PM6600 datasheet(31/43 Pages) STMICROELECTRONICS

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PM6600

Operation description

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7.5

Soft-start

The Soft-Start function is required to perform a correct start-up of the system, controlling the

inrush current required to charge the output capacitor and to avoid output voltage overshoot.

The Soft-Start duration is set connecting an external capacitor between the SS pin and

ground. This capacitor is charged with a 5

µA constant current, forcing the voltage on the SS

pin to ramp up. When this voltage increases from zero to nearly 1.2 V, the current limit of the

power-MOSFET is proportionally released to its final value. In addition, during the initial part

of the Soft-Start, the switching frequency of the boost converter is reduced to half of the

nominal value to permit to use inductors with lower saturation current value; the nominal

switching frequency is restored after the SS pin voltage has crossed 0.8 V. In this mode, the

current runaway is avoided.

Figure 51.

Soft-start sequence waveforms in case of floating ROWs

During the soft-start phase it is also performed the floating ROWs detection. In presence of

one or more floating ROWs, the error amplifier is unbalanced and the output voltage

increases; when it reaches the Floating ROW Detection (FRD) threshold (93 % of the OVP

threshold), the floating ROWs are managed according to Table 3 (see Section 9 on page 36).

After the SS voltage reaches a 2.4 V threshold, the start-up finishes and all the protections

turn active. The soft-start capacitor CSS can be calculated according to equations 12.

Equation 13

Where ISS = 5 µA and tSS is the desired Soft-start duration.

AVCC

tss

SS pin voltage

Protections turn active

1.2V

0.8V

Current limit

EN pin voltage

100%

Output voltage

OVP

93% of OVP

Floating ROWs detection

2.4V

Nominal switching

frequency release

)

(

min

max

OUT

−

⋅

≅

−

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