Datasheet

bq24616
SLUSA49B –APRIL 2010– REVISED OCTOBER 2011 www.ti.com
MOSFET is turned on again. The 80-ns low-side MOSFET on-time is required to ensure the bootstrap capacitor
is always recharged and able to keep the high-side power MOSFET on during the next cycle. This is important
for battery chargers, where unlike regular dc-dc converters, there is a battery load that maintains a voltage and
can both source and sink current. The 80-ns low-side pulse pulls the PH node (connection between high- and
low-side MOSFETs) down, allowing the bootstrap capacitor to recharge up to the REGN LDO value. After 80-ns,
the low-side MOSFET is kept off to prevent negative inductor current from occurring.
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 at around
80 ns to provide refresh charge for the bootstrap capacitor when the bootstrap capacitor voltage drops below 4.2
V. 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 and potentially causes 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 the battery instead of the adapter.
Input Undervoltage Lockout (UVLO)
The system must have a minimum VCC voltage to allow proper operation. This VCC voltage could come from
either the input adapter or the battery, because 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 on the IC are disabled, and the
gate-drive bias to ACFET and BATFET is disabled. ACFET is OFF and BATFET is ON.
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. BATOVP
also suspends the safety timer.
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.
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