LTC4098/LTC4098-1
19
40981fc
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 R
PROG
,
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 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 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%
I
BAT
< C/10 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 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
