LT3651EUHE-4.1#PBF Datasheet LT3651EUHE-4.2#PBF, LT3651EUHE-4.1#PBF, LT3651IUHE-4.1#PBF | P16-P18

LT3651-4.1/LT3651-4.2

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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 V

, 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 batter

y 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 R

SENSE

The C/10 threshold current corresponds to 9mV across

SENSE

. 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 V

BAT

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 (C

TIMER

) is connected from the TIMER

pin to ground. The timer cycle end-of-cycle (t

EOC

) occurs

based on C

TIMER

following the relation:

TIMER

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

BAT

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 I

MAX

as set by R

SENSE

. The precondition current corresponds

to 14mV across R

SENSE

Precondition mode is engaged while the voltage on the BAT

pin is below the precondition threshold (V

BAT(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.

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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 BA

T 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 T

emperature Fault: NTC

The L

T3651 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

capacitor (C

) 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

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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 (C

)

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 (C

BAT

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.

is also possible to lower the sense resistance (with a

reduction in R

RNG/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 V

there is a 1% improvement in efficiency for every 200kHz

reduction in frequency (100kHz to 1MHz); At 28V V

, 1%

for every 100kHz.

Of course all of these must be experimentally confirmed

in the actual charger.

BOOST

365142 F08

CHARGE

DISCHARGE

LT3651

BOOST

SENSE

BAT

BATTERY

Figure 8

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LT3651EUHE-4.1#PBF

Mfr. #:

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Manufacturer:

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Description:

Battery Management Monolithic 4A High Voltage Li-Ion Battery Charger

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