ltc3557-ltc3557-1 Linear Technology Corporation, ltc3557-ltc3557-1 Datasheet - Page 16

• Manufacturer Part Number: ltc3557-ltc3557-1
• Description: Usb Power Manager With Li-ion Charger And Three Step-down Regulators
• Manufacturer: Linear Technology Corporation

LTC3557/LTC3557-1
or high ambient thermal conditions and allows the user
to push the limits of the power handling capability with a
given circuit board design without risk of damaging the
LTC3557/LTC3557-1 or external components. The benefi t
of the LTC3557/LTC3557-1 thermal regulation loop is that
charge current can be set according to actual conditions
rather than worst-case conditions with the assurance that
the battery charger will automatically reduce the current
in worst-case conditions.
Charge Status Indication
The CHRG pin indicates the status of the battery charger.
Four possible states are represented by CHRG which
include 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 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, 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 charger enters constant voltage
mode and the charge current has dropped to one-tenth
of the programmed value, the CHRG pin is released (high
impedance). The CHRG pin does not respond to the C/10
threshold if the LTC3557/LTC3557-1 is in input current
limit. This prevents false end of charge indications due
to insuffi cient power available to the battery charger. 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
OPERATION
16
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 Output Pin
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 V
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 LTC3557/LTC3557-1 is
a 3-terminal PowerPath product where system load is
always prioritized over battery charging. Due to excessive
system load, there may not be suffi cient 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.
Bad Battery
I
BAT
NTC Fault
Charging
STATUS
< C/10
FREQUENCY
35kHz
35kHz
0Hz
0Hz
TRKL
MODULATION
1.5Hz at 50%
6.1Hz at 50%
FREQUENCY
, typically 2.8V, for 1/2 hour),
0Hz (Lo-Z)
0Hz (Hi-Z)
(BLINK)
6.25% or 93.75%
12.5% or 87.5%
DUTY CYCLE
100%
0%
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