percentage
of Programmed IIN(MAX)
9
ICHG Current Limit
(VSENSE – VBAT) vs Temperature
120
11
8
36518284 G03
36518284 G02
C/10 Threshold (VSENSE – VBAT)
vs Temperature
7
101.0
100
9
VSENSE – VBAT (mV)
ICHG(MAX) (%)
100.5
80
100.0
60
40
8
99.5
20
–25
75
0
25
50
TEMPERATURE (°C)
100
0
125
0
0.2
0.4
0.6
0.8
1.0
Charge Current vs VBAT as a
Percentage of Programmed
ICHG(MAX)
100
80
80
60
125
36518284 G06
60
40
40
20
20
6
100
120
LT3651-8.4
5
75
0
25
50
TEMPERATURE (°C)
Maximum Input Current
vs VILIM as a Percentage
of Programmed IIN(MAX)
100
0
–25
36518284 G05
36518284 G04
120
99.0
–50
1.2
VRNG/SS (V)
IIN(MAX) (%)
7
–50
ICHG (%)
VSENSE – VBAT (mV)
10
7
8
9
0
0
0.2
VBAT (V)
36518284 G07
0.4
0.6
0.8
VILIM (V)
1.0
1.2
36518284 G08
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LT3651-8.2/LT3651-8.4
TYPICAL PERFORMANCE CHARACTERISTICS
Topside Switch VON
vs Temperature
Input Current Limit Voltage
Threshold vs Temperature
2.0
700
–50
ISW = 4A
–100
RILIM OPEN
RILIM = 10k
0
–0.5
600
VSW (mV)
0.5
550
–150
500
–200
–1.0
450
–1.5
–2.0
–50 –25
ISW = 4A
650
1.0
VIN – VSW (mV)
∆(VCLP – VCLN) (mV)
1.5
Bottom Side Switch VON
vs Temperature
75
50
25
TEMPERATURE (°C)
0
100
400
–50 –25
125
50
25
75
0
TEMPERATURE (˚C)
100
36518284 G09
100
125
26518284 G11
24
ISW = 4A
40
IBST/ISW (mA/A)
23
30
20
22
21
10
0
1
2
3
4
20
5
2
3
ISW (A)
4
5
6
7
8
VBST – VIN (V)
36518284 G14
36518284 G13
Oscillator Frequency
vs Temperature
1.0
75
0
25
50
TEMPERATURE (°C)
Boost Drive vs Boost Voltage
Boost Drive vs Switch Current
IBST/ISW (mA/A)
–25
36518284 G10
50
0
–250
–50
125
Timer Resistor (RT)
vs Period and Frequency
400
RT = 54.9k
300
RT (kΩ)
FREQUENCY DEVIATION (%)
350
0.5
0
250
200
150
–0.5
100
–1.0
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
50
1
1000
2
500
26518284 G15
3
4
333
250
PERIOD (µs)
FREQUENCY (kHz)
5
200
6
167
36518284 G16
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LT3651-8.2/LT3651-8.4
PIN FUNCTIONS
NTC (Pin 1): Battery Temperature Monitor Pin. This
pin is used to monitor battery temperature. Typically a
10kΩ NTC (negative temperature coefficient) thermistor
(B = 3380) is embedded with the battery and connected
from the NTC pin to ground. The pin sources 50µA into
the resistor and monitors the voltage across the thermistor, regulating charging based on the voltage. If this
function is not desired, leave the NTC pin unconnected.
ACPR (Pin 2): Open-Collector AC Present Status Pin.
This pin sinks current to indicate that VIN is valid and the
charger is on. Typically a resistor pull-up is used on this
pin. This pin can be pulled up to voltages as high as VIN
when disabled, and can sink currents up to 10mA when
enabled.
BAT (Pin 3): Battery Voltage Monitor Pin. This pin monitors battery voltage. A Kelvin connection is made to the
battery from this pin and a decoupling capacitor (CBAT)
is placed from this pin to ground.
The charge function operates to achieve the final float
voltage at this pin. The auto-restart feature initiates a new
charging cycle when the voltage at the BAT pin falls 2.5%
below this float voltage. Once the charge cycle is terminated, the input bias current of the BAT pin is reduced to
<0.1µA to minimize battery discharge while the charger
remains connected.
SENSE (Pin 4): Charge Current Sense Pin. The charge
current is monitored with a sense resistor (RSENSE) connected between this pin and the BAT pin. The inductor
current flows through RSENSE to the battery. The voltage
across this resistor sets the average charge current. The
maximum average charge current (IMAX) corresponds to
95mV across the sense resistor.
BOOST (Pin 5): Bootstrapped Supply Rail for Switch
Drive. This pin facilitates saturation of the high side switch
transistor. Connect a 1µF or greater capacitor from the
BOOST pin to the SW pin. The operating range of this pin
is 0V to 8.5V, referenced to the SW pin when the switch is
high. 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.
GND (Pins 6, 23, 31, 37): Ground. These pins are the
ground pins for the part. Pins 31 and 37 must be connected
together. Pins 6 and 23 are connected via the leadframe to
the exposed backside Pin 37. Solder the exposed backside
to the PCB for good thermal and electrical connection.
SW (Pins 7, 11-18, 22, 38): Switch Output Pin. These
pins are the output of the charger switches. An inductor is
connected between these pins and the SENSE pin. When
the switcher is active, the inductor is charged by the high
side switch from VIN and discharged by the bottom side
switch to GND. Solder the exposed backside, Pin 38, to
the PCB for good thermal connection.
NC (Pins 8-10,19-21): No Connect. These pins can be left
floating (not connected).
TIMER (Pin 24): End-Of-Cycle Timer Programming Pin.
A capacitor on this pin to ground determines the full
charge end-of-cycle time. Full charge end-of-cycle time is
programmed with this capacitor. A 3 hour charge cycle is
obtained with a 0.68µF capacitor. This timer also controls
the bad battery fault that is generated if the battery does not
reach the precondition threshold voltage within one-eighth
of a full cycle (22.5 minutes for a 3 hour charge cycle).
The timer based termination 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 rate, or approximately 10% of maximum charge rate.
