LTC4098-1 datasheet(19/32 Pages) LINER

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LTC4098/LTC4098-1

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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 charge

current drops to 1/10th the value programmed by RPROG,

the CHRG pin is released (Hi-Z). The CHRG pin does not

respond to the C/10 threshold if the LTC4098/LTC4098-1

are in VBUS 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 giving 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%

IBAT < C/10

0Hz

0Hz (Hi-Z)

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 human

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 LTC4098/LTC4098-1 are 3-terminal PowerPath

products, 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. In

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.

OPERATION

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