LT3651IUHE-4.1#PBF - ANALOG DEVICES

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Monolithic 4A Wide Input

1 Cell Li-Ion Battery Charger

n Industrial Handheld Instruments

n 12V to 24V Automotive and Heavy Equipment

n Desktop Cradle Chargers

n Notebook Computers

6.5V to 32V Single Cell 4A Charger

Efficiency, Power Loss vs DCIN

n Wide Input Voltage Range: Up 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 <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-Pin

QFN Package

The LT

3651 is a 1 cell 4A Li-Ion/Polymer battery charger

that operates with input voltages up to 32V. 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 termina-

tion 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 pro-

grammable (charging continues below C/10 until timeout).

Once charging is terminated, the LT3651 supply current

drops to 85µA into a standby mode.

The LT3651 offers 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. Ex-

cessive die temperature reduces charge current. Charge

current is also reduced to maintain constant input current

to prevent excessive input loading.

The LT3651 is available in a 5mm × 6mm 36-pin QFN

package.

DCIN (V)

EFFICIENCY (%)

POWER LOSS (W)

15 25

365142 TA01b

3.2

3.6

4.0

4.4

2.8

2.4

2.0

10 20 30

VBAT = 3.9

EFF

PLOSS

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and

PowerPath is a trademark of Linear Technology Corporation. All other trademarks are the

property of their respective owners.

TIMER

BOOST

SENSE

BAT

ILIM RNG/SS GND

NTC

CLP CLN

1µF

100µF BATTERY

365142 TA01a

6.5µH

WÜRTH 744314650

SYSTEM

LOAD

24mΩ

301k

DCIN

6.5V TO 32V

1N5819

LT3651-4.2

VIN

22µF

FAULT

CHRG

ACPR

SHDN

10V

Si7611DN

100k

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS

(Note 1)

11 12 13 14

TOP VIEW

UHE PACKAGE

36-LEAD (5mm × 6mm) PLASTIC QFN

15 16 17 18

36 35 34 33 32 31 30 29

1NTC

ACPR

BAT

SENSE

BOOST

GND

ILIM

SHDN

CHRG

FAULT

TIMER

GND

RNG/SS

VIN

GND

CLN

CLP

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-4.1#PBF LT3651EUHE-4.1#TRPBF 365141 36-Lead (5mm × 6mm) Plastic QFN –40°C to 125°C

LT3651IUHE-4.1#PBF LT3651IUHE-4.1#TRPBF 365141 36-Lead (5mm × 6mm) Plastic QFN –40°C to 125°C

LT3651EUHE-4.2#PBF LT3651EUHE-4.2#TRPBF 365142 36-Lead (5mm × 6mm) Plastic QFN –40°C to 125°C

LT3651IUHE-4.2#PBF LT3651IUHE-4.2#TRPBF 365142 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/

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

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

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 MAX UNITS

VIN Operating Range l6.5 32 V

VIN OVLO Threshold VIN Rising 32 35 40 V

VIN OVLO Hysteresis 1.1 V

VIN UVLO Threshold VIN Rising l4.6 4.8 V

VIN UVLO Hysteresis 0.2 V

Battery Float Voltage, VBAT(FLT) LT3651-4.1

4.08

4.05 4.1 4.12

4.14 V

LT3651-4.2

4.18

4.16 4.2 4.22

4.24 V

Battery Recharge Voltage Hysteresis Threshold Voltage Relative to VBAT(FLT) –105 mV

Battery Precondition Threshold Voltage,

VBAT(PRE)

LT3651-4.1 VBAT Rising

LT3651-4.2 VBAT Rising 2.85

2.9 V

Battery Precondition Threshold Hysteresis Threshold Voltage Relative to VBAT(PRE) 70 mV

Operating VIN Supply Current CC/CV Mode, Top Switch On, ISW = 0A

Standby Mode

Shutdown (SHDN = 0V)

8.6

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 = 0A, 2.5V < (VBOOST – VSW) < 4.5V 40 mA

