AN3093 datasheet(23/29 Pages) STMICROELECTRONICS

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AN3093

Modes of operation

Doc ID16539 Rev 1

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Appendix A

Modes of operation

A.1

Constant voltage operation

The IC is specifically designed to work in primary regulation and the output voltage is

sensed through a voltage partition of the auxiliary winding, just before the auxiliary rectifier

diode.

Figure 21 shows the internal schematic of the constant voltage mode and the external

connections.

Figure 21.

Voltage control principle: internal schematic

In order to achieve a tight output regulation, the signal on the ZCD/FB pin is sampled and

held at the end of the transformer demagnetization to obtain an accurate image of the output

voltage, and it is compared with the error amplifier internal reference.

The internal error amplifier delivers an output current proportional to the voltage imbalance

of the two outputs: the output generates the control voltage that is compared with the

voltage across the sense resistor, thus modulating the cycle-by-cycle peak drain current.

The output voltage can be set by choosing RFB according to Equation 4:

Equation 4

Where nSEC and nAUX are the secondary and auxiliary number of turns, respectively.

The RZCD value can be defined based on the application parameters and can be calculated

according to Equation 6.

REF

AUX

SEC

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

OUT

REF

–

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

ZCD

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