Datasheet

bq24600

SLUS891A –FEBRUARY 2010– REVISED OCTOBER 2011

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At very low currents during non-synchronous operation, there may be a small amount of negative inductor

current during the 80-ns recharge pulse. The charge should be low enough to be absorbed by the input

capacitance. Whenever the converter goes into zero-percent duty cycle, the high-side MOSFET does not turn on,

and the low-side MOSFET does not turn on (only 80-ns recharge pulse) either, and there is almost no discharge

from the battery.

During the DCM mode, the loop response automatically changes and has a single-pole system at which the pole

is proportional to the load current, because the converter does not sink current, and only the load provides a

current sink. This means at very low currents the loop response is slower, as there is less sinking current

available to discharge the output voltage.

Cycle-by-Cycle Charge Undercurrent Protection

If the SRP-SRN voltage decreases below 5 mV (the charger is also forced into non-synchronous mode when the

average SRP-SRN voltage is lower than 1.25 mV), the low side FET is turned off for the remainder of the

switching cycle to prevent negative inductor current. During DCM, the low-side FET only turns on for around 80

ns when the bootstrap capacitor voltage drops below 4.2 V to provide refresh charge for the bootstrap capacitor.

This is important to prevent negative inductor current from causing a boost effect in which the input voltage

increases as power is transferred from the battery to the input capacitors and leads to an overvoltage stress on

the VCC node, potentially causing damage to the system.

Input Overvoltage Protection (ACOV)

ACOV provides protection to prevent system damage due to high input voltage. Once the adapter voltage

reaches the ACOV threshold, charge is disabled and the system is switched to battery instead of adapter.

Input Undervoltage Lock-Out (UVLO)

The system must have a minimum VCC voltage to allow proper operation. This VCC voltage could come from

either input adapter or battery, if a conduction path exists from the battery to VCC through the high-side NMOS

body diode. When VCC is below the UVLO threshold, all circuits in the IC are disabled.

Battery Overvoltage Protection

The converter does not allow the high-side FET to turn on until the BAT voltage goes below 102% of the

regulation voltage. This allows one-cycle response to an overvoltage condition, such as occurs when the load is

removed or the battery is disconnected. An 8-mA current sink from SRP/SRN to GND is on only during charge

and allows discharging the stored output-inductor energy that is transferred to the output capacitors.

Cycle-by-Cycle Charge Overcurrent Protection

The charger has secondary cycle-to-cycle overcurrent protection. It monitors the charge current and prevents the

current from exceeding 160% of the programmed 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.

Thermal Shutdown Protection

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

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

self-protects whenever the junction temperature exceeds the TSHUT threshold of 145°C. The charger stays off

until the junction temperature falls below 130°C, then the charger soft-starts again if all other enable charge

conditions are valid. Thermal shutdown also suspends the safety timer.

Product Folder Link(s): bq24600