BOOST Switch Drive IBOOST/ISW

, ISW = 4A 25 mA/A

Precondition Current Sense Voltage VSENSE – VBAT , VBAT = 2.5V 14 mV

Input Current Limit Voltage VCLP – VCLN, ILIM Open l70 95 115 mV

CLP Input Bias Current 120 nA

CLN Input Bias Current 36 µA

ILIM Bias Current l43 50 57 µA

System Current Limit Programming Gain VILIM/(VCLP – VCLN), VILIM = 0.5V 11.5 V/V

Maximum Charge Current Sense Voltage VSENSE – VBAT

, VBAT = 3.5V, VRNG/SS > 1.1V l88 95 103 mV

C/10 Trigger Sense Voltage VSENSE – VBAT l4.5 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

RNG/SS Bias Current l44 50 56 µA

Charge Current Limit Programming Gain VRNG/SS/(VSENSE – VBAT), VRNG/SS = 0.5V l8.5 10.8 12.5 V/V

NTC Range Limit (High) VNTC Rising l1.25 1.36 1.45 V

NTC Range Limit (Low) VNTC Falling l0.27 0.29 0.31 V

NTC Threshold Hysteresis % of Threshold 10 %

NTC Disable Impedance Minimum External Impedance to GND l150 470 kΩ

NTC Bias Current VNTC = 0.75V l46.5 50 53.5 µA

Shutdown Threshold VSHDN Rising l1.15 1.20 1.23 V

Shutdown Hysteresis 95 mV

SHDN Input Bias Current –10 nA

Status Low Voltage VCHRG, VFAULT

, VACPR, Load = 10mA l0.45 V

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

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 is tested under pulse loaded conditions such that

TJ = TA. The LT3651E is 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

LT3651I is 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.

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.

PARAMETER CONDITIONS MIN TYP MAX UNITS

TIMER Charge/Discharge Current 25 µA

TIMER Disable Threshold l0.1 0.25 V

Full Charge Cycle Time-Out 3 Hour

Precondition Timeout 22.5 Minute

Timer Accuracy l–13 13 %

Switcher Operating Frequency, fORT = 50kΩ

RT = 250kΩ 1.1

250 MHz

kHz

Minimum SW On-Time, tON(MIN) 150 ns

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.

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

TYPICAL PERFORMANCE CHARACTERISTICS

Battery Float Voltage

vs Temperature

VIN Standby Mode Current

vs Temperature

SENSE and BAT Pin Current

vs BAT Voltage, VSENSE = VBAT

Maximum Charge Current vs

VRNG/SS as a Percentage of

Programmed IMAX

IMAX Current Limit

(VSENSE – VBAT ) vs Temperature

Thermal Foldback–Charge Current

vs Temperature as a Percentage

of Programmed ICHG(MAX)

TEMPERATURE (°C)

–50 –25

∆VBAT(FLT) (%)

0.5

125

365142 G01

–0.5

–1.0 0100755025

1.0

TEMPERATURE (°C)

–50

IVIN (µA)

–25 025 50

365142 G02

100

VBAT (V)

–350

CURRENT (µA)

–300

–200

–150

–100

150

1.0 2.0 2.5 4.5

365142 G03

–250

100

–50

0.5 1.5 3.0 3.5 4.0

ISENSE

LT3651-4.2

IBAT

VRNG/SS (V)

ICHG(MAX) (%)

120

0.2 0.4 0.6 0.8

365142 G04

1.0 1.2

100

TEMPERATURE (°C)

–50

99.0

VSENSE – VBAT (mV)

99.5

100.0

100.5

101.0

–25 0 25 50

365142 G05

75 100 125

TEMPERATURE (°C)

100

ICHG(MAX) (%)

100

120

110 120 130 140

365142 G06

Maximum Input Current vs VILIM

as a Percentage of Programmed

IIN(MAX)

Input Current Limit Voltage

Threshold vs Temperature

VILIM (V)

IIN(MAX) (%)

120

0.2 0.4 0.6 0.8

365142 G07

1.0 1.2

100

TEMPERATURE (°C)

–50

∆(VCLP – VCLN) (mV)

1.5

365142 G08

–1.0

–25 0 50

–1.5

–2.0

2.0

1.0

0.5

–0.5

75 100 125

RILIM OPEN

RILIM = 10k

Charge Current vs VBAT as a

Percentage of Programmed

ICHG(MAX)

