Designing a fixed-off-time PFC

AN3112

20/36

Doc ID 16820 Rev 3

Pin 1 (INV) is connected both to the inverting input of the E/A and to the OVP circuitry. A

resistive divider is 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 its nominal value, by the resistors ratio of the

feedback output divider. RoutH and RoutL are then selected considering the desired

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

example for a 50 mW output divider dissipation:

Equation 39

With the commercial value selected RoutH = 3 M

Ω:

Equation 40

Equation 41

RoutL = 62 k

Ω in parallel to a 27 kΩ can be selected. Please note that for RoutH a resistor

with a suitable voltage rating (>400 V) is needed, or more resistors in series must be used.

Pin 6 (PFC_OK - feedback failure protection): The PFC_OK pin is dedicated to monitoring

the output voltage with 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 Vout max of 430 V and selecting a 50 µA current flowing into

the divider:

Equation 42

By selecting a commercial value of 51k

Ω:

Equation 43

Connecting in series, two 3.3 M

Ω resistors and one 2.2 MΩ resistor, a total value of 8.8 MΩ

can be obtained.

Note that both feedback dividers connected to the L6564 pin #1 (INV) and pin #6 (PFC_OK)

can be selected without any constraints. The unique criterion is that both dividers must sink

mW

50

)

V

5

.

2

V

(

R

2

OUT

outH

−

=

Ω

=

−

=

M

160

.

3

mW

50

)

V

5

.

2

V

400

(

R

2

outH

1

V

5

.

2

V

R

R

out

outL

outH

−

=

159

1

V

5

.

2

V

400

R

R

outL

outH

=

−

=

159

R

R

outH

Ω

=

Ω

=

