HVLED007TR datasheet(25/33 Pages) STMICROELECTRONICS

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DS12866 Rev 1

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HVLED007

Application information

Vcc supply voltage is high enough to define its state) to reset the COMP voltage.

Figure 11 shows the functional schematic of the OVP circuit.

The OVP detection level VoutOVP can be defined by properly selecting the lower resistor

Rovp of the divider that connects the pin ZCD to the auxiliary winding according the

formula:

Equation 18

where Nsec and Naux are the secondary and auxiliary turns number respectively. The Rzcd

value is defined by other application parameters (see "Section 4.8: Zero current detection

and triggering block (pin ZCD); starter").

Figure 11. Functional schematic of the OVP function

4.11

Soft-restart function

This function provides a smooth converter restart limiting the initial primary peak current

whenever the IC is temporarily stopped during burst-mode operation (i.e. as VCOMP <

VCOMP_BM, see "Section 4.5: Control input for isolated feedback and optocoupler driving

(pin COMP)") or after an overvoltage detection (see "Section 4.10: Overvoltage protection

(OVP function)").

This goal is met by turning on a MOSFET switch connected between pin CT and ground that

discharges the external shaping capacitor Ct.

Firstly, the discharge switch is kept in the ON-state as long as the IC is in UVLO (provided its

Vcc supply voltage is high enough to define its state) to reset the CT voltage at start-up.

Secondly, the discharge switch is turned on as long as the IC is stopped and is turned off

when the idle period ends and the gate-drive is turned on. The schematic of Figure 12

shows the block diagram of the circuit implementing the above-mentioned functionality.

Rovp =

VZCD_OVP

N aux

N sec

------------ VoutOVP VZCD_OVP

–

------------------------------------------------------------------ Rzcd

VZCD_OVP

Rzcd

Rovp

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