BQ24735_17 Datasheet, PDF (17/46 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
The bq24735 has three loops of regulation: input current, charge current and charge voltage. The three loops are
brought together internally at the error amplifier. The maximum voltage of the three loops appears at the output
of the error amplifier EAO. An internal saw-tooth ramp is compared to the internal error control signal EAO (see
Functional Block Diagram) to vary the duty-cycle of the converter. The ramp has offset of 200 mV in order to
allow 0% duty-cycle.
When the battery charge voltage approaches the input voltage, EAO signal is allowed to exceed the saw-tooth
ramp peak in order to get a 100% duty-cycle. If voltage across BTST and PHASE pins falls below 4.3 V, a
refresh cycle starts and low-side N-channel power MOSFET is turned on to recharge the BTST capacitor. It can
achieve duty cycle of up to 99.5%.
9.3.6 Input Overcurrent Protection (ACOC)
The bq24735 cannot maintain the input current level if the charge current has been already reduced to zero.
After the system current continues increasing to the 3.33× of input current DAC set point (with 4.2-ms blank-out
time), ACFET/RBFET is latches off and an adapter removal and system shutdown is required to force ACDET <
0.6 V to reset IC. After IC reset from latch off, ACFET/RBFET can be turned on again.
The ACOC function threshold can be set to 3.33× of input DPM current or disable this function through SMBus
command (ChargeOption() bit [1]).
9.3.7 Charge Overcurrent Protection (CHGOCP)
The bq24735 has a cycle-by-cycle peak overcurrent protection. The device monitors the voltage across SRP and
SRN, and prevents the current from exceeding of the threshold based on the DAC charge current set point. The
high-side gate drive turns off for the rest of the cycle when the overcurrent is detected, and resumes when the
next cycle starts.
The charge OCP threshold is automatically set to 6 A, 9 A, and 12 A on a 10-mΩ current-sensing resistor based
on charge current register value. This prevents the threshold to be too high which is not safe or too low which
can be triggered in normal operation. Proper inductance should be selected to prevent OCP triggered in normal
operation due to high inductor current ripple.
9.3.8 Battery Overvoltage Protection (BATOVP)
The bq24735 will not allow the high-side and low-side MOSFET to turn on when the battery voltage at SRN
exceeds 104% of the regulation voltage set-point. If BATOVP last more than 30 ms, the charger is completely
disabled. This allows quick response to an overvoltage condition – such as occurs when the load is removed or
the battery is disconnected. A 4-mA current sink from SRP to GND is on only during BATOVP and allows
discharging the stored output inductor energy that is transferred to the output capacitors. Setting ChargeVoltage()
register value to 0 V will not trigger BATOVP function.
9.3.9 Battery Shorted to Ground (BATLOWV)
The bq24735 will limit inductor current if the battery voltage on SRN falls below 2.5 V after 1-ms charge is reset.
After 4-5 ms, the charge is resumed with soft start if all the enable conditions in Enable and Disable Charging are
satisfied. This prevents any overshoot current in inductor which can saturate inductor and may damage the
MOSFET. The charge current is limited to 0.5 A on 10-mΩ current-sensing resistor when BATLOWV condition
persists and LSFET remains off. The LSFET turns on only for a refreshing pulse to charge the BTST capacitor.
9.3.10 Thermal Shutdown Protection (TSHUT)
The QFN package has low thermal impedance, which provides good thermal conduction from the silicon to the
ambient, to keep junctions temperatures low. As added level of protection, the charger converter turns off for self-
protection whenever the junction temperature exceeds the 155°C. The charger stays off until the junction
temperature falls below 135°C. During thermal shutdown, the REGN LDO current limit is reduced to 16 mA.
Once the temperature falls below 135°C, charge can be resumed with soft start.
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