PM8903A datasheet(13/33 Pages) STMICROELECTRONICS

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PM8903A

Device description

Doc ID 024147 Rev 1

13/33

To properly supply the power section the following is advised:

●

Bypass VIN pins to PGND pins as close as possible to the IC package with high quality

MLCC capacitors (at least 10 µF).

●

Connect the bootstrap capacitor (typically a 100 nF ceramic capacitor rated to stand

VIN voltage) from the BOOT pin to the PHASE pin to supply the HS driver.

Caution:

Do not connect an external bootstrap diode. The IC already integrates an active bootstrap

diode to charge the bootstrap capacitor, saving the cost of this external component.

The PM8903A embodies an anti-shoot-through and adaptive deadtime control to minimize

low-side body diode conduction time and consequently reduce power losses:

●

When the voltage at the PHASE pin drops (to check high-side MOSFET turn-off), the

LS MOSFET is suddenly switched on

●

When the gate driving voltage of LS drops (to check low-side MOSFET turn-off), the

HS MOSFET is suddenly switched on.

If the current flowing in the inductor is negative, voltage on the PHASE pin never drops. A

watchdog controller is implemented to allow the LS MOSFET to turn on even in this case,

allowing the negative current of the inductor to recirculate. This mechanism allows the

system to regulate even if the current is negative (if pulse-skipping is disabled).

5.2

Startup and shutdown management

The PM8903A monitors the supply voltage on both VCC and VIN pins. Once both VCC and

VIN voltages are above the respective UVLO (undervoltage lockout) thresholds and the EN

pin is high, the device waits for 0.5 ms (typ.) and then begins the soft-start.

Figure 9.

PM8903A soft-start sequence

TSS =0.9 ms

2.8V

VIN UVLO

VIN

2.7V

VCC

VCC UVLO

0.6V

EN THRESHOLD

1.5V

VREF

0.5 ms

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