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Battery Shorted (Battery Undervoltage) Protection

Charge Overcurrent Protection

Thermal Shutdown Protection

Adapter Detected Status Register (ACGOOD Pin)

Input Over-Power Protection (ACOP)

bq24751A

SLUS756 – SEPTEMBER 2007

The bq24751A has a BAT SHORT comparator monitoring the output battery voltage (BAT). If the voltage falls

below 2.9 V per cell (5.8 V for 2 cells, 8.7 V for 3 cells, 11.6 V for 4 cells), a battery-short status is detected.

Below the BAT_SHORT threshold, the charger reduces the charge current to 1/8th of the programmed charging

current (0.1

×SRSET/VDAC)/8 = C/8 down to zero volts on BAT pin.. This lower current is used as a pre-charge

current for over-discharged battery packs. Above the BAT_SHORT threshold (plus hysteresis, the charge current

resumes at the programmed value (0.1

×SRSET/VDAC).

The BAT_SHORT comparator also serves as a depleted-battery alarm during a LEARN cycle. If the selector is in

a LEARN cycle, and the battery voltage falls bellow the BAT_SHORT threshold, the selector disconnects the

battery from the system and connects the adapter to the system in order to protect the battery pack. If battery

voltage increases, and LEARN is still logic high, then the selector disconnects the adapter from the system and

reconnects the battery to the system.

The charger has a secondary overcurrent protection feature. It monitors the charge current, and prevents the

current from exceeding 145% of regulated charge current. The high-side gate drive turns off when the

overcurrent is detected, and automatically resumes when the current falls below the overcurrent threshold.

The QFN package has low thermal impedance, which provides good thermal conduction from the silicon to the

ambient, to keep junction temperatures low. As an added level of protection, the charger converter turns off and

self-protects when the junction temperature exceeds the TSHUT threshold of 155

°C. The charger stays off until

the junction temperature falls below 135

°C.

One status output is available, and it requires an external pullup resistor to pull the pin to the system digital rail

for a high level.

ACGOOD goes low when ACDET is above 2.4 V and the 500-ms delay time is over. It indicates that the adapter

voltage is high enough for normal operation.

The ACOC/ACOP circuit provides a reliable layer of safety protection that can complement other safety

measues. ACOC/ACOP helps to protect from input current surge due to various conditions including:

• Adapter insertion and system selector connecting adapter to system where system capacitors need to charge

• Learn mode exit when adapter is reconnected to the system; system load over-current surge

• System shorted to ground

• Battery shorted to ground

• Phase shorted to ground

• High-side FET shorted from drain to source (SYSTEM shorted to PH)

• BATFET shorted from drain to source (SYSTEM shorted to BAT)

Several examples of the circuit protecting from these fault conditions are shown below.

For designs using the selector functions, an input overcurrent (ACOC) and input over-power protection function

(ACOP) is provided. The threshold is set by an external capacitor from the ACOP pin to AGND. After the adapter

is detected (ACDET pin > 2.4V), there is a 700-ms delay before ACGOOD is asserted low, and Q3 (BATFET) is

turned-off. Then Q1/Q2 (ACFET) are turned on by the ACDRV pin. When Q1/Q2 (ACFET) are turned on, the

ACFET allows operation in linear-regulation mode to limit the maximum input current, ACOC, to a safe level. The

ACOC current limit is 1.5 times the programmed DPM input current limit set by the ratio of SRSET/VDAC. The

maximum allowable current limit is 100 mV across ACP – ACN (10 A for a 10-m

Ω sense resistor).

The first 2 ms after the ACDRV signal begins to turn on, ACOC may limit the current; but the controller is not

allowed to latch off in order to allow a reasonable time for the sytem voltage to rise.

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