LTC4088EDE-2#TRPBF Datasheet LTC4088EDE-1#PBF, LTC4088EDE-2#PBF, LTC4088EDE-1#TRPBF | P13-P15

LTC4088-1/LTC4088-2

40881fc

operaTion

When a battery charge cycle begins, the battery charger

first determines if the battery is deeply discharged. If the

battery voltage is below V

TRKL

, typically 2.85V, an automatic

trickle charge feature sets the battery charge current to

10% of the programmed value. If the low voltage persists

for more than 1/2 hour, the battery charger automatically

terminates and indicates, via the CHRG pin, that the battery

was unresponsive.

Once the battery voltage is above V

TRKL

, the charger be-

gins charging in full power constant-current mode. The

current delivered to the battery will try to reach 1031V/

PROG

. Depending on available input power and external

load conditions, the battery charger may or may not be

able to charge at the full programmed rate. The external

load will always be prioritized over the battery charge

current. The USB current limit programming will always

be observed and only additional power will be available to

charge the battery. When system loads are light, battery

charge current will be maximized.

Charge Termination

The battery charger has a built-in safety timer. Once the

voltage on the battery reaches the pre-programmed float

voltage of 4.200V, the charger will regulate the battery

voltage there

and the charge current will decrease naturally.

Once the charger detects that the battery has reached

4.200V, the 4-hour safety timer is started. After the safety

timer expires, charging of the battery will discontinue and

no more current will be delivered.

Automatic Recharge

Once the battery charger terminates, it will remain off

drawing only microamperes of current from the battery.

If the portable product remains in this state long enough,

the battery will eventually self discharge. To ensure that the

battery is always topped off, a charge cycle will automati-

cally begin when the battery voltage falls below V

RECHRG

(typically 4.1V). In the event that the safety timer is running

when the battery voltage falls below V

RECHRG

, it will reset

back to zero. To prevent brief excursions below V

RECHRG

from resetting the safety timer, the battery voltage must be

below V

RECHRG

for more than 1.5ms. The charge cycle and

safety timer will also restart if the V

BUS

UVLO cycles low

and then high (e.g., V

BUS

is removed and then replaced)

or if the charger is momentarily disabled using the D2 pin.

Charge Current

The charge current is programmed using a single resistor

from PROG to ground. 1/1031

th of the battery charge cur-

rent

is delivered to PROG, which will attempt to servo to

1.000V. Thus, the battery charge current will try to reach

1031 times the current in the PROG pin. The program

resistor and the charge current are calculated using the

following equations:

PROG

1031V

CHG

, I

CHG

1031V

PROG

In either the constant-current or constant-voltage charging

modes, the voltage at the PROG pin will be proportional

to the actual charge current delivered to the battery. The

charge current can be determined at any time by monitoring

the PROG pin voltage and using the following equation:

BAT

PROG

• 1031

In many cases, the actual battery charge current, I

BAT

will be lower than the programmed current, I

CHG

, due

to limited input power available and prioritization to the

system load drawn from V

OUT

Charge Status Indication

The CHRG pin indicates the status of the battery charger.

Four possible states are represented by CHRG which

include charging, not charging (or float charge current

less than programmed end of charge indication current),

unresponsive battery and battery temperature out of range.

The signal at the CHRG pin can be easily recognized as

one of the above four states by either a human or a mi-

croprocessor. An open-drain output, the CHRG pin can

drive an indicator LED through a current limiting resistor

for human interfacing or simply a pull-up resistor for

microprocessor interfacing.

LTC4088-1/LTC4088-2

40881fc

operaTion

To make the CHRG pin easily recognized by both humans

and microprocessors, the pin is either a DC signal of ON

for charging, OFF for not charging or it is switched at high

frequency (35kHz) to indicate the two possible faults. While

switching at 35kHz, its duty cycle is modulated at a slow

rate that can be recognized by a human.

When charging begins, CHRG is pulled low and remains

low for the duration of a normal charge cycle. When charg-

ing is complete, as determined by the criteria set by the

C/X pin, the CHRG pin is released (Hi-Z). The CHRG pin

does not respond to the C/X threshold if the LTC4088-1/

LTC4088-2 is in V

BUS

current limit. This prevents false end

of charge indications due to insufficient power available to

the battery charger. If a fault occurs while charging, the

pin is switched at 35kHz. While switching, its duty cycle

is modulated between a high and low value at a very low

frequency. The low and high duty cycles are disparate

enough to make an LED appear to be on or off thus giv-

ing the appearance of “blinking”. Each of the two

faults

has

its own unique “blink” rate for human recognition as

well as two unique duty cycles for machine recognition.

Table 2 illustrates the four possible states of the CHRG

pin when the battery charger is active.

Table 2. CHRG Signal

STATUS

FREQUENCY

MODULATION

(BLINK) FREQUENCY

DUTY

CYCLES

Charging 0Hz 0Hz (Low Z) 100%

BAT

< C/X 0Hz 0Hz (Hi-Z) 0%

NTC Fault 35kHz 1.5Hz at 50% 6.25% or 93.75%

Bad Battery 35kHz 6.1Hz at 50% 12.5% or 87.5%

Notice that an NTC fault is represented by a 35kHz pulse

train whose duty cycle toggles between 6.25% and 93.75%

at a 1.5Hz rate. A human will easily recognize the 1.5Hz

rate as a “slow” blinking which indicates the out of range

battery temperature while a microprocessor will be able

to decode either the 6.25% or 93.75% duty cycles as an

NTC fault.