FAULT (Pin 25): Open-Collector Fault Status Output. This
pin indicates charge cycle fault conditions during a battery
charging cycle. Typically a resistor pull-up is used on this
pin. This status pin can be pulled up to voltages as high
as VIN when disabled, and can sink currents up to 10mA
when enabled. A temperature fault causes this pin to be
pulled low. If the internal timer is used for termination,
a bad battery fault also causes this pin to be pulled low.
If no fault conditions exist, the FAULT pin remains high
impedance.
CHRG (Pin 26): Open-Collector Charger Status Output.
This pin indicates the battery charging status. Typically
a resistor pull-up is used on this pin. This status pin can
be pulled up to voltages as high as VIN when disabled,
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LT3651-8.2/LT3651-8.4
PIN FUNCTIONS
and can sink currents up to 10mA when enabled. CHRG
is pulled low during a battery charging cycle. When the
charge cycle is terminated, the CHRG pin becomes high
impedance. If the internal timer is used for termination,
the pin stays low during the charging cycle until the charge
current drops below a C/10 rate, or approximately 10%
of the maximum charge current. A temperature fault also
causes this pin to be pulled low.
SHDN (Pin 27): Shutdown Pin. This pin can be used for
precision UVLO functions. When this pin rises above the
1.20V threshold, the part is enabled. The pin has 95mV of
voltage hysteresis. When in shutdown mode, all charging
functions are disabled. When the SHDN pin is pulled below
0.4V, the IC enters a low current shutdown mode where
the VIN pin current is reduced to 17µA. Typical SHDN pin
input bias current is 10nA. Connect the pin to VIN if the
shutdown function is not desired.
ILIM (Pin 28): Input Current Limit Programming. This pin
allows for setting and dynamic adjustment of the system
input current limit, and can be used to employ a soft-start
function. The voltage on this pin sets the maximum input
current by setting the maximum voltage across the input
current sense resistor, placed between CLP and CLN.
The effective range on the pin is 0V to 1V. 50µA is sourced
from this pin usually to a resistor (RILIM) to ground. VIILIM
represents approximately 11 times the maximum voltage
across the input current sense resistor. If no RILIM is used
the part will default to maximum input current.
ply to the CLP pin, connecting a sense resistor from the
CLP pin to the CLN pin and then connecting CLN to VIN.
The system load is then delivered from the CLN pin. The
LT3651-8.2/LT3651-8.4 servo the maximum charge current required to maintain programmed maximum system
current. The system current limit is set as a function of
the voltage on the ILIM pin and the input current sense
resistor. This function is disabled by shorting CLP, CLN
and VIN together.
VIN (Pins 32, 33, 34): Charger Input Supply. These pins
provide power for the LT3651-8.2/LT3651-8.4. Charge
current for the battery flows into these pins. IVIN is less
than 100µA after charge termination. Connect the pins
together.
RNG/SS (Pin 35): Charge Current Range and Soft-Start
Pin. This pin allows for setting and dynamic adjustment
of the maximum charge current, and can be used to employ a soft-start function. The voltage on this pin sets the
maximum charge current by setting the maximum voltage
across the charge current sense resistor, RSENSE , placed
between SENSE and BAT.
The effective range on the pin is 0V to 1V. 50µA is sourced
from this pin usually to a resistor (RRNG/SS) to ground.
VRNG/SS represents approximately 10 times the maximum
voltage across the charge current sense resistor. If no RRNG/
SS is used the part will default to maximum charge current.
Soft-start functionality for input current can be implemented with a capacitor (CILIM) from ILIM to ground. The
soft-start capacitor and the programming resistor can be
implemented in parallel.
Soft-start functionality for charge current can be implemented by connecting a capacitor (CRNG/SS) from RNG/SS
to ground. The soft-start capacitor and the programming
resistor can be implemented in parallel. The RNG/SS pin
is pulled low during fault conditions, allowing graceful
recovery from faults if CRNG/SS is used.
CLP/CLN (Pin 29/Pin 30): System Current Limit Positive
and Negative Input. System current levels are monitored
by connecting a sense resistor from the input power sup-
RT (Pin 36): Switcher Oscillator Timer Set Pin. A resistor from this pin to ground sets the switcher oscillator
frequency. Typically this is 54.9k for fOSC = 1MHz.
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LT3651-8.2/LT3651-8.4
BLOCK DIAGRAM
A12
RT
TIMER OSC
+
–
A11
+
–
COUNT
RESET
COUNT
VC
TJ
A9
0.3V
REV CUR
INHIBIT
C-EA
SENSE
RS
– +
+
BAT
ITH
RNG/SS
10RS
A8
0.1V
+
+
0.15V
A7
PRECONDITION
NTC
VINT
2.7V
A6
×2.25
0.29V
A1
STANDBY
A4
A2
1.3V
+
+
+
1.2V
8.2V*
8.0V**
+
–
5.65V†
SHDN
27
TERMINATE
ACPR
A3
– +
1
NTC
–
+
35
50µA
1V
50µA 1.36V
3
SS/RESET
C/10
+
–
4
V-EA
TERMINATE
SS/RESET
STATUS
FAULT
RS
+
–
CHRG
VIN
125°C
STANDBY
COUNT
RESET
MODE
ENABLE (TIMER
OR C/10)
CONTROL LOGIC
SW
7, 11-18, 22, 38
A14
+
–
25
+
A10
RIPPLE COUNTER
26
A13
35V
OSC
0.2V
TIMER
+
–
24
R
LATCH
S Q
5
VIN
32, 33, 34
+
–
36
+
–
CLP
+
–
+
–
29
+
CLN
8.7V
OVLO
+ –
30
ILIM
BOOST
–
+
28
UVLO
+
–
STANDBY
50µA
2.4V
+
–
A5
2
0.7V
46µA
GND
6, 23, 31, 37
*VBAT(FLT): 8.2V FOR LT3651-8.2, 8.4V FOR LT3651-8.4
**VBAT(FLT) – ∆VRECHRG: 8V FOR LT3651-8.2, 8.2V FOR LT3651-8.4
†V
BAT(PRE): 5.65V FOR LT3651-8.2, 5.8V FOR LT3651-8.4
365148284 BD
36518284fa
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9
LT3651-8.2/LT3651-8.4
OPERATION
Overview
The LT3651-8.2/LT3651-8.4 are complete Li-Ion battery
chargers, addressing wide input voltage and high currents
(up to 4A). High charging efficiency is produced with a
constant frequency, average current mode synchronous
step-down switcher architecture.
