LTC3576-1_15 Datasheet, PDF (24/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go Triple Step-Down DC/DCs

LTC3576/LTC3576-1
OPERATION
Likewise, the USB current limit programming will always
be observed and only additional power will be available to
charge the battery. When system loads are light, battery
charge current will be maximized.
Charge Termination
The battery charger has a built-in safety timer. When the
voltage on the battery reaches the pre-programmed float
voltage, the battery charger will regulate the battery volt-
age and the charge current will decrease naturally. Once
the battery charger detects that the battery has reached
the float voltage, 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 automatically
begin when the battery voltage falls below the recharge
threshold which is typically 100mV less than the charger’s
float voltage. In the event that the safety timer is running
when the battery voltage falls below the recharge threshold,
it will reset back to zero. To prevent brief excursions below
the recharge threshold from resetting the safety timer, the
battery voltage must be below the recharge threshold for
more than 1ms. 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 I²C port.
Charge Current
The charge current is programmed using a single resis-
tor from PROG to ground. 1/1030th 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
1030 times the current in the PROG pin. The program
resistor and the charge current are calculated using the
following equation:
I_CHG
=
V_PROG
R_PROG
• 1030
24
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
•1030
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.
The Battery Charger Flow Chart illustrates the battery
charger’s algorithm.
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
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”.
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