VBAT (V)

2.5

ICHG (%)

100

120

3.0 3.5 4.0 4.5

365142 G09

LT3651-4.2

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

TYPICAL PERFORMANCE CHARACTERISTICS

TEMPERATURE (°C)

–50

VSENSE – VBAT (mV)

–25 0 25 50

365142 G10

75 100 125

Switch Drive (IBST/ISW)

vs Temperature Boost Drive vs Boost Voltage

Boost Switch Drive

vs Switch Current

Oscillator Frequency

vs Temperature

Timer Resistor (RT)

vs Period and Frequency

C/10 Threshold (VSENSE –VBAT )

vs Temperature

Top Side Switch VON

vs Temperature Bottom Side VON vs Temperature

TEMPERATURE (˚C)

–50

VIN – VSW (mV)

600

650

700

25 75

365142 G11

550

500

–25 0 50 100 125

450

400

ISW = 4A

TEMPERATURE (°C)

–50

–250

VSW (mV)

–200

–150

–100

–50

–25 0 25 50

365142 G12

75 100 125

ISW = 4A

TEMPERATURE (°C)

–50

IBST/ISW (mA/A)

–25 0 25 50

365142 G13

75 100 125

ISW = 4A

VBST – VIN (V)

2.5

IBST/ISW (mA/A)

6.5

365142 G14

10 3.5 4.5 5.5 7.5

ISW = 4A

ISW (A)

IBST/ISW (mA/A)

1 2 3 4

365142 G15

TEMPERATURE (°C)

–50

–1.0

FREQUENCY DEVIATION (%)

–0.5

0.5

1.0

–25 0 25 50

365142 G16

75 100 125

RT = 54.9k

PERIOD (µs)

FREQUENCY (kHz)

1000

300

350

400

200

365142 G17

250

200

500 3

333 4

250 6

167

150

100

RT (kΩ)

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

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 therm-

istor, 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 moni-

tors 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 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 4): Charge Current Sense Pin. The charge

current is monitored with a sense resistor (RSENSE) con-

nected 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 2V to 4.5V, 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 this pin 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.

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.

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

PIN FUNCTIONS

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,

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.

Soft-start functionality for input current can be imple-

mented with a capacitor (CILIM) from ILIM to ground. The

soft-start capacitor and the programming resistor can be

implemented in parallel.

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-

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 servos 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. Charge current for the

battery flows into this pin. 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 em-

ploy 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 charge current can be imple-

mented 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.

RT (Pin 36): Switcher Oscillator Timer Set Pin. A resis-

tor from this pin to ground sets the switcher oscillator

frequency. Typically this is 54.9k for fOSC = 1MHz.

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

BLOCK DIAGRAM

–

ILIM

50µA

30 CLN

A12

A11

0.25V

A14

0.3V

A10

C-EA

125°C

STANDBY

ITH

A13

VIN

REV CUR

INHIBIT

OVLO

UVLO

STANDBY

OSC

29 CLP

36 RT

24 TIMER

26 CHRG

25 FAULT

1NTC

RESET

ENABLE MODE

(TIMER

OR C/10)

TERMINATE

SS/RESET

COUNT

STATUS

CONTROL LOGIC

COUNT

RESET

COUNT

RIPPLE COUNTER

TIMER OSC

LATCH

35V

4.6V

–

+–

–

0.1V

1V 0.15V

2.9V

C/10

NTC

PRECONDITION

–

V-EA

–

10RS

50µA

SS/RESET

+ +

4.2V 4.1V

2.4V

365142 BD

TERMINATE

1.2V

+ +

–

ACPR

–

1.36V

0.29V

0.7V

GND

VBAT(FLT): 4.1V FOR LT3651-4.1, 4.2V FOR LT3651-4.2

VBAT(FLT)-VRECHRG: 4.0V FOR LT3651-4.1, 4.1V FOR LT3651-4.2

VBAT(PRE): 2.85V FOR LT3651-4.1, 2.9V FOR LT3651-4.2

6, 23, 31, 37

1.3V

STANDBY

VINT

2.7V

×2.25

–

46µA

50µA

SHDN 27

RNG/SS 35

BAT 3

SENSE 4

BOOST 5

7, 11-18, 22, 38

VIN

32, 33, 34

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

OPERATION

Overview

The LT3651 is a complete Li-Ion battery charger, addressing

wide input voltage (up to 32V) 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 re-

sistor 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 con-

stant 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 de-

creases to one-tenth the programmed maximum charge

current (C/10 termination). Alternately, termination can

be time based through the use of an internal program-

mable 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 is not actively

charging, the IC automatically enters a low current standby

mode in which supply bias currents are reduced to 100µA.

