STCMB1 datasheet(34/59 Pages) STMICROELECTRONICS

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Application information

STCMB1

34/59

DocID028958 Rev 4

The network connected to the RFmin pin generally comprises three branches:

A resistor RFmin connected between the pin and ground that determines the minimum

oscillator frequency

A resistor RFmax connected between the pin and the collector of the (emitter-grounded)

phototransistor that transfers the feedback signal from the secondary side back to the

primary side; while in the operation, the phototransistor will modulate the current

through this branch - hence modulating the oscillator frequency - to perform output

voltage regulation; the value of RFmax determines the maximum oscillation frequency

when the phototransistor is fully saturated

An R-C series circuit (CSS + RSS) connected between the pin and ground that enables

to set up a frequency shift at the start-up. Note that the contribution of this branch is

zero during the steady state operation.

Because of the time shift control implementation, the on-time of the half bridge is defined not

only by the oscillator ramp, but also by the time the current takes to arrive to zero from the

switched value, since the oscillator ramp is actually set by the signal at the ISEN_HB pin

crossing zero.

In the end, the oscillator frequency is always twice the half bridge frequency, being its period

given by the sum of the time the tank current, sensed at the ISEN_HB pin, takes from its

switched value to its zero crossing and the time length of the ramp, that is defined by the

external capacitor on the CF pin and by the current the control loop pulls from the RFMIN

pin.

From this point, we will refer to the switching frequency of the half-bridge, it being

understood that the oscillator frequency is always twice as much.

Figure 19. Oscillator waveforms and their relationship with gate-driving signals

STCMB1 数据表(PDF) 34 Page - STMicroelectronics