BQ24765 Datasheet, PDF (25/36 Pages) Texas Instruments – SMBus-Controlled Multi-Chemistry Battery Charger With Integrated Power MOSFETs

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bq24765
SLUS999 – NOVEMBER 2009
The fixed frequency oscillator keeps tight control of the switching frequency under all conditions of input voltage,
battery voltage, charge current, and temperature, simplifying output filter design and keeping it out of the audible
noise region. The type III compensation provides phase boost near the cross-over frequency, giving sufficient
phase margin.
CONTINUOUS CONDUCTION MODE AND DISCONTINUOUS CONDUCTION MODE
In Continuous Conduction Mode (CCM), the inductor current always flows to charge battery, and the charger
always operates in synchronized mode. At the beginning of each clock cycle, high-side n-channel power
MOSFET turns on, and the turn-on time is set by the voltage on the EAO pin. After the high-side power MOSFET
turns off, the low-side n-channel power MOSFET turns on. During CCM, the low-side n-channel power MOSFET
stays on until the end of the clock cycle. The internal gate drive logic ensures there is break-before-make
switching to prevent shoot-through currents. During the 25ns dead time where both FETs are off, the back-diode
of the low-side power MOSFET conducts the inductor current. Having the low-side FET turn-on keeps the power
dissipation low, and allows safely charging at high currents. With type III compensation, the loop has a fixed
2-pole system.
As the ripple valley current gets close to zero, charger operation goes to non-synchronized mode. During
non-synchronous operation, after the high-side n-channel power MOSFET turns off, and after the
break-before-make dead-time, the low-side n-channel power MOSFET will turn-on 40ns. After the 40ns blank out
time is over, if V(CSOP-CSON) voltage falls below UCP threshold (typical 10mV), 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. After the low-side MOSFET turns off, the inductor current flows through back-gate diode until it reaches
zero. The negative inductor current is blocked by the diode, and the inductor current will become discontinuous.
This mode is called Discontinuous Conduction Mode (DCM).
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. At very low currents during non-synchronous operation, there may be a
small amount of negative inductor current during the 40ns 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 (no 40ns recharge pulse) either, and there is no discharge from the battery;
unless the BOOT to PHASE voltage discharges below 4V. In that case, it pulses once to recharge the boot-strap
capacitor.
REFRESH BTST CAPACITOR
If the BOOT pin to PHASE pin voltage falls below 4V for more than 3 cycles, then the high-side n-channel power
MOSFET is turned off and the low-side n-channel power MOSFET is turned on for 40ns to pull the PHASE node
down and recharge the BOOT capacitor. The 40ns low-side MOSFET on-time is required protect from ringing
noise, and to ensure the bootstrap capacitor is always recharged and able to keep the high-side power MOSFET
on during the next cycle.
UCP (Charge Under-Current): USING SENSE RESISTOR
In bq24765, the cycle-by-cycle UCP allows using very small inductors seamlessly, even if they have large ripple
current. Every cycle when the low-side MOSFET turns-on, if the CSOP-CSON voltage falls below 10mV (inductor
current falls below 1A if using 10mΩ sense resistor), the low-side MOSFET is latched off until the next cycle
begins and resets the latch.
The converter automatically detects when to turn-off the low-side MOSFET every cycle. The inductor current
ripple is given by
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