If the battery voltage drops 2.5% from the full charge float

voltage, the LT3651 engages 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 contains 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 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.

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 uses an average current mode control loop

architecture to control average charge current. The LT3651

senses charger output voltage via the BAT pin. The dif-

ference between this voltage and the internal float volt-

age 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 im-

posed 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.

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

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

OPERATION

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 pre-

condition 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 transi-

tions 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 config-

ured 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 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 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 detec-

tion. 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 imped-

ance and the FAULT pin is pulled low.

When the LT3651 terminates 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.

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

APPLICATIONS INFORMATION

OSC Frequency

A precision resistor to ground sets the LT3651 switcher

oscillator frequency, fOSC, permitting user adjustability

of the frequency value. Typically this frequency is in the

200kHz to 1MHz range. Power consideration may neces-

sitate 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

fOSC MHz

( )

kΩ

( )

Set RT to 54.9k for 1MHz operation.

VIN Input Supply

The LT3651 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 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:

CIN(BULK) =

MAX

• V

BAT

fOSC MHz

( )

•∆VIN • VIN

µF

(

)

where ∆VIN is the input ripple, IMAX 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 8V VIN(MIN),

IMAX = 4A and a 1MHz oscillator frequency:

CIN(BULK) =

4 • 4.2

1• 0.1• 8

=21µF

The capacitor must have an adequate ripple current rating.

RMS ripple current, ICVIN(RMS) is approximated by:

ICVIN(RMS) ≈ICHARGE(MAX) •VBAT

VIN









•VIN

VBAT

– 1

which has a maximum at VIN = 2 • VBAT

, where ICVIN(RMS)

= ICHARGE(MAX)/2. In the example above that requires a

capacitor RMS rating of 2A.

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 con-

necting a 1µF capacitor from the BOOST pin to the SW

pin. Operating range of the BOOST pin is 2V to 4.5V

, as

referenced to the SW pin.

The boost capacitor is normally charged via a diode con-

nected 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).

If an external supply that is greater than the input is avail-

able (VBOOST – VIN > 2V), it may be used in place of the

bootstrap capacitor and diode.

VIN ,VBOOST Start-Up Requirement and Blocking

The LT3651 operates with a VIN range up to 32V. The

charger begins a charging cycle when the detected battery

voltage is below the 4.0V/4.1V auto-restart float voltage

and the part is enabled.

When VIN is below 6.3V and the BOOST capacitor is un-

charged, 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 in-

ductor. However in order to facilitate start-up, the LT3651

enables 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. Because of potential issues when operating at very

high duty cycles that often occur at start-up, it is highly

recommended that the part SHDN pin be used to enable

part start-up once VIN is above 6.3V.

To prevent battery discharge with a shorted input supply,

a blocking Schottky diode or FET is recommended in se-

ries with the input as shown in the Typical Applications.

Of course the voltage drops associated with the blocking

diode or FET as well as the input current sense resistor (if

used) and IR drops in the power path need to be accounted

for. Input currents increase at lower voltages because the

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

duty cycle increases. Adding a soft-start capacitor to the

RNG/SS pin may be helpful as well at start-up

BAT Output Decoupling

It is recommended that the LT3651 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:

CBAT ≈20µF +350µF

VIN(MIN)











The voltage rating on CBAT must meet or exceed the bat-

tery float voltage.

RSENSE: Charge Current Programming

The LT3651 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

IMAX(AVG)

where IMAX(AVG) is the maximum average charge current.

RSENSE is 24mΩ for a 4A charger.

