L6599AD datasheet(21/35 Pages) STMICROELECTRONICS

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L6599A

Application information

Doc ID 15308 Rev 7

21/35

7.3

Soft-start

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

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

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

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

With the L6599A converter, soft-startup is simply realized with the addition of an R-C series

circuit from pin 4 (RFmin) to ground (see

Figure 27, left).

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 phototransistor is cut off (as long as the output voltage is not too

far away from the regulated value):

Equation 4

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

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

imposed 5 times by selecting the constants RSS·CSS. Before reaching 2 V on Css, the

output voltage should be already close to the regulated value and the feedback loop already

taken over, so that it is the optocoupler phototransistor to determine the operating frequency

from that moment onwards.

During this frequency sweep phase the operating frequency decays following the

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

gets slower and slower. This counteracts the non-linear frequency dependence of the tank

circuit that makes the converter power capability change little as frequency is away from

resonance and change very quickly as frequency approaches resonance frequency (see

Figure 27, right).

Figure 27.

Soft-start circuit (left) and power vs. frequency curve in a resonant

half bridge (right)

As a result, the average input current smoothly increases, without the peaking that occurs

with linear frequency sweep, and the output voltage reaches the regulated value with almost

no overshoot.

()

min

start

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L6599AD Datasheet(PDF) 21 Page - STMicroelectronics