The charger includes the necessary circuitry to allow for
programming and control of constant current, constant
voltage (CC/CV) charging with both current only and timer
termination. High charging efficiency is achieved by the
switcher by using a bootstrapped supply for low switch
drop for the high side driver and a MOSFET for the low
side (synchronous) switch.
Maximum charge current is set with an external sense resistor in series with the inductor and is adjustable through
the RNG/SS pin. Total system input current is monitored
with an input sense resistor and is used to maintain constant input current by regulating battery charge current.
It is adjustable through the ILIM pin.
If the battery voltage is low, charge current is automatically
reduced to 15% of the programmed current to provide
safe battery preconditioning. Once the battery voltage
climbs above the battery precondition threshold, the IC
automatically increases the maximum charge current to
the full programmed value.
Charge termination can occur when charge current decreases to one-tenth the programmed maximum charge
current (C/10 termination). Alternately, termination can
be time based through the use of an internal programmable charge cycle control timer. When using the timer
termination, charging continues beyond the C/10 level to
“top-off” a battery. Charging typically terminates three
hours after initiation. When the timer-based scheme is
used, bad battery detection is also supported. A system
fault is triggered if a battery stays in precondition mode
for more than one-eighth of the total charge cycle time.
Once charging is terminated and the LT3651-8.2/
LT3651‑8.4 are not actively charging, the IC automatically
enters a low current standby mode in which supply bias
currents are reduced to <85µA. If the battery voltage drops
2.5% from the full charge float voltage, the LT3651-8.2/
LT3651-8.4 engage an automatic charge cycle restart. The
IC also automatically restarts a new charge cycle after a
bad battery fault once the failed battery is removed and
replaced with another battery.
After charging is completed the input bias currents on the
pins connecting to the battery are reduced to minimize
battery discharge.
The LT3651-8.2/LT3651-8.4 contain provisions for a battery temperature monitoring circuit. Battery temperature
is monitored by using a NTC thermistor located with the
battery. If the battery temperature moves outside a safe
charging range of 0°C to 40°C the charging cycle suspends
and signals a fault condition.
The LT3651-8.2/LT3651-8.4 contain two digital opencollector 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.
A precision undervoltage lockout is possible by using a
resistor divider on the shutdown pin (SHDN). The input
supply current is 17µA when the IC is in shutdown.
General Operation (See Block Diagram)
The LT3651-8.2/LT3651-8.4 use an average current mode
control loop architecture to control average charge current.
The LT3651-8.2/LT3651-8.4 sense charger output voltage
via the BAT pin. The difference between this voltage and the
internal float voltage reference is integrated by the voltage
error amplifier (V‑EA). The amplifier output voltage (ITH)
corresponds to the desired average voltage across the
inductor sense resistor, RSENSE, connected between the
SENSE and BAT pins. The ITH voltage is divided down by
a factor of 10, and provides a voltage offset on the input
of the current error amplifier (C‑EA). The difference between this imposed voltage and the current sense resistor
voltage is integrated by C-EA. The resulting voltage (VC)
provides a voltage that is compared against an internally
generated ramp and generates the switch duty cycle that
controls the charger’s switches.
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LT3651-8.2/LT3651-8.4
OPERATION
The ITH error voltage corresponds linearly to average current sensed across the inductor current sense resistor.
Maximum charge current is controlled by clamping the
maximum voltage of ITH to 1V. This limits the maximum
current sense voltage (voltage across RSENSE) to 95mV
setting the maximum charge current. Manipulation of
maximum charge current is possible through the RNG/SS
and ILIM pins (see the RNG/SS: Dynamic Charge Current
Adjust, RNG/SS: Soft-Start and ILIM Control sections).
If the voltage on the BAT pin (VBAT) is below VBAT(PRE), A7
initiates the precondition mode. During the precondition
interval, the charger continues to operate in constant current mode, but the ITH clamp is reduced to 0.15V reducing
charge current to 15% of the maximum programmed value.
As VBAT approaches the float voltage (VFLOAT) the voltage
error amp V-EA takes control of ITH and the charger transitions into constant voltage (CV) mode. As this occurs, the
ITH voltage falls from the limit clamp and charge current is
reduced from the maximum value. When the ITH voltage
falls below 0.1V, A8 signals C/10. If the charger is configured for C/10 termination the charge cycle is terminated.
Once the charge cycle is terminated, the CHRG status
pin becomes high impedance and the charger enters low
current standby mode.
The LT3651-8.2/LT3651-8.4 contain 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 in three hours, but
can be configured for any amount of time by setting an
appropriate timing capacitor value (CTIMER). When timer
termination is used, the charge cycle does not terminate
after 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 will continue to
source low currents. At the programmed end of the cycle
time the charge cycle stops and the part enters standby
mode. If the battery did not achieve at least 97.5% of the
full float voltage at the end-of-cycle, charging is deemed
unsuccessful and another full-timer cycle is initiated.
Use of the timer function also enables bad battery detection. This fault condition is achieved if the battery does
not respond to preconditioning and the charger remains
in (or enters) precondition mode after one-eighth 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 LT3651-8.2/LT3651-8.4 terminate a charging
cycle, whether through C/10 detection or by reaching
timer end-of-cycle, 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 overvoltage condition and rails low.
When the voltage error amp output drops below 0.3V,
the IC enters standby mode, where most of the internal
circuitry is disabled and the VIN bias current is reduced
to <100µA. When the voltage on the BAT 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.
The system current limit allows charge current to be
reduced in order to maintain a constant input current.
Input current is measured via a resistor (RCL) that is
placed between the CLP and CLN pins. Power is applied
through this resistor and is used to supply both VIN of the
chip and other system loads. An offset produced on the
inputs of A12 sets the threshold. When that threshold is
achieved, ITH is reduced, lowering the charge current thus
maintaining the maximum input current.
