PM8800A

PWM section

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The PM8800A can work also with a clock reference provided by an external source whose

external resistor is mandatory also when the reference is provided from outside the IC.

The external source must coupled to the RT with a 100 pF capacitor and have a minimum

peak amplitude of 2.8 V. Minimum pulse width of 50 ns has to be assured for proper

operation.

When synchronized the PWM frequency is equal to the external clock reference.

6.3

Soft start

The soft-start feature of the PM8800A allows the load voltage to ramp-up in a safe and

controlled manner. This is achieved by a 10

µA internal current source charging an external

PWM pulse, since the reference of the error amplifier is clamped with this value. Duration of

the start-up time and external capacitor are linked by the following relation:

SS voltage is actively kept at 2.3 V by the internal control circuitry, which manages also

over-current and fault conditions.

Figure 16.

Typical soft start waveform

Ch1 = 5Vout, Ch2 = soft start, Ch4 = I input

t

()

0.23 C

()

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=