AN3009 datasheet(28/36 Pages) STMICROELECTRONICS

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Designing a transition mode PFC

AN3009

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Doc ID 16032 Rev 4

VZCDH at 5.7 V and VZCDL at 0 V are the upper and lower ZCD clamp voltages of the L6564.

Considering the highest value of the two calculations, an RZCD equal to 68 k

Ω has been

selected as the limiting resistor.

The actual value can then be tuned by trying to make the activation of the MOSFET occur

right on the valley of the drain voltage (which is resonating because the boost inductor has

run out of energy – as shown in

Figure 13).This minimizes the power dissipation at turn-on.

Figure 13.

Optimum MOSFET activation

Pin 8 (GND): this pin acts as the current return for both the signal’s internal circuitry and for

the gate drive current. When layouting the printed circuit board, these two paths should run

separately.

Pin 9 (GD): this pin is the output of the driver. It can drive an external MOSFET with a

600 mA source and a 800 mA sink capability.

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

voltages in case the pin is supplied with a high Vcc. To avoid undesired switch-ons 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 maximum on the pin (when Isink = 2 mA), with Vcc > VCC_ON. This allows omitting

the "bleeder" resistor connected between the gate and the source of the external MOSFET

used for this purpose.

Pin 10 (Vcc): this pin is the supply of the device. It is externally connected to the start-up

circuit (normally, one resistor is connected to the rectified mains) and to the self-supply

circuit.

Whatever the configuration of the self-supply system, a capacitor must be 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 work and consumes less than 90 µA (typ.) from Vcc. This allows

the use of high-value start-up resistors (in the hundred k

Ω), which reduces power

consumption and optimizes system efficiency at low loads, especially in wide-range mains

applications.

When operating, the current consumption (of the device only, not of the gate drive) rises to a

value that depends on the operating conditions but never exceeds 6 mA.

The device keeps on working 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 (rated at 20 mA) is

activated in order to clamp the voltage. Remember that during normal operation, the internal

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