50µA of current is sourced from ILIM to a resistor (RILIM)
that is placed from that pin to ground. The voltage on ILIM
determines the regulating voltage across RCL. 1V on ILIM
corresponds to 95mV across RCL. The ILIM pin clamps
internally to 1V maximum.
If the junction temperature of the die becomes excessive,
A10 activates decreasing ITH and reduces charge current.
This reduces on-chip power dissipation to safe levels but
continues charging.
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11
LT3651-8.2/LT3651-8.4
APPLICATIONS INFORMATION
OSC Frequency
A precision resistor to ground sets the LT3651-8.2/
LT3651‑8.4 switcher oscillator frequency, fOSC, permitting user adjustability of the frequency value. Typically
this frequency is in the 200kHz to 1MHz range. Power
consideration may necessitate lower frequency operation
especially if the charger is operated with very high voltages.
Adjustability also allows the user to position switching
harmonics if their system requires.
The timing resistor, RT , value is set by the following:
RT =
54.9
(kΩ)
fOSC (MHz )
VIN Input Supply
The LT3651-8.2/LT3651-8.4 are 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 required to minimize voltage glitches on VIN. The VIN decoupling capacitor (CVIN)
absorbs all input switching ripple current in the charger.
Size is determined by input ripple voltage with the following equation:
ICHG(MAX) • VBAT
fOSC (MHz ) • ∆VIN • VIN
(µF )
where ∆VIN is the input ripple, ICHG(MAX) is the maximum
charge current and f is the oscillator frequency. A good
starting point for ∆VIN is 0.1V. Worst-case conditions
are with VBAT high and VIN at minimum. So for a 15V
VIN(MIN), IMAX = 4A and a 1MHz oscillator frequency:
CIN(BULK) =
4 • 8.2
= 22µF
1• 0.1• 15
The capacitor must have an adequate ripple current rating.
RMS ripple current, ICVIN(RMS) is approximated by:
V 
ICVIN(RMS) ≈ICHG(MAX) •  BAT  •
 VIN 
Boost Supply
The BOOST bootstrapped supply rail drives the internal
switch and facilitates saturation of the high side switch
transistor. The BOOST voltage is normally created by
connecting a 1µF capacitor from the BOOST pin to the
SW pin. Operating range of the BOOST pin is 2V to 8.5V,
as referenced to the SW pin.
The boost capacitor is normally charged via a diode connected from the battery or an external source through the
low side switch. Rate the diode average current greater
than 0.1A and its reverse voltages greater than VIN(MAX).
Set RT to 54.9k for 1MHz operation.
CIN(BULK)
which has a maximum at VIN = 2 • VBAT , where ICVIN(RMS)
= ICHG(MAX)/2. In the example above that requires a capacitor RMS rating of 2A.
VIN
–1
VBAT
If an external supply that is greater than the input is available (VBOOST – VIN > 2V), it may be used in place of the
bootstrap capacitor and diode.
VIN ,VBOOST Start-Up Requirement
The LT3651-8.2/LT3651-8.4 operate with a VIN range of
9V to 32V. The charger begins a charging cycle when the
detected battery voltage is below the auto-restart float
voltage and the part is enabled.
When VIN is below 10.5V and the BOOST capacitor is
uncharged, the high side switch would normally not have
sufficient head room to start switching. During normal
operation the low side switch is deactivated when charge
current is very low to prevent reverse current in the inductor. However in order to facilitate start-up, the LT36518.2/LT3651-8.4 enable the switch if VBOOST voltage is
low. This allows initial charging of the BOOST capacitor
which then permits the high side switch to saturate and
efficiently operate. The boost capacitor charges to full
potential after a few cycles.
The design should consider that as the switcher turns on
and input current increases, input voltage drops due to
source input impedance and input capacitance. This potentially allows the input voltage to drop below the internal
VIN UVLO turn-on and thus disrupt normal behavior and
potentially stall start-up. If an input current sense resis36518284fa
12
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LT3651-8.2/LT3651-8.4
APPLICATIONS INFORMATION
tor is used, its drop must be considered as well. These
problems are worsened because input current is largest
at low input voltage. Pay careful attention to drops in the
power path. Adding a soft-start capacitor to the RNG/SS
pin and setting UVLO to 9V with the SHDN pin is required
at low VIN.
BAT Output Decoupling
It is recommended that the LT3651-8.2/LT3651-8.4 charger output have a decoupling capacitor. If the battery can
be disconnected from the charger output this capacitor is
required. The value of this capacitor (CBAT) is related to
the minimum operational VIN voltage such that:
 350µF 
CBAT ≈ 20µF + 

 VIN(MIN) 
The voltage rating on CBAT must meet or exceed the battery float voltage.
RSENSE: Charge Current Programming
The LT3651-8.2/LT3651-8.4 charger is configurable to
charge at average currents as high as 4A (see Figure 1).
If RNG/SS maximum voltage is not limited, the inductor
sense resistor, RSENSE, has 95mV across it at maximum
charge current so:
RSENSE =
0.095V
Inductor Selection
The primary criteria for inductor value selection in the
LT3651-8.2/LT3651-8.4 charger is the ripple current created during switching. Ripple current, ∆IMAX, is typically
set within a range of 25% to 35% of the maximum charge
current, IMAX. This percentage typically gives a good compromise between losses due to ripple and inductor size.
An approximate formula for inductance is:
L=
 V +V 
VBAT + VF
•  1– BAT F  (µH)
∆IMAX • fOSC (MHz ) 
VIN + VF 
Worse-case ripple is at high VIN and high VBAT . VF is the
forward voltage of the synchronous switch (approximately
0.14V at 4A). Figure 2 shows inductance for the case of a
4A charger. The inductor must have a saturation current
equal to or exceeding the maximum peak current in the
inductor. Peak current is ICHG(MAX) + ∆ICHG(MAX)/2.
Magnetics vendors typically specify inductors with maximum RMS and saturation current ratings. Select an inductor
that has a saturation current rating at or above peak current,
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 approximately:
VBAT
ICHG(MAX)
where ICHG(MAX) is the maximum average charge current.