Inductor Selection

The primary criteria for inductor value selection in the

LT3651 charger is the ripple current created during switch-

ing. 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=VBAT +VF

∆IMAX • fOSC MHz

( )

• 1– VBAT +VF

VIN +VF









µH

( )

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 IMAX + ∆IMAX/2.

Magnetics vendors typically specify inductors with maxi-

mum RMS and saturation current ratings. Select an induc-

tor that has a saturation current rating at or above peak

current, and an RMS rating above IMAX. 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

fOSC(MHz)

• 1– VBAT

VIN(MAX)













V •µs

( )

APPLICATIONS INFORMATION

Figure 2. Inductance (L) vs Maximum VIN

Figure 1. Programming Maximum Charge Current Using RSENSE

VIN(MAX) (V)

L (µH)

365142 F02

010 15 20 30

IMAX = 4A

fOSC = 1MHz

25% TO 35% RIPPLE

BOOST

SENSE

RSENSE

LT3651

BAT

365142 F01

LT3651-4.1/LT3651-4.2

365142ff

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System Input Current Limit

The LT3651 contains 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.

A resistor, RCL, is placed between the input supply and the

system and charger loads as shown in Figure 3.

The LT3651 sources 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:

IINPUT(MAX) =

ILIM

11.5 • RCL =

50µA • R

ILIM

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.

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 time 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.

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.

APPLICATIONS INFORMATION

Figure 3. Input Current Limit Configuration

SYSTEM LOAD CURRENT (A)

INPUT CURRENT

CHARGE

CURRENT

(VIN REFERRED)

2365142 F04

CURRENT (A)

Figure 4. Input Current Limit for 4A Maximum Charger

and 6A System Current Limit

CLP

CLN

SYSTEM LOAD

INPUT

SUPPLY

VIN

RILIM

RCL

LT3651

ILIM

365142 F03

LT3651-4.1/LT3651-4.2

365142ff

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Note the LT3651 internally integrates the input limit

signals. This should normally provide sufficient filtering

and reduce the sensitivity to current spikes. For the best

accuracy take care in provide good Kelvin connections

from RCL to CLP, CLN.

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 cor-

responds to approximately ten times the maximum volt-

age across the charge current sense resistor, RSENSE. The

defining equations for charge current are:

IMAX(RNG/SS) =

RNG/SS

10.8 • R

SENSE

50µA • R

RNG/SS

10.8 • R

SENSE

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 48mV 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 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 charg-

ing 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.

APPLICATIONS INFORMATION

LT3651

RNG/SS

10k

365142 F05

LOGIC HIGH = HALF CURRENT

Figure 5. Using the RNG/SS Pin for

Digital Control of Maximum Charge Current

LT3651

RNG/SS

SERVO

REFERENCE

365142 F06

–

Figure 6. Driving the RNG/SS Pin

with a Current-Sink Active Servo Amplifier

LT3651

RNG/SS

CRNG/SS

365142 F07

Figure 7. Using the RNG/SS Pin for Soft-Start

LT3651-4.1/LT3651-4.2

365142ff

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Status Pins

The LT3651 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 cur-

rent at greater than a C/10 rate, or one-tenth of the pro-

grammed 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

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)

C/10 Termination

The LT3651 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 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.

When C/10 termination is used, a LT3651 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 LT3651 enters 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 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 end-of-cycle (tEOC) occurs

based on CTIMER following the relation:

CTIMER =

EOC

(Hrs)

• 0.68 µF

(

)

so a typical 3 hour timer end-of-cycle would use a 0.68µF

capacitor.

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 terminates the charge cycle and enters

standby mode.

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 success-

ful 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 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

incorporates 2.5% of threshold for hysteresis to prevent

mode glitching.

APPLICATIONS INFORMATION

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APPLICATIONS INFORMATION

When the internal timer is used for termination, bad bat-

tery 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 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 pre-

condition 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. A new charge cycle is started by con-

necting another battery to the charger output.

Battery Temperature Fault: NTC

The LT3651 can accommodate battery temperature moni-

toring 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

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 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).

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 contains a thermal foldback protection fea-

ture 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 will suspend charging and enter standby mode

until the overtemperature condition is relieved.

