bq24735
SLUSAK9 –SEPTEMBER 2011
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Both CCM and DCM are synchronous operation with LSFET turn-on every clock cycle. If the average charge
current goes below 125mA on 10mΩ current sensing resistor or the battery voltage falls below 2.5V, the LSFET
keeps turn-off. The battery charger operates in non-synchronous mode and the current flows through the LSFET
body diode. During non-synchronous operation, the LSFET turns on only for a refreshing pulse to charge the
BTST capacitor. If the average charge current goes above 250mA on 10mΩ current sensing resistor, the LSFET
exits non-synchronous mode and enters synchronous mode to reduce LSFET power loss.
Input Over Current 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 3.33X of input current DAC set point (with 4.2ms blank out
time), ACFET/RBFET is latches off and an adapter removal and system shutdown is required to force ACDET <
0.6V to reset IC. After IC reset from latch off, ACFET/RBFET can be turned on again.
The ACOC function threshold can be set to 3.33x of input DPM current or disable this function via SMBus
command (ChargeOption() bit [1]).
Charge Over Current Protection (CHGOCP)
The bq24735 has a cycle-by-cycle peak over current protection. It 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 over current is detected, and resumes when the
next cycle starts.
The charge OCP threshold is automatically set to 6A, 9A, and 12A on a 10mΩ 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.
Battery Over Voltage 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 over 30ms, charger is completely disabled.
This allows quick response to an over-voltage condition – such as occurs when the load is removed or the
battery is disconnected. A 4mA 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. Set ChargeVoltage() register value
to 0V will not trigger BATOVP function.
Battery Shorted to Ground (BATLOWV)
The bq24735 will limit inductor current if the battery voltage on SRN falls below 2.5V. After 1ms charge is reset.
After 4-5 ms the charge is resumed with soft-start if all the enable conditions in the “Enable and Disable
Charging” sections are satisfied. This prevents any overshoot current in inductor which can saturate inductor and
may damage the MOSFET. The charge current is limited to 0.5A on 10mΩ current sensing resistor when
BATLOWV condition persists and LSFET keeps off. The LSFET turns on only for a refreshing pulse to charge
BTST capacitor.
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 shut down, the REGN LDO current limit is reduced to 16mA.
Once the temperature falls below 135°C, charge can be resumed with soft start.
EMI Switching Frequency Adjust
The charger switching frequency can be adjusted ±18% to solve EMI issue via SMBus command.
ChargeOption() bit [9]=0 disable the frequency adjust function. To enable frequency adjust function, set
ChargeOption() bit[9]=1. Set ChargeOption() bit [10]=0 to reduce switching frequency, set bit[10]=1 to increase
switching frequency.
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