RSENSE is 24mΩ for a 4A charger.
fOSC(MHz)


V
•  1– BAT  ( V • µs )
 VIN(MAX) 
4
SW
3
L (µH)
BOOST
LT3651-8.2
LT3651-8.4
SENSE
2
RSENSE
BAT
1
IMAX = 4A
fOSC = 1MHz
25% TO 35% RIPPLE
+
365142 F01
0
9 10
Figure 1. Programming Maximum Charge Current Using RSENSE
15
20
VIN(MAX) (V)
25
30
36512 F02
Figure 2. Inductance (L) vs Maximum VIN
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13
LT3651-8.2/LT3651-8.4
APPLICATIONS INFORMATION
Acceptable power inductors are available from several
manufacturers such a Würth Elektronik, Vishay, Coilcraft
and TDK.
System Input Current Limit
The LT3651-8.2/LT3651-8.4 contain a PowerPath control
feature to help manage supply load currents. The charger
adjusts charger output current in response to a system
load so as to maintain a constant input supply load. If
overall input supply current exceeds the programmed
maximum value the charge current is diminished in an
attempt to keep supply current constant. One application
where this is helpful is if you have a current limited input
supply. Setting the maximum input current limit below
the supply limit prevents supply collapse.
For example, say you want a maximum input current of
2A and the charger is designed for 4A maximum average
charge current, which is 1A VIN referred (4A times duty
cycle). Using the full ILIM range, the maximum voltage
across RCL is 95mV. So RCL is set at 95mV/2A = 48mΩ.
When the system load exceeds 1A (= 2A – 1A) charge
current is reduced such that the total input current stays
at 2A. When the system load is 2A the charge current is 0.
This feature only controls charge current so if the system
load exceeds the maximum limit and no other limitation
is designed, the input current exceeds the maximum
desired, though the charge current reduces to 0A. When
the input limiter reduces charge current it does not impact
the internal system timer if used. See Figure 4.
A resistor, RCL, is placed between the input supply and the
system and charger loads as shown in Figure 3.
IINPUT(MAX) =
VILIM
50µA •RILIM
=
11.5 •RCL
11.5 •RCL
The programming range for ILIM is 0V to 1V. Voltages higher
than 1V have no effect on the maximum input current. The
default maximum sense voltage is 95mV and is obtained
if RILIM is greater than 20k or if the pin is left open.
INPUT
SUPPLY
CLP
LT3651-8.2
LT3651-8.4
CLN
RCL
CURRENT (A)
The LT3651-8.2/LT3651-8.4 source 50µA from the ILIM pin,
so a voltage is developed by simply connecting a resistor
to ground. The voltage on the ILIM pin corresponds to
11.5 times the maximum voltage across the input sense
resistor (RCL). Input current limit is defined by:
3
INPUT CURRENT
2
CHARGE
CURRENT
(VIN REFERRED)
1
0
0
2
1
SYSTEM LOAD CURRENT (A)
365142 F04
Figure 4. Input Current Limit for 4A Maximum Charger
and 6A System Current Limit
If reduced voltage overhead or better efficiency is required
then reduce the maximum voltage across RCL. So for
instance, a 10k RILIM sets the maximum RCL voltage to
43mV. This reduction comes at the expense of slightly
increased limit variation.
Note the LT3651-8.2/LT3651-8.4 internally integrate the
input limit signals. This should normally provide sufficient
filtering and reduce the sensitivity to current spikes. For
the best accuracy take care to provide good Kelvin connections from RCL to CLP, CLN.
SYSTEM LOAD
VIN
ILIM
RLIM
365142 F03
Figure 3. Input Current Limit Configuration
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LT3651-8.2/LT3651-8.4
APPLICATIONS INFORMATION
Further flexibility is possible by dynamically altering the
ILIM pin. Different resistor values could be switched in
to create unique input limit conditions. The ILIM pin can
also be tied to a servo amplifier for other options. See the
information in the following section concerning IRNG/SS
programming for examples.
RNG/SS: Dynamic Current Adjust
The RNG/SS pin gives the user the capability to adjust
maximum charge current dynamically. The part sources
50µA from the pin, so connecting a resistor to ground
develops a voltage. The voltage on the RNG/SS pin corresponds to ten times the maximum voltage across the
charge current sense resistor, RSENSE. The defining equations for charge current are:
IMAX(RNG/SS) =
VRNG/SS
50µA •RRNG/SS
=
10.8 •RSENSE 10.8 •RSENSE
IMAX(RNG/SS) is the maximum charge current.
The programming range for RNG/SS is 0V to 1V. Voltages
higher than 1V have no effect on the maximum charge
current. The default maximum sense voltage is 95mV
and is obtained if RRNG/SS is greater than 20k or if the
pin is left open.
For example, say you want to reduce the maximum charge
current to 50% of the maximum value. Set RNG/SS to 0.5V
(50% of 1V), imposing a 46mV maximum sense voltage.
Per the above equation, 0.5V on RNG/SS requires a 10k
resistor. If the charge current needs to be dynamically
adjustable then Figure 5 shows one method.
Active servos can also be used to impose voltages on the
RNG/SS pin, provided they can only sink current. Active
circuits that source current cannot be used to drive the
RNG/SS pin. An example is shown in Figure 6.
RNG/SS: Soft-Start
Soft-start functionality is also supported by the RNG/SS
pin. The 50µA sourced from the RNG/SS pin can linearly
charge a capacitor, CRNG/SS, connected from the RNG/
SS pin to ground (see Figure 7). The maximum charge
current follows this voltage. Thus, the charge current
increases from zero to the fully programmed value as the
LT3651-8.2
LT3651-8.4
LT3651-8.2
LT3651-8.4
RNG/SS
RNG/SS
10k
+
–
LOGIC HIGH = HALF CURRENT
SERVO
REFERENCE
365142 F06
365142 F05
Figure 5. Using the RNG/SS Pin for
Digital Control of Maximum Charge Current
Figure 6. Driving the RNG/SS Pin
with a Current-Sink Active Servo Amplifier
LT3651-8.2
LT3651-8.4
RNG/SS
CRNG/SS
365142 F07
Figure 7. Using the RNG/SS Pin for Soft-Start
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15
LT3651-8.2/LT3651-8.4
APPLICATIONS INFORMATION
capacitor charges from 0V to 1V. The value of CRNG/SS is
calculated based on the desired time to full current (tSS)
following the relation:
CRNG/SS = 50µA • tSS
The RNG/SS pin is pulled to ground internally when charging is terminated so each new charging cycle begins with
a soft-start cycle. RNG/SS is also pulled to ground during
bad battery and NTC fault conditions, so a graceful recovery
from these faults is possible.