Layout Considerations

The LT3651 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

and BAT traces together and keep the traces as short as

possible. Shielding these signals from switching noise

LT3651-4.1/LT3651-4.2

365142ff

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APPLICATIONS INFORMATION

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 compo-

nent 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).

Power Considerations

The LT3651 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.

Consideration should be given for power dissipation and

overall efficiency in a LT3651 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 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 3.6V, with 1MHz os-

cillator, 24mΩ sense resistor and 3.3µH/20mΩ inductor.

A 1% increase in efficiency represents a 0.2W 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 12.5mΩ 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 in-

creasing 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.

VIN

BOOST

365142 F08

CIN

CHARGE DISCHARGE

LT3651

CBOOST

RSENSE

CBAT BATTERY

Figure 8

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

TYPICAL APPLICATIONS

6.5V to 32V 4A Charger with High Voltage Current Foldback

SHDN

ACPR

FAULT

CHRG

BOOST

SENSE

BAT

ILIM RNG/SS GND

NTC

CLP

RCL

100Ω

CLN

1µF

CBAT

100µF

BATTERY

365142 TA02a

3.3µH

DCIN

SBM540

RSENSE

24mΩ

54.9k

SMAZ18

18V

200k

1N5819

LT3651

VIN

CIN

22µF

TIMER

Maximum Charge Current vs DCIN

DCIN (V)

MAXIMUM CHARGE CURRENT (A)

10 15 20 25

365142 TA02b

30 35

Maximum Charge Current vs DCIN

12V to 32V 4A Charger with Low Voltage Current Foldback

Using the RNG/SS Pin

SHDN

ACPR

FAULT

CHRG

BOOST

SENSE

BAT

ILIM RNG/SS GND

NTC

CLP

SBM540

CLN

1µF

5.1k

68k

SMAZ9V1

9.1V

CBAT

100µF BATTERY

365142 TA03a

3.3µH

SYSTEM

LOAD

RSENSE

24mΩ

54.9k

DCIN

1N5819LT3651

VIN

CIN

22µF

TIMER

DCIN (V)

MAXIMUM CHARGE CURRENT (A)

365142 TA03b

015 20 25 35

6.5V to 32V 4A Charger with Approximately Constant Input Power

SHDN

ACPR

FAULT

CHRG

BOOST

SENSE

BAT

ILIM RNG/SS GND

NTC

CLP

RCL

50mΩ

CLN

1µF

0.1µF 22k

180k

20k

8.2V

CBAT

100µF BATTERY

365142 TA05a

3.3µH

SYSTEM

LOAD

RSENSE

24mΩ

54.9k

DCIN

1N5819LT3651

VIN

CIN

22µF

6.2V RT

TIMER

SBM540

Input Power vs DCIN

DCIN (V)

INPUT POWER (W)

10 15 20 25

365142 TA05b

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

UHE Package

Variation: UHE36MA

36-Lead Plastic QFN (5mm × 6mm)

(Reference LTC DWG # 05-08-1753 Rev A)

5.00 ± 0.10

NOTE:

1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE

2. DRAWING NOT TO SCALE

3. ALL DIMENSIONS ARE IN MILLIMETERS

PIN 1

TOP MARK

(SEE NOTE 6)

BOTTOM VIEW—EXPOSED PAD

4.50 REF

6.00 ± 0.10

R = 0.125

TYP

0.25 ± 0.05

2.54 ± 0.10

3.45

± 0.10

(UHE36MA) QFN 0410 REV A

0.50 BSC

3.45

± 0.10

1.52 ± 0.10

2.54 ± 0.05 1.52

± 0.05

3.45 ± 0.05 3.45 ± 0.05

0.75 ± 0.05

0.00 – 0.05

0.200 REF

RECOMMENDED SOLDER PAD LAYOUT

APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

3.50 REF

0.40 ±0.10

0.70 ± 0.05

0.50 BSC

4.50 REF

3.50 REF

4.10 ± 0.05

5.50 ± 0.05

5.10 ± 0.05

6.50 ± 0.05

0.25 ± 0.05

PACKAGE

OUTLINE

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

PIN 1 NOTCH

R = 0.30 TYP

OR 0.35 × 45°

CHAMFER

R = 0.10

TYP

0.25 ± 0.05

0.76 ± 0.05

PACKAGE DESCRIPTION

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

REVISION HISTORY

REV DATE DESCRIPTION PAGE NUMBER

A 01/11 Revised Pin 34 to VIN in Pin Configuration, Pin Functions and Block Diagram 2, 6, 7, 8

