Datasheet

bq24620
www.ti.com SLUS893A –MARCH 2010– REVISED OCTOBER 2011
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
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, which 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 battery is switched to the system 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 input the adapter orthe 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 105% 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 PGND 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 a 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.
TEMPERATURE QUALIFICATION
The controller continuously monitors battery temperature by measuring the voltage between the TS pin and
GND. A negative temperature coefficient thermistor (NTC) and an external voltage divider typically develop this
voltage. The controller compares this voltage against its internal thresholds to determine if charging is allowed.
To initiate a charge cycle, the battery temperature must be within the V
LTF
to V
HTF
thresholds. If battery
temperature is outside of this range, the controller suspends charge and the safety timer and waits until the
battery temperature is within the V
LTF
to V
HTF
range. During the charge cycle, the battery temperature must be
within the V
LTF
to V
TCO
thresholds. If the battery temperature is outside of this range, the controller suspends
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