Status Pins
The LT3651-8.2/LT3651-8.4 report 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 than a C/10 rate, or one-tenth of the programmed maximum charge current. The FAULT pin signals
bad battery and NTC faults. These pins are binary coded,
and signal state following the table below. On indicates
the pin pulled low, and Off indicates pin high impedance.
Table 1. Status Pins State Table
STATUS PINS STATE
When C/10 termination is used, a LT3651-8.2/LT3651-8.4
charger sources 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 LT3651-8.2/LT3651‑8.4
enter standby mode. The CHRG status pin follows the
charge 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 LT3651-8.2/LT3651-8.4 support 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 end-of-cycle
(tEOC) occurs based on CTIMER following the relation:
CHARGER STATUS
CTIMER =
tEOC (Hrs)
• 0.68 (µF )
3
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
so a typical 3 hour timer end-of-cycle would use a 0.68µF
capacitor.
On
On
NTC Fault (Pause)
C/10 Termination
The LT3651-8.2/LT3651-8.4 support a low current based
termination scheme, where a battery charge cycle terminates when the current output from the charger falls to
below one-tenth the maximum current, as programmed
with RSENSE. The C/10 threshold current corresponds to
9mV across RSENSE. This termination mode is engaged
by shorting the TIMER pin to ground.
The CHRG status pin continues to signal charging at a
C/10 rate, regardless of which termination scheme is
used. When timer termination is used, the CHRG status
pin is pulled low during a charge cycle until the charger
output current falls below the C/10 threshold. The charger
continues to “top off” the battery until timer end-of-cycle,
when the LT3651-8.2/LT3651-8.4 terminate the charge
cycle and enters standby mode.
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APPLICATIONS INFORMATION
Termination at the end of the timer cycle only occurs if the
charge cycle was successful. A successful charge cycle
occurs when the battery is charged to within 2.5% of the
full-charge float voltage. If a charge cycle is not successful at end-of-cycle, 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 re-engages and starts
charging.
Precondition and Bad Battery Fault
A LT3651-8.2/LT3651-8.4 charger has a precondition
mode, in which charge current is limited to 15% of the
programmed IMAX, as set by RSENSE. The precondition
current corresponds to 14mV across RSENSE.
Precondition mode is engaged while the voltage on the
BAT pin is below the precondition threshold (VBAT(PRE)).
Once the BAT voltage rises above the precondition threshold, normal full-current charging can commence. The
LT3651-8.2/LT3651-8.4 incorporate 2.5% of threshold
for hysteresis to prevent mode glitching.
When the internal timer is used for termination, bad battery detection is engaged. This fault detection feature
is designed to identify failed cells. A bad battery fault is
triggered when the voltage on BAT remains below the
precondition threshold for greater than one-eighth of a full
timer cycle (one-eighth end-of-cycle). A bad battery fault
is also triggered if a normally charging battery re-enters
precondition mode after one-eighth end-of-cycle.
When a bad battery fault is triggered, the charge cycle
is suspended, so the CHRG status pin becomes high
impedance. The FAULT pin is pulled low to signal a fault
detection. The RNG/SS pin is also pulled low during this
fault, to accommodate a graceful restart, in the event that
a soft-start function is incorporated (see the RNG/SS:
Soft-Start section).
Cycling the charger’s power or SHDN function initiates a
new charge cycle, but a LT3651-8.2/LT3651-8.4 charger
does not require a reset. Once a bad battery fault is detected, a new timer charge cycle initiates when the BAT pin
exceeds the precondition threshold voltage. During a bad
battery fault, 1mA is sourced from the charger. Removing
the failed battery allows the charger output voltage to rise
and initiate a charge cycle reset. In that way removing a
bad battery resets the LT3651-8.2/LT3651-8.4. A new
charge cycle is started by connecting another battery to
the charger output.
Battery Temperature Fault: NTC
The LT3651-8.2/LT3651-8.4 can accommodate battery temperature monitoring by using an NTC (negative temperature
coefficient) 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 LT3651-8.2/
LT3651-8.4 trigger 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 corresponds
to 2.5°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 condition
is relieved. The RNG/SS pin is also pulled low during this
fault, to accommodate a graceful restart in the event that
a soft-start function is being incorporated (see the RNG/
SS: Soft-Start section).
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For more information www.linear.com/LT3651-8.2
17
LT3651-8.2/LT3651-8.4
APPLICATIONS INFORMATION
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 (0TC)
resistor will increase the effective temperature threshold
to 45°C.
Thermal Foldback
The LT3651-8.2/LT3651-8.4 contain a thermal foldback
protection feature that reduces maximum charger output
current if the internal IC junction temperature approaches
125°C. In most cases, on-chip temperature servos such
that any overtemperature conditions are relieved with only
slight reductions in maximum charge current.
In some cases, the thermal foldback protection feature
can reduce charge currents below the C/10 threshold. In
applications that use C/10 termination (TIMER = 0V), the
LT3651-8.2/LT3651-8.4 suspend charging and enters
standby mode until the overtemperature condition is
relieved.
and BAT traces together and keep the traces as short as
possible. Shielding these signals from switching noise
with ground is recommended. Make Kelvin connections
to the battery and sense resistor.
Keep high current paths and transients isolated from
battery ground, to assure an accurate output voltage
reference. Effective grounding is 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 8 illustrates the high current, high speed
current loops. When the top switch is enabled (charge
loop), current flows from the input bypass capacitor (CIN)
through the switch and inductor to the battery positive
terminal. When the top switch is disabled (discharge loop),
current to the battery positive terminal is provided from
ground through the synchronous switch. In both cases,
these switched currents return to ground via the output
bypass capacitor (CBAT).
BOOST
Layout Considerations
The LT3651-8.2/LT3651-8.4 switch node has rise and fall
times that are typically less than 10ns to maximize conversion efficiency. These fast switch times require care in the
board layout to minimize noise problems. The philosophy
is to keep the physical area of high current loops small (the
inductor charge/discharge paths) to minimize magnetic
radiation. Keep traces wide and short to minimize parasitic
inductance and resistance and shield fast switching voltage nodes (SW, BOOST) to reduce capacitive coupling.
