DocID028766 Rev 1

15/24

STCH02

Application information

24

3.5

Constant voltage operation

The device is specific for secondary feedback. The FB pin is connected to an optocoupler

which transmits the error signal from the regulation loop located on the secondary side of

the converter. Typically, a TS431 is used as a voltage reference.

The FB pin is driven directly by the phototransistor's collector to modulate the duty cycle.

The voltage coming from the FB pin is compared with the voltage across the sense resistor,

controlling the peak drain current cycle-by-cycle.

Figure 8. Voltage control principle: internal schematic

3.6

Constant current operation

The voltage of the auxiliary winding is fed into the internal CC block trough the ZCD pin to

achieve an output constant current regulation.

Equation 1 can be used to define to output current in CC mode.

Equation 1

This formula shows that the average output current does not depend anymore on the input

or the output voltage, neither on transformer inductance values. The external parameters

S E N S E

I

S E C

P R I

O U T

R

2

K

N

N

I

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