B 03/11 Revised entire data sheet to add LT3651-4.1 1-22

C 10/12 Revised specification for Battery Float Voltage

Clarified Figure 3 Current Limit pin label

D 03/13 Clarified input voltage range

Clarified start-up functionality

1, 3, 10

E 04/13 Changed VIN to DCIN for Typical Application and curve

Modified VIN, VBOOST Start-Up section

Added LTC4000, removed LTC4007/8 from Related Parts section

12, 13

F 11/13 Change conditions and typical values

Change to 2V under Boost Pin 5 section

Modify A11 in Block Diagram

Overview section-change supply bias current to 100µA

Modify RT equation

Modify RNG/SS section

Modify Typical Applications circuits

19, 22

LT3651-4.1/LT3651-4.2

365142ff

For more information www.linear.com/LT3651-4.1

 LINEAR TECHNOLOGY CORPORATION 2010

LT 1113 REV F • PRINTED IN USA

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT3651-4.1

PART NUMBER DESCRIPTION COMMENTS

LT1511 3A Constant Current/Constant Voltage

Battery Charger High Efficiency, Minimum External Components to Fast Charge Lithium,

NIMH and NiCd Batteries, 24-Lead SO Package

LT1513 SEPIC Constant or Programmable

Current/Constant Voltage Battery Charger Charger Input Voltage May Be Higher, Equal to or Lower Than Battery Voltage,

500kHz Switching Frequency, DD-Pak and TO-220 Packages

LT3650 2A Monolithic Li-Ion Battery Charger 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

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

Battery Charger Standalone 9.0 ≤ VIN ≤ 32V, 40V Abs Max, 1MHz, Programmable Charge Current,

Timer or C/10 Termination, 5mm × 6mm QFN-36 Package

LT3652 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

LTC4000 High Voltage High Current Controller for

Battery Charging and Power Management Complete High Performance Battery Charger When Paired with a DC/DC Converter,

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

LTC4002 Standalone Li-Ion Switch Mode

Battery Charger Complete Charger for 1- or 2-Cell Li-Ion Batteries, Onboard Timer Termination,

Up to 4A Charge Current, 10-Lead DFN and SO-8 Packages

LTC4006 Small, High Efficiency, Fixed Voltage

Li-Ion Battery Charger with Termination Complete Charger for 2-, 3- or 4-Cell Li-Ion Batteries, AC Adapter Current Limit

and Thermistor Sensor, 16-Lead Narrow SSOP Package

LTC4009/LTC4009-1

LTC4009-2 High Efficiency, Multi-Chemistry

Battery Charger Complete Charger for 1- to 4-Cell Li-Ion Batteries or 4- to 18-Cell Nickel Batteries,

Up to 93% Efficiency, 20-Lead (4mm × 4mm) QFN Package, LTC4009-1 for 4.1V

Float Voltage, LTC4009-2 for 4.2V Float Voltage

LTC4012/LTC4012-1/

LTC4012-2/LTC4012-3 4A, High Efficiency, Multi-Chemistry

Battery Charger with PowerPath Control PowerPath Control, Constant-Current/Constant-Voltage Switching Regulator 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

RELATED PARTS

TYPICAL APPLICATION

6.5V to 32V 4A Charger with 3-Hour Charge Timeout, 6.3A Input

Current Limit, 10ms Soft-Start and Battery Temperature Monitoring

ACPR

FAULT

CHRG

TIMER

BOOST

SENSE

BAT

ILIM GND

RNG/SS

NTC

CLP

SHDN SW

RCL

16mΩ

CLN

1µF

CTIMER

0.68µF

CBAT

100µF

0.47µF NTC B

10k BATTERY

365142 TA04

3.3µH

SYSTEM

LOAD

RSENSE

24mΩ

54.9k

DCIN

VLOGIC

CONTROLLER 1N5819

LT3651

VIN

CIN

22µF

1µF

50k50k50k50k

SBM540