Datasheet

Not Recommended for New Designs
bq24725
www.ti.com SLUS702A –JULY 2010–REVISED NOVEMBER 2010
Battery Shorted to Ground (BATLOWV)
The bq24725 will disable charge for 1ms if the battery voltage on SRN falls below 2.5V. After 1ms reset, 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 the 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.
If frequency is reduced, for a fixed inductor the current ripple is increased. The inductor value must be carefully
selected, so that it will not trigger a cycle-by-cycle peak over current protection even for the worst condition such
as higher input voltage, 50% duty cycle, lower inductance and lower switching frequency.
Inductor Short, MOSFET Short Protection
The bq24725 has a unique short circuit protection feature. Its cycle-by-cycle current monitoring feature is
achieved through monitoring the voltage drop across R
DS(on)
of the MOSFETs after a certain amount of blanking
time. In case of MOSFET short or inductor short circuit, the over current condition is sensed by two comparators
and two counters will be triggered. After seven times of short circuit events, the charger will be latched off and
ACFET and RBFET is turned off to disconnect adapter from system. BATFET is turned on to connect battery
pack to system. To reset the charger from latch-off status, the IC VCC pin must be pulled below UVLO or the
ACDET pin must be pulled below 0.6V. This can be achieved by removing the adapter and shut down the
operation system . The low side MOSFET short circuit voltage drop threshold is fixed to typical 110mV. The high
side MOSFET short circuit voltage drop threshold can be adjusted via SMBus command. ChargeOption() bit[8:7]
= 00, 01, 10, 11 set the threshold 300mV, 500mV, 700mV and 900mV respectively.
Due to the certain amount of blanking time to prevent noise when MOSFET just turns on, the cycle-by-cycle
charge over current protection may detect high current and turn off MOSFET first before the short circuit
protection circuit can detect short condition because the blanking time has not finished. In such a case the
charge may not be able to detect shorts circuit and counter may not be able to count to seven then latch off.
Instead the charge may continuously keep switching with very narrow duty cycle to limit the cycle-by-cycle
current peak value. However, the charger should still be safe and will not cause failure because the duty cycle is
limited to a very short of time and MOSFET should be still inside the safety operation area. During a soft start
period, it may takes long time instead of just seven switching cycles to detect short circuit based on the same
blanking time reason.
Table 8. Component List for Typical System Circuit of Figure 1
PART DESIGNATOR QTY DESCRIPTION
C1, C2, C3, C13, C14, C16 6 Capacitor, Ceramic, 0.1µF, 25V, 10%, X7R, 0603
C4 1 Capacitor, Ceramic, 100pF, 25V, 10%, X7R, 0603
C5, C6 2 Capacitor, Ceramic, 1µF, 25V, 10%, X7R, 0603
C7 1 Capacitor, Ceramic, 0.047µF, 25V, 10%, X7R, 0603
C8, C9, C10, C11 4 Capacitor, Ceramic, 10µF, 25V, 10%, X7R, 1206
C15 1 Capacitor, Ceramic, 0.01µF, 25V, 10%, X7R, 0603
C17 1 Capacitor, Ceramic, 2200pF, 25V, 10%, X7R, 0603
Ci 1 Capacitor, Ceramic, 2.2µF, 25V, 10%, X7R, 1210
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