DS8816A Datasheet, PDF(14/21 Page) Richtek Technology Corporation – Dual-Phase PWM Controller with PWM-VID Reference

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DS8816A Datasheet, PDF (14/21 Pages) Richtek Technology Corporation – Dual-Phase PWM Controller with PWM-VID Reference

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RT8816A

Enable and Disable

The EN pin is a high impedance input that allows power

sequencing between the controller bias voltage and

another voltage rail. The RT8816A remains in shutdown if

the EN pin is lower than 550mV. When the EN voltage

rises above the 1.2V high level threshold, the RT8816A

will begin a new initialization and soft-start cycle.

Power On Reset (POR), UVLO

Power On Reset (POR) occurs when VPVCC rises above

to approximately 4.1V (typical), the RT8816A will reset

the fault latch circuit and prepare for PWM operation.

When the VPVCC is lower than 3.8V (typical), the Under

Voltage Lockout (UVLO) circuitry inhibits switching by

keeping UGATE and LGATE low.

Soft-Start

The RT8816A provides internal soft-start function and

external soft-start function. The soft-start function is used

to prevent large inrush current and output voltage overshoot

while the converter is being powered up. The soft-start

function automatically begins once the chip is enabled.

There is a delay time around 200us from EN goes high to

VOUT begins to ramp-up.

If external capacitor from OCSET/SS pin to GND is

removed, the internal soft-start function will be chosen.

An internal current source charges the internal soft-

start capacitor so that the internal soft-start voltage

ramps up linearly. The output voltage will track the

internal soft-start voltage during the soft-start interval.

After the internal soft- start voltage exceeds the REFIN

voltage, the output voltage no longer tracks the internal

soft-start voltage but follows the REFIN voltage.

Therefore, the duty cycle of the UGATE signal as well

as the input current at power up are limited.

The soft-start process is finished until the internal SSOK

go high and protection is not triggered. Figure 6 shows

the internal soft-start sequence.

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PVCC

VOUT

Internal SS

Internal SSOK

VREFIN

UGATE

LGATE

PGOOD

Enable

delay time

Soft-start

Normal

Soft

Discharged

Figure 6. Internal Soft-Start Sequence

The RT8816A also provides a proximate external soft-start

function, and the external soft-start sequence is shown in

Figure 7, an additional capacitor can be connected from

OCSET/SS pin to GND. The external capacitor will be

charged by internal current source to build soft-start

voltage ramp. If external soft-start function is chosen, the

external soft-start time should be set longer than internal

soft-start time to avoid output voltage tracking the internal

soft-start ramp, the external soft-start time setting is shown

in Figure 7, the recommend external soft-start slew rate

is 0.1V/ms to 0.4V/ms.

PVCC

VOUT

External SS

External SSOK

120 % VREFIN