BQ24600_1 Datasheet, PDF (15/31 Pages) Texas Instruments – Stand-Alone Synchronous Switch-Mode Li-Ion or Li-Polymer Battery Charger with Low Iq

bq24600
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SLUS891 – FEBRUARY 2010
At very low currents during non-synchronous operation, there may be a small amount of negative inductor
current during the 80ns 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 80ns 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 Under Current Protection
If the SRP-SRN voltage decreases below 5mV (The charger is also forced into non-synchronous mode when the
average SRP-SRN voltage is lower than 1.25mV), the low side FET will be turned off for the remainder of the
switching cycle to prevent negative inductor current. During DCM, the low-side FET will only turn on for at around
80ns when the bootstrap capacitor voltage drops below 4.2V 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 lead to an over-voltage
stress on the VCC node and potentially cause damage to the system.
Input Over-Voltage 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 Under-Voltage-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 Over-Voltage Protection
The converter will 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 over-voltage condition – such as occurs when the load is removed
or the battery is disconnected. An 8mA 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 Over-Current Protection
The charger has a secondary cycle-to-cycle over-current 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
over-current is detected, and automatically resumes when the current falls below the over-current 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 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 will soft-start again if all other enable charge
conditions are valid. Thermal shutdown will also suspend the safety timer.
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