The switched node (SW pin) trace should be kept as
short as possible to minimize high frequency noise. The
VIN capacitor (CIN) should be placed close to the IC to
minimize this switching noise. Short, wide traces on these
nodes minimize stray inductance and resistance. Keep the
BOOST decoupling capacitor in close proximity to the IC to
minimize ringing from trace inductance. Route the SENSE
VIN
CBOOST
CIN
LT3651-8.2
LT3651-8.4
RSENSE
SW
CHARGE
+
DISCHARGE
CBAT
BATTERY
365142 F08
Figure 8
Power Considerations
The LT3651-8.2/LT3651-8.4 packaging is 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.
36518284fa
18
For more information www.linear.com/LT3651-8.2
LT3651-8.2/LT3651-8.4
APPLICATIONS INFORMATION
Consideration should be given for power dissipation and
overall efficiency in a LT3651-8.2/LT3651-8.4 charger. A
detailed analysis is beyond the scope of the data sheet,
however following are general guidelines.
The major components of power loss are: conduction and
transition losses of the LT3651-8.2/LT3651-8.4 switches;
losses in the inductor and sense resistors; and AC losses
in the decoupling capacitors. Switch conduction loss is
fixed. Transition losses are adjustable by changing switcher
frequency. Higher input voltages cause an increase in
transition losses, decreasing overall efficiency. However
transition losses are inversely proportional to switcher
oscillator frequency so lowering operating frequency
reduces these losses. But lower operating frequency
usually requires higher inductance to maintain inductor
ripple current (inversely proportional). Inductors with
larger values typically have more turns, increasing ESR
unless you increase wire diameter making them physically
larger. So there is an efficiency and board size trade-off.
Secondarily, inductor AC losses increase with frequency
and lower ripple reduces AC capacitor losses.
The following simple rules of thumb assume a charge
current of 4A and battery voltage of 7.5V, with 1MHz oscillator, 24mΩ sense resistor and 3.3µH/20mΩ inductor.
A 1% increase in efficiency represents a 0.35W reduction
in power loss at 85% overall efficiency. One way to do
this is to decrease resistance in the high current path. A
reduction of 0.2W at 4A requires a 22mΩ reduction in
resistance. This can be done by reducing inductor ESR.
It is also possible to lower the sense resistance (with a
reduction in RRNG/SS as well), with a trade-off of slightly
less accurate current accuracy. All high current board
traces should have the lowest resistance possible. Addition
of input current limit sense resistance reduces efficiency.
Charger efficiency drops approximately linearly with increasing frequency all other things constant. At 15V VIN
there is a 1% improvement in efficiency for every 200kHz
reduction in frequency (100kHz to 1MHz); At 28V VIN, 1%
for every 100kHz.
Of course all of these must be experimentally confirmed
in the actual charger.
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For more information www.linear.com/LT3651-8.2
19
LT3651-8.2/LT3651-8.4
TYPICAL APPLICATIONS
9V to 32V 4A Charger with High Voltage Current Foldback
SBM540
CIN
22µF
SMAZ24
18.2V
CLP
RT
54.9k
CLN
VIN
SHDN
SW
ACPR
FAULT
BOOST
CHRG
LT3651-8.2/LT3651-84
SENSE
NC
RT
TIMER
ILIM RNG/SS
5
VIN
MAXIMUM CHARGE CURRENT (A)
RIL
1k
120k
Maximum Charge Current vs VIN
1µF
3.3µH
1N5819
RSENSE
24mΩ
BAT
NTC
GND
CBAT
100µF
+
4
3
2
1
2-CELL
Li-Ion
BATTERY
0
5
10
15
365142 TA02a
20
VIN (V)
30
25
35
3651 TA02b
12V to 32V 4A Charger with Low Voltage Current Foldback
Using the RNG/SS Pin
SMAZ9V1
9.1V
RT
54.9k
CLP
TIMER
ILIM
CIN
22µF
VIN
SHDN
SW
ACPR
FAULT
BOOST
CHRG
LT3651-82/LT3651-84
NC
SENSE
RT
68k
5.1k
CLN
BAT
NTC
RNG/SS GND
5
TO
SYSTEM
LOAD
MAXIMUM CHARGE CURRENT (A)
SBM540
VIN
1µF
3.3µH
1N5819
RSENSE
24mΩ
CBAT
100µF
+
Maximum Charge Current vs VIN
2-CELL
Li-Ion
BATTERY
365142 TA03a
4
3
2
0
10
15
20
25
VIN (V)
30
35
3651 TA03b
1µF
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20
For more information www.linear.com/LT3651-8.2
LT3651-8.2/LT3651-8.4
TYPICAL APPLICATIONS
9V to 32V 4A Charger with Approximately Constant Input Power
8.2V
CLP
180k
20k
6.2V
180k
RT
54.9k
CLN
VIN
SHDN
SW
ACPR
FAULT
BOOST
CHRG
LT3651-8.2/LT3651-8.4
NC
SENSE
RT
TIMER
ILIM RNG/SS
CIN
22µF
BAT
NTC
GND
25
TO
SYSTEM
LOAD
24
23
INPUT POWER (W)
RSENSE
50mΩ
SBM540
VIN
1µF
3.3µH
1N5819
RSENSE
24mΩ
CBAT
100µF
22
21
20
19
18
17
+
2-CELL
Li-Ion
BATTERY
365142 TA05a
0.1µF
Input Power vs VIN
16
15
5
22k
10
15
20
VIN (V)
25
30
35
365142 TA05b
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For more information www.linear.com/LT3651-8.2
21
LT3651-8.2/LT3651-8.4
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
UHE Package
Variation: UHE36MA
36-Lead Plastic QFN (5mm × 6mm)
(Reference LTC DWG # 05-08-1753 Rev A)
0.70 ±0.05
1.52
±0.05
2.54 ±0.05
5.50 ±0.05
0.25 ±0.05
4.10 ±0.05
3.50 REF
3.45 ±0.05
3.45 ±0.05
PACKAGE
OUTLINE
0.76 ±0.05
0.25 ±0.05
0.50 BSC
4.50 REF
5.10 ±0.05
6.50 ±0.05
RECOMMENDED SOLDER PAD LAYOUT
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
5.00 ±0.10
0.00 – 0.05
0.200 REF
R = 0.10
TYP
PIN 1 NOTCH
R = 0.30 TYP
OR 0.35 × 45°
CHAMFER
3.50 REF
35
36
0.40 ±0.10
PIN 1
TOP MARK
(SEE NOTE 6)
1
2
2.54 ±0.10
6.00 ±0.10
3.45
±0.10
4.50 REF
1.52 ±0.10
3.45
±0.10
(UHE36MA) QFN 0410 REV A
0.75 ±0.05
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
0.25 ±0.05
R = 0.125
TYP
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm 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
36518284fa
22
For more information www.linear.com/LT3651-8.2
LT3651-8.2/LT3651-8.4
REVISION HISTORY
REV
DATE
DESCRIPTION
A
07/15
Modified Typical Application circuit.
