PM8903 datasheet(15/34 Pages) STMICROELECTRONICS

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PM8903

Device description

Doc ID 022748 Rev 1

15/34

The PM8903 implements the soft-start by gradually increasing the internal reference from

0 V to 0.6 V in a 1024 switching clock (0.79 ms typ.), linearly charging the output capacitors

to the final regulation voltage in closed loop regulation. The soft-start prevents high inrush

current from power supply rail.

5.2.1

Low-side-less startup

In order to avoid any kind of negative undershoot and dangerous return from the load during

startup, the PM8903 performs a special sequence in enabling the LS driver to switch: during

the soft-start phase, the LS driver results disabled (LS = OFF) until the first PWM pulse

occurs. This avoids the dangerous negative spike on the output voltage that may happen if

starting over a pre-biased output.

As long as the output voltage is biased to a voltage higher than the programmed one, the

control loop does not provide the HS pulse that enables LS. In this case LS is enabled at the

end of the soft-start time and, if the device is allowed to sink (PSKIP disabled), it discharges

the output to the final regulation value.

This particular feature of the device masks the LS turn-on only from the control loop point of

view: protection has higher priority and can turn on the LS MOSFET if an overvoltage event

is detected.

5.2.2

Soft-off

The PM8903 implements the soft-off sequence turning off both HS and LS MOSFETs and

connecting the integrated bleeding resistor (100

Ω) between the PHASE and PGND pin.

When small load currents are applied to the converter, the soft-off sequence allows the

discharging of the output voltage within a maximum time (TSO) that depends only on the

output capacitance value.

The PM8903 begins the soft-off sequence, and remains in a latched state, if one of the

following conditions occurs:

●

VCC voltage falls below UVLO threshold

●

OVP (overvoltage protection)

●

UVP (undervoltage protection)

●

OCP (overcurrent protection)

●

EN pin is pulled low

Cycle EN or VCC to recover from latched state with a new soft-start sequence.

5.3

Output voltage monitoring and protection

The PM8903 monitors the output voltage status through the FB pin and compares the

voltage on this pin with the internal reference in order to provide over and undervoltage

protection as well as PGOOD signal.

5.3.1

Overvoltage protection

Overvoltage protection is active as soon as the device is enabled and both VCC and VIN

voltages are above the respective undervoltage lockout levels.

5 100 C

OUT

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