Designing a fixed-off-time PFC

AN3112

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Doc ID 16820 Rev 3

Equation 69

Equation 70

For example, a commercial value of the limiting resistor of 1 k

Ω and a speed-up capacitor of

100 pF can be selected for this application.

Pin 8 (GND) acts as the current return both for the signal internal circuitry and for the gate

drive current. When laying out the printed circuit board, these two paths should run

separately.

Pin 9 (GD) is the output of the driver. The pin is able to drive an external MOSFET with

600 mA source and 800 mA sink capability.

The high-level voltage of this pin is clamped at about 12 V to avoid excessive gate voltages

if the pin is supplied with a high Vcc. To avoid undesired switch-on of the external MOSFET

because of some leakage current when the supply of the L6564 is below the UVLO

threshold, an internal pull-down circuit holds the pin low. The circuit guarantees 1.1 V

bleeder-resistor connected between the gate and the source of the external MOSFET used

to this purpose.

Pin 10 (Vcc) is the supply of the device which is externally connected to the start-up circuit

(usually, one resistor connected to the rectified mains) and to the self-supply circuit.

Whatever the configuration of the self-supply system, a capacitor is connected between this

pin and ground.

To start the L6564, the voltage must exceed the start-up threshold (12 V typ.). Below this

value the device does not

and consumes less than 90 µA (typ.) from Vcc. This allows the use of high value start-up

resistors (in the hundreds k

Ω), which reduces power consumption and optimizes system

efficiency at low load, especially in wide range mains applications.

When operating, the current consumption (of the device only, not considering the gate drive

current) rises to a value depending on the operating conditions but never exceeding 6 mA.

The device continues to work as long as the supply voltage is over the UVLO threshold (13

V max). If the Vcc voltage exceeds 22.5 V an internal zener diode, 20 mA rated, is activated

in order to clamp the voltage. Please remember that during normal operation the internal

zener doesn’t have to clamp the voltage, because in this case the power consumption of the

device increases considerably and its junction temperature also increases. The suggested

operating condition, for safe operation of the device, is below the minimum clamping voltage

of the pin.

Ω

<

<

Ω

k

1

Rs

726

pF

144

Cs

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