L6599 datasheet(21/36 Pages) STMICROELECTRONICS

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L6599

Application information

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7.3

Soft-start

Generally speaking, purpose of soft-start is to progressively increase converter's power

capability when it is started up, so as to avoid excessive inrush current. In resonant

converters the deliverable power depends inversely on frequency, then soft- start is done by

sweeping the operating frequency from an initial high value until the control loop takes over.

With the L6599 converter's soft start-up is simply realized with the addition of an R-C series

circuit from pin 4 (RFmin) to ground (see Figure 27).

Initially, the capacitor CSS is totally discharged, so that the series resistor RSS is effectively

in parallel to RFmin and the resulting initial frequency is determined by RSS and RFmin only,

since the optocoupler's phototransistor is cut off (as long as the output voltage is not too far

away from the regulated value):

The CSS capacitor is progressively charged until its voltage reaches the reference voltage

(2V) and, consequently, the current through RSS goes to zero. This conventionally takes 5

time constants RSS·CSS but, before that time, the output voltage will have got close to the

regulated value and the feedback loop taken over, so that it will be the optocoupler's

phototransistor to determine the operating frequency from that moment onwards.

During this frequency sweep phase the operating frequency will decay following the

exponential charge of CSS, that is, initially it will change relatively quickly but the rate of

change will get slower and slower. This counteracts the non-linear frequency dependence

of the tank circuit that makes converter's power capability change little as frequency is away

from resonance and change very quickly as frequency approaches resonance frequency

(see Figure 28).

Figure 27. Soft-start circuit

start

3CF

R F

min

()

⋅⋅

------------------------------------------------------------------------

L6599

Css

RFmin

RSS

CSS

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