AN4149 datasheet(24/43 Pages) STMICROELECTRONICS

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L4984D biasing circuitry

AN4149

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DocID023523 Rev 2

L4984D biasing circuitry

The following sections describe the selection of the circuitry around the L4984D.

3.1

Feedback and OVP

Pin 1 (INV): this pin is connected both to the inverting input of the E/A and to the OVP

circuitry. A resistive divider has to be connected between the boost regulated output voltage

and this pin. The internal reference on the non-inverting input of the E/A is 2.5 V (typ.), the

output voltage (Vout) of the PFC pre-regulator is set at the nominal value by the resistor ratio

of the feedback output divider. RoutH and RoutL will be then selected considering the desired

nominal output voltage and the desired output power dissipated on the output divider.

For example for a 25 mW output divider dissipation:

Equation 70

Please note that for RoutH a resistor with a suitable voltage rating (> 400 V) is needed, or

more resistors in series have to be used. Here three 2.2 M

Ω resistors in series have been

selected.

Equation 71

Equation 72

For RoutL a value of 160 kΩ in parallel to a 56 kΩ has been selected.

Pin 2 (COMP): this pin is the output of the E/A that is fed to one of the inputs of the

multiplier. A feedback compensation network is placed between this pin and INV. It has to be

designed in with a narrow bandwidth in order to avoid that the system rejects the output

voltage ripple (100 Hz) that would bring high distortion of the input current waveform. A

theoretical criterion to define the compensation network value is to set the E/A bandwidth

(BW) below 20 Hz. The compensated two-pole feedback network selection for this 350 W

FOT PFC has been described in detail in Section 4: FOT PFC boost control loop.

Pin 6 (PFC_OK - Feedback failure protection): PFC_OK pin has been dedicated to

monitor the output voltage of a separate resistor divider. This divider is selected so that the

voltage at the pin reaches 2.5 V if the output voltage exceeds a preset value (Vovp), usually

larger than the maximum Vout that can be expected, also including worst-case load/line

transients. For a maximum output voltage Voutmax of 430 V and selecting a 50 μA current

flowing in to the divider:

()

out

outH

−

()

−

outH

320

400

−

out

outL

outH

159

400

−

outL

outH

159

out

outL

159

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