BQ24640 Datasheet, PDF (14/25 Pages) Texas Instruments – High-Efficiency Synchronous Switch-Mode Super Capacitor Charger

bq24640
SLUSA44 – MARCH 2010
www.ti.com
During non-synchronous operation, the body-diode of lower-side MOSFET can conduct the positive inductor
current after the high-side n-channel power MOSFET turns off. When the load current decreases and the
inductor current drops to zero, the body diode will be naturally turned off and the inductor current will become
discontinuous. This mode is called Discontinuous Conduction Mode (DCM). During DCM, the low-side n-channel
power MOSFET will turn on when the bootstrap capacitor voltage drops below 4.2V, then the low-side power
MOSFET will turn off and stay off until the beginning of the next cycle, where the high-side power MOSFET is
turned on again. The 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.
At very low currents during non-synchronous operation, there may be a small amount of negative inductor
current during the 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 recharge pulse) either, and there is almost no discharge from the
output.
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.
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.
OUTPUT OVER-VOLTAGE PROTECTION
The converter will not allow the high-side FET to turn-on until the output 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. An 8mA current sink from SRP/SRN to GND is on during charge and allows discharging 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.
TEMPERATURE QUALIFICATION
The controller continuously monitors load 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 temperature must be within the V(LTF) to V(HTF) thresholds. If temperature is outside of this
range, the controller suspends charge and waits until the temperature is within the V(LTF) to V(HTF) range.
During the charge cycle the temperature must be within the V(LTF) to V(TCO) thresholds. If temperature is
outside of this range, the controller suspends charge and waits until the temperature is within the V(LTF) to
V(HTF) range. The controller suspends charge by turning off the PWM charge FETs. If the TS function is not
required, R9 and R10 can be the same value so the voltage on TS is 1.65V with VREF as the reference supply.
14
Submit Documentation Feedback
Product Folder Link(s): bq24640
Copyright © 2010, Texas Instruments Incorporated