LTC3556
18
3556f
drive an indicator LED through a current limiting resistor
for human interfacing or simply a pull-up resistor for
microprocessor interfacing.
To make the CHRG pin easily recognized by both humans
and microprocessors, the pin is either Low for charging,
High for not charging, or it is switched at high frequency
(35kHz) to indicate the two possible faults, unresponsive
battery and battery temperature out of range.
When charging begins, CHRG is pulled low and remains
low for the duration of a normal charge cycle. When charg-
ing is complete, i.e., the BAT pin reaches 4.200V and the
charge current has dropped to one tenth of the programmed
value, the CHRG pin is released (Hi-Z). If a fault occurs,
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.
The CHRG pin does not respond to the C/10 threshold if
the LTC3556 is in V
BUS
current limit. This prevents false
end of charge indications due to insufficient power avail-
able to the battery charger.
Table 1 illustrates the four possible states of the CHRG
pin when the battery charger is active.
Table 1. CHRG Signal
STATUS FREQUENCY
MODULATION
(BLINK) FREQUENCY DUTY CYCLES
Charging 0Hz 0Hz (Lo-Z) 100%
Not Charging 0Hz 0Hz (Hi-Z) 0%
NTC Fault 35kHz 1.5Hz at 50% 6.25%, 93.75%
Bad Battery 35kHz 6.1Hz at 50% 12.5%, 87.5%
An NTC fault is represented by a 35kHz pulse train whose
duty cycle alternates 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 battery fault.
Note that the LTC3556 is a 3-terminal PowerPath prod-
uct where 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 trickle
charge threshold voltage within the bad battery timeout
period. In this case, the battery charger will falsely indicate
a bad battery. 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.
NTC Thermistor
The battery temperature is measured by placing a nega-
tive temperature coefficient (NTC) thermistor close to the
battery pack.
To use this feature, connect the NTC thermistor, R
NTC
, be-
tween the NTC pin and ground and a resistor, R
NOM
, from
V
BUS
to the NTC pin. 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 LTC3556 and
will have to be budgeted for USB compliance.
The LTC3556 will pause charging when the resistance of
the NTC thermistor drops to 0.54 times the value of R25
or approximately 54k. For 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 LTC3556 is
also designed to pause charging when the value of the
NTC thermistor increases to 3.25 times the value of R25.
For Vishay “Curve 1” this resistance, 325k, corresponds
to approximately 0°C. The hot and cold comparators each
OPERATION
