STCMB1 datasheet(35/59 Pages) STMICROELECTRONICS

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DocID028958 Rev 4

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STCMB1

Application information

In spite of the time-shift control, the calculation of the oscillator components can be easily

done taking into account the limits of the operating frequency range of the converter, used

for the resonant converter calculation. Therefore, after fixing CF in the hundred pF or in the

nF range (consistent with the maximum source capability of the RFmin pin and trading this

off against the total consumption of the device), the value of RFmin and RFmax will be

selected so that the oscillator frequency is able to cover the entire range needed for

regulation, from the minimum value fmin (at minimum input voltage and maximum load) to

the maximum value fmax (at maximum input voltage and minimum load):

Equation 19

VCEsat is the saturation voltage of the phototransistor.

The above RFmax formula has to be considered for calculation in case the resonant stage

does not need working in the burst mode, in that case the STBY pin (#10) will have to be

connected to the RFmin (pin #9).

If the burst mode has to be implemented Equation 20 has to be used.

Equation 20

7.19

HB: Soft-start

The soft-start function is simply realized by the addition of an R-C series circuit from the pin

9 (RFmin) to ground (see Figure 20, left). The middle point of the RC is also connected to

the pin 8 (CSS).

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):

Equation 21

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

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

5 times constants RSS · CSS however, the soft-start phase really ends when the output

voltage has got close to the regulated value and the feedback loop taken over, so that the

operating frequency is essentially determined by the current sunk by the optocoupler's

phototransistor.

Rfmin

3VCF f



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

min

Rfmax

2V VCEsat

–

3VCF 2fmax fmin

–





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

RfmaxBM

4V CF 2fmax fmin

–





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



start

min





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