If a battery is found to be unresponsive to charging (i.e.,

its voltage remains below 2.85V for 1/2 hour), the CHRG

pin gives the battery fault indication. For this fault, a hu-

man would easily recognize the frantic 6.1Hz “fast” blink of

the LED while a microprocessor would be able to decode

either the 12.5% or 87.5% duty cycles as a bad cell fault.

Because the LTC4088-1/LTC4088-2 is a 3-terminal Power-

Path product, system load is always prioritized over battery

charging. Due to excessive system load, there may not

be sufficient power to charge the battery beyond the bad

cell threshold voltage within the bad

cell timeout period.

this case the battery charger will falsely indicate a bad

cell. System software may then reduce the load and reset

the battery charger to try again.

Although very improbable, it is possible that a duty cycle

reading could be taken at the bright-dim transition (low

duty cycle to high duty cycle). When this happens the

duty cycle reading will be precisely 50%. If the duty cycle

reading is 50%, system software should disqualify it and

take a new duty cycle reading.

C/X Determination

The current exiting the C/X pin represents 1/1031th of

the battery charge current. With a resistor from C/X to

ground that is X/10 times the resistor at the PROG pin,

the CHRG pin releases when the battery current drops to

C/X. For example, if C/10 detection is desired, R

C/X

should

be made equal to R

PROG

. For C/20, R

C/X

would be twice

PROG

. The current threshold at which CHRG will change

state is given by:

BAT

C/X

• 103

With this design, C/10 detection can be achieved with only

one resistor rather than a resistor for both the C/X pin and

the PROG pin. Since both of these pins have 1/1031 of

the battery charge current in them, their voltages will be

equal when they have the same resistor value. Therefore,

rather than using two resistors, the C/X pin and the PROG

pin can be connected together and the resistors can be

paralleled to a single resistor of 1/2 of the program resistor.

LTC4088-1/LTC4088-2

40881fc

operaTion

NTC Thermistor

The battery temperature is measured by placing a negative

temperature coefficient (NTC) thermistor close to the bat-

tery pack. The NTC circuitry is shown in the Block Diagram.

To use this feature, connect the NTC thermistor, R

NTC

between the NTC pin and ground and a bias resistor, R

NOM

from V

BUS

to NTC. R

NOM

should be a 1% resistor with

a value equal to the value of the chosen NTC thermistor

at 25°C (R25). A 100k thermistor is recommended since

thermistor current is not measured by the LTC4088-1/

LTC4088-2 and will have to be considered for USB com-

pliance.

The LTC4088-1/LTC4088-2 will pause charging when the

resistance of the NTC thermistor drops to 0.54 times the

value of R25 or approximately 54k (for a Vishay “Curve

1” thermistor, this corresponds to approximately 40°C).

If the battery charger is in constant voltage (float) mode,

the safety timer also pauses until the thermistor indicates

a return to a valid temperature. As the temperature drops,

the resistance of the NTC thermistor rises. The LTC4088-1/

LTC4088-2 is also designed to pause charging when the

value of the NTC thermistor increases to 3.25 times the

value of R25. For a Vishay

“Curve 1” thermistor, this

resistance

, 325k, corresponds to approximately 0°C. The

hot and cold comparators each have approximately 3°C

of hysteresis to prevent oscillation about the trip point.

Grounding the NTC pin disables all NTC functionality.

Thermal Regulation

To prevent thermal damage to the IC or surrounding

components, an internal thermal feedback loop will

automatically decrease the programmed charge current

if the die temperature rises to approximately 110°C.

Thermal regulation protects the LTC4088-1/LTC4088-2

from excessive temperature due to high power operation

or high ambient thermal conditions, and allows the user

to push the limits of the power handling capability with

a given circuit board design without risk of damaging

the LTC4088-1/LTC4088-2 or external components. The

benefit of the LTC4088-1/LTC4088-2 thermal regulation

loop is that charge current can be set according to actual

conditions rather than worst-case conditions for a given

application with the assurance that the charger will au-

tomatically reduce the current in worst-case conditions.

Shutdown Mode

The input switching regulator is enabled whenever V

BUS

is above the UVLO voltage and the LTC4088-1/LTC4088-

2 is not in one of the two USB suspend modes (500µA

or 2.5mA).

The ideal diode is

enabled at all times and cannot be

disabled.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24

LTC4088EDE-2#TRPBF

Mfr. #:

Buy LTC4088EDE-2#TRPBF

Manufacturer:

Analog Devices / Linear Technology

Description:

Battery Management Hi Eff Bat Chr/USB Pwr Manager w/ Reg Ou

Lifecycle:

New from this manufacturer.

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LTC4088EDE-2#TRPBF

LTC4088EDE-2#TRPBF

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