BQ24721 Datasheet, PDF (29/48 Pages) Texas Instruments – ADVANCED MULTI-CHEMISTRY AND MULTI-CELL SYNCHRONOUS SWITCH-MODE CHARGER AND SYSTEM POWER SELECTOR

bq24721
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SLUS683 – NOVEMBER 2005
The ISYNSET pin is internally regulated to 1 V. When the R_(SYNSET) resistor is connected to AGND, it sets an
ISYNSET current equal to 1 V/R_(SYNSET). The ISYNSET current internally flows through a 500 Ω creating a
voltage at which the voltage across R_(SENSE) is compared. The ISYN charge current threshold is the voltage
divided by the R_(SENSE) sense resistor value. The R_(SYNSET) resistor value is calculated by:
1 V x 500 W
R_(SYNSET) = _{I(SYN) x R(SENSE)}
(3)
where ISYN is the charge current threshold at which the converter changes to synchronous operation.
Synchronous versus Nonsynchronous Operation
The charger operates in non-synchronous mode when the sensed charge current is below the ISYNSET
programmed value. When above the ISYNSET programmed value, the charger operates in synchronous mode.
During synchronous mode, the low-side n-channel power MOSFET is on, when the high-side n-channel power
MOSFET is off. The internal gate drive logic ensures there is break-before-make switching to prevent
shoot-through currents. During the 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. During Synchronous mode the inductor current is always flowing and
operates in Continuous Conduction Mode (CCM) creating a fixed two-pole system. 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 turns on for around 80 ns, then the low-side power MOSFET turns off
and stays off until the beginning of the next cycle, where the high-side power MOSFET is turned on again. The
80 ns low-side MOSFET on-time is done 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 MOSFET) down, allowing
the bootstrap capacitor to recharge up to the REGN LDO value. After the 80 ns, the low-side MOSFET is kept off
to prevent negative inductor current from occurring. The inductor current is blocked by the off low-side MOSFET,
and the inductor current becomes 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 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 (no 80 ns recharge pulse), so there is no discharge from the battery.
Battery Regulation Loop
The BAT pin is used to sense the battery voltage and should be connected as close to the battery as possible, or
directly on the output capacitor. A 0.1-µF ceramic capacitor from BAT to AGND is recommended added as close
the BAT pin as possible to decouple high frequency noise.
Charge Current Regulation Loop
The SRP and SRN pins are used to sense across the sense resistor. The charge current DAC is set for a 10-mΩ
sense resistor; however, resistors of other values can also be used. The larger the sense resistance, the larger
the sensed voltage, and the higher the regulation accuracy, but at the expense of higher conduction losses.
Input Current Regulation Loop (DPM)
The ACP and ACN pins are used to sense across the sense resistor. The input current DAC is set for a 10-mΩ
sense resistor; however, resistors of other values can also be used. The larger the sense resistance, the larger
the sensed voltage, and the higher the regulation accuracy, but at the expense of higher conduction losses.
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