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STCH03 Pin connection and functions
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4 Pin connection and functions
Figure 2. Pin connection (top view)
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Table 4. Pin functions
No. Name Function
1HV
High-voltage start-up. This pin is able to withstand 650 V and is tied directly to the rectified mains
voltage. When the voltage on the pin reaches the HV
START
voltage (50 V typ.) a 7 mA internal
current source charges the capacitor connected between V
DD
and GND to start-up the IC. When the
voltage on the V
DD
pin reaches the turn-on threshold (17 V typ.) the generator is shut down.
2 NC Not internally connected. Provision for clearance on the PCB to meet safety requirements.
3FB
Control input for duty cycle control. A voltage set 65 mV below the threshold V
FBB
activates the
burst-mode operation. A level close to the threshold V
FBH
means that we are approaching the cycle-
by-cycle overcurrent set point.
4ZCD
The transformer demagnetization sensing for quasi-resonant operation and input/output voltage
monitor. A negative-going edge triggers the MOSFET turn-on. The current sourced by the pin during
the MOSFET ON time is monitored for an image of the input voltage to the converter, in order to
compensate the internal delay of the current sensing circuit and achieve a CC regulation
independent of the mains voltage. At the same time the pin voltage is sampled-and-held right at the
end of the transformer demagnetization to get an accurate image of the output voltage to be used for
overvoltage protection (OVP) and undervoltage protection (UVP) sensing. Please note that
maximum I
ZCD
sink/source current must not exceed 3 mA over the entire voltage range. No
capacitor is allowed between the pin and the auxiliary winding of the transformer.
5 SENSE
Input to the PWM comparators. The current flowing in the MOSFET is sensed through a resistor
connected between the pin and GND. The resulting voltage is compared with an internal reference
(0.75 V typ.) to determine MOSFET turn-off. The pin is equipped with 380 ns blanking time after the
gate-drive output goes high for improved noise immunity. If a second comparison level located at
1 V, is exceeded, the IC stops and restarts after V
DD
has dropped below V
DDR
(4.5 V typ.).
6GND
Circuit ground reference and current return for both the signal part of the IC and the gate-drive. All of
the ground connections of the bias components should be tied to an interconnect going to this pin
and kept separate from any pulsed current return.
7 GD Gate-driver with the totem pole output stage for the external power MOSFET.
8V
DD
Supply voltage of the device. An electrolytic capacitor, connected between this pin and ground, is
initially charged by the internal high-voltage start-up generator. It is recommended to place a small
bypass capacitor (0.1 µF typ.) connected between the pin and GND might be useful to get a clean
bias voltage for the signal part of the IC. To improve the ruggedness of the pin during electrical fast
transient events, an RC low-pass filter can be also connected on the pin.
