BQ24735_17 Datasheet, PDF (21/46 Pages) Texas Instruments – 1- to 4-Cell Li+ Battery SMBus Charge Controller for Supporting Turbo Boost Mode With N-Channel Power MOSFET Selector

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bq24735
SLUSAK9B – SEPTEMBER 2011 – REVISED APRIL 2015
Programming (continued)
9.5.2 Battery LEARN Cycle
A battery LEARN cycle can be activated through SMBus command (ChargeOption() bit [6] = 1 enable LEARN
cycle, bit [6] = 0 disable LEARN cycle). When LEARN is enabled with ACFET/RBFET connected, the system
power selector logic is overdriven to switch to battery by turning off ACFET/RBFET and turning on BATFET.
LEARN function allows the battery to discharge in order to calibrate the battery gas gauge over a complete
discharge/charge cycle. The controller automatically exits LEARN cycle when the battery voltage is below battery
depletion threshold, and the system switches back to adapter input by turning off BATFET and turning on
ACFET/RBFET. After LEARN cycle, the LEARN bit is automatically reset to 0. The battery depletion threshold
can be set to 59.19%, 62.65%, 66.55%, and 70.97% of voltage regulation level through SMBus command
(ChargeOption() bit [12:11]).
9.5.3 Charge Time-out
The bq24735 includes a watchdog timer to terminate charging if the charger does not receive a write
ChargeVoltage() or write ChargeCurrent() command within 175 s (adjustable through ChargeOption() command).
If a watchdog time-out occurs all register values keep unchanged but charge is suspended. Write
ChargeVoltage() or write ChargeCurrent() commands must be resent to reset watchdog timer and resume
charging. The watchdog timer can be disabled, or set to 44 s, 88 s or 175 s through SMBus command
(ChargeOption() bit [14:13]). After watchdog time-out write ChargeOption() bit [14:13] to disable watchdog timer
also resume charging.
9.5.4 High-Accuracy Current-Sense Amplifier
As an industry standard, high-accuracy current-sense amplifier (CSA) is used to monitor the input current or the
charge current, selectable through SMBUS (ChargeOption() bit [5] = 0 select the input current, bit [5] = 1 select
the charge current) by host. The CSA senses voltage across the sense resistor by a factor of 20 through the
IOUT pin. Once VCC is above UVLO and ACDET is above 0.6 V, CSA turns on and IOUT output becomes valid.
To lower the voltage on current monitoring, a resistor divider from IOUT to GND can be used and accuracy over
temperature can still be achieved.
A 100-pF capacitor connected on the output is recommended for decoupling high-frequency noise. An additional
RC filter is optional, if additional filtering is desired.
NOTE
Adding filtering also increases response delay.
9.5.5 EMI Switching Frequency Adjust
The charger switching frequency can be adjusted ±18% to solve the EMI issue through SMBus command.
ChargeOption() bit [9] = 0 disables the frequency adjust function. To enable frequency adjust function, set
ChargeOption() bit [9] = 1. Set ChargeOption() bit [10] = 0 to reduce switching frequency, and set bit [10] = 1 to
increase switching frequency.
If frequency is reduced for a fixed inductor, the current ripple is increased. Inductor value must be carefully
selected so that it will not trigger cycle-by-cycle peak overcurrent protection, even for the worst conditions such
as higher input voltage, 50% duty cycle, lower inductance, and lower switching frequency.
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