LTC3566 Datasheet, PDF (15/28 Pages) Linear Technology – High Effi ciency USB Power Manager Plus 1A Buck-Boost Converter

LTC3566
OPERATION
Charge Termination
The battery charger has a built-in safety timer. When
the voltage on the battery reaches the pre-programmed
float voltage of 4.200V, the battery charger will regulate
the battery voltage and the charge current will decrease
naturally. Once the battery charger detects that the battery
has reached 4.200V, the four hour safety timer is started.
After the safety timer expires, charging of the battery will
discontinue and no more current will be delivered.
Automatic Recharge
After the battery charger terminates, it will remain off
drawing only microamperes of current from the battery.
If the portable product remains in this state long enough,
the battery will eventually self discharge. To ensure that
the battery is always topped off, a charge cycle will au-
tomatically begin when the battery voltage falls below
4.1V. In the event that the safety timer is running when
the battery voltage falls below 4.1V, it will reset back to
zero. To prevent brief excursions below 4.1V from reset-
ting the safety timer, the battery voltage must be below
4.1V for more than 1.3ms. The charge cycle and safety
timer will also restart if the V_BUS UVLO cycles low and
then high (e.g. V_BUS, is removed and then replaced) or if
the battery charger is cycled on and off by the CHRGEN
digital I/O pin.
Charge Current
The charge current is programmed using a single resis-
tor from PROG to ground. 1/1022^th of the battery charge
current is sent to PROG which will attempt to servo to
1.000V. Thus, the battery charge current will try to reach
1022 times the current in the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
R_PROG
=
1022V
I_CHG
, I_CHG
=
1022V
R_PROG
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
=
V_PROG
R_PROG
•
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
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 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
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