PAGE NUMBER
1
Modified Efficiency/Power Loss curve.
1
Changed typical values of Boost Supply Current/ Switch Drive.
3
Modified Typical Performance Characteristic curves.
6
Clarified GND Pin Function description.
7
36518284fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection
of itsinformation
circuits as described
herein will not infringe on existing patent rights.
For more
www.linear.com/LT3651-8.2
23
LT3651-8.2/LT3651-8.4
TYPICAL APPLICATION
9V to 32V 4A Charger with 3-Hour Charge Timeout, 6.3A Input Current
Limit, 10ms Soft-Start and Battery Temperature Monitoring
RIL
16mΩ
SBM540
VIN
50k
50k
50k
CIN
22µF
1µF
VLOGIC
50k
CLP
SHDN
VIN
SW
CLN
LT3651-8.2
NC LT3651-8.4
ACPR
TO
CONTROLLER
FAULT
CHRG
CTIMER
0.68µF
0.47µF
3.3µH
1N5819
SENSE
BAT
NTC
ILIM RNG/SS GND
TIMER
1µF
BOOST
RT
RT
54.9k
TO
SYSTEM
LOAD
RSENSE
24mΩ
CBAT
100µF
NTC B
10k
+
2-CELL
Li-Ion
BATTERY
3651 TA04
RELATED PARTS
PART NUMBER
DESCRIPTION
LT3651-4.1/LT3651-4.2 Monolithic 4A Switch Mode Synchronous
1-Cell Li-Ion Battery Charger
LT3650
2A Monolithic Li-Ion Battery Charger
LT3652/LT3652HV
LTC4000
LTC4002
LTC4006
LTC4007
LTC4008
LTC4009/LTC4009-1
LTC4009-2
LTC4012/LTC4012-1/
LTC4012-2/LTC4012-3
COMMENTS
Standalone, 4.75 ≤ VIN ≤ 32V (40V Abs Max), 1MHz, 4A, Programmable Charge
Current Timer or V/10 Termination 5mm × 6mm QFN-36 Package
High Efficiency, Wide Input Voltage Range Charger, Time or Charge Current
Termination, Automatic Restart, Temperature Monitoring, Programmable Charge
Current, Input Current Limit, 12-Lead DFN and MSOP Packages
Power Tracking 2A Battery Charger
Input Supply Voltage Regulation Loop for Peak Power Tracking in (MPPT)
Solar Applications, Standalone, 4.95V ≤ VIN ≤ 32V (40V Abs Max), 1MHz, 2A
Programmable Charge Current, Timer or C/10 Termination, 3mm × 3mm DFN-12
Package and MSOP-12 Packages. LT3652HV Version Up to VIN = 34V
High Voltage High Current Controller for Complete High Performance Battery Charger When Paired with a DC/DC Converter
Battery Charging and Power Management Wide Input and Output Voltage Range: 3V to 60V ±0.25% Accurate Programmable
Float Voltage, Programmable C/X or Timer Based Charge Termination NTC Input for
Temperature Qualified Charging, 28-Lead 4mm × 5mm QFN or SSOP Packages
Complete Charger for 1- or 2-Cell Li-Ion Batteries, Onboard Timer Termination,
Standalone Li-Ion Switch Mode
Battery Charger
Up to 4A Charge Current, 10-Lead DFN and SO-8 Packages
Small, High Efficiency, Fixed Voltage
Complete Charger for 2-, 3- or 4-Cell Li-Ion Batteries, AC Adapter Current Limit
Li-Ion Battery Charger with Termination
and Thermistor Sensor, 16-Lead Narrow SSOP Package
Complete Charger for 3- or 4-Cell Li-Ion Batteries, AC Adapter Current Limit,
High Efficiency, Programmable Voltage
Battery Charger with Termination
Thermistor Sensor and Indicator Outputs, 24-Lead SSOP Package
Complete Charger for 2- to 6-Cell Li-Ion Batteries or 4- to 18-Cell Nickel Batteries,
4A, High Efficiency, Multi-Chemistry
Battery Charger
Up to 96% Efficiency, 20-Lead SSOP Package
Complete Charger for 1- to 4-Cell Li-Ion Batteries or 4- to 18-Cell Nickel Batteries,
High Efficiency, Multi-Chemistry
Battery Charger
Up to 93% Efficiency, 20-Lead (4mm × 4mm) QFN Package, LTC4009-1 for 4.1V
Float Voltage, LTC4009-2 for 4.2V Float Voltage
4A, High Efficiency, Multi-Chemistry
PowerPath Control, Constant-Current/Constant-Voltage Switching Regulator
Battery Charger with PowerPath Control Charger, Resistor, Voltage/Current Programming, AC Adapter Current Limit and
Thermistor Sensor and Indicator Outputs, 1 to 4-cell Li, Up to 18-cell Ni, SLA and
SuperCap Compatible; 4mm × 4mm QFN-20 Package; LTC4012-1 Version for 4.1V
Li Cells, LTC4012-2 Version for 4.2V Li Cells, LTC4012-3 Version Has Extra GND Pin
36518284fa
24 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT3651-8.2
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT3651-8.2
LT 0715 REV A • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2012
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