LTC3555/LTC3555-X
19
3555fe
For more information www.linear.com/LTC3555
1022 times the current in the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
R
PROG
=
I
, I
CHG
=
R
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. There-
fore, the actual charge current can be determined at any
time by monitoring the PROG pin voltage and using the
following equation:
I
BAT
=
PROG
R
• 1022
In many cases, the actual battery charge current, I
BAT
, will
be lower than I
CHG
due to limited input power available
and prioritization with 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 in-
clude charging, not charging, 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.
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 low
and high 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 LTC3555 family is in V
BUS
current limit. This prevents
false end-of-charge indications due to insufficient power
available 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% 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 LTC3555 family is a three terminal PowerPath
product 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
OPERATION
