LTC3586/LTC3586-1
18
3586fb
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
charging is complete, i.e., the BAT pin reaches the float
voltage 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 LTC3586/LTC3586-1 are in V
BUS
current limit. This
prevents false end-of-charge indications due to insufficient
power available to the battery charger.
Table 3 illustrates the four possible states of the CHRG
pin when the battery charger is active.
Table 3. 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% to 93.75%
Bad Battery 35kHz 6.1Hz at 50% 12.5% to 87.5%
An NTC fault is represented by a 35kHz pulse train whose
duty cycle varies 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 LTC3586/LTC3586-1 are 3-terminal
PowerPath products where system load is always pri-
oritized 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
,
between the NTC pin and ground and a resistor, R
NOM
,
from V
BUS
to the NTC pin. R
NOM
should be a 1% resis-
tor with a value equal to the value of the chosen NTC
thermistor at 25°C (R25). A 100k thermistor is recom-
mended since thermistor current is not measured by the
LTC3586/LTC3586-1 and will have to be budgeted for USB
compliance.
The LTC3586/LTC3586-1 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
batter
y 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 LTC3586/
LTC3586-1 are 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 have approximately 3°C of hysteresis
to prevent oscillation about the trip point. Grounding the
NTC pin disables the NTC charge pausing function.
operaTion
