STCH02 Datasheet, PDF (13/24 Pages) STMicroelectronics – Offline PWM controller for ultra-low standby adapters

STCH02
Application information
At converter power-down the system will the lose the regulation as soon as the input voltage
falls below HV_START. This prevents converter's restart attempts and ensures monotonic
output voltage decay at system power-down.
Figure 6. Timing diagram: normal power-up and power-down sequences
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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.
This blanking time is dependent on the voltage on the FB pin: it is T_BLANK = 24 µs for
V_FB = 0.6 V, and decreases linearly down to T_BLANK = 3.8 µs for V_FB  1.65 V.
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.
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