DocID028766 Rev 1

13/24

STCH02

Application information

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At converter power-down the system will the lose the regulation as soon as the input voltage

output voltage decay at system power-down.

Figure 6. Timing diagram: normal power-up and power-down sequences

3.4

Zero current detection and triggering block

The zero current detection (ZCD) and triggering blocks switch on the power MOSFET if

a negative-going edge falling below 50 mV is applied to the ZCD pin. To do so, the triggering

block must be previously armed by a positive-going edge exceeding 100 mV.

This feature is used to detect transformer demagnetization for the QR operation, where the

signal for the ZCD input is obtained from the transformer's auxiliary winding used also to

supply the IC.

The triggering block is blanked after the MOSFET's turn-off to prevent any negative-going

edge that follows leakage inductance demagnetization from triggering the ZCD circuit

erroneously.

The voltage on the pin is both top and bottom limited by a double clamp. The upper clamp is

typically located at 3 V, while the lower clamp is located at -60 mV. The interface between

the pin and the auxiliary winding will be a resistor divider. Its resistance ratio as well as the

individual resistance values will be properly chosen (see Section 3.10: Overvoltage

protection on page 18 and Section 3.7: Voltage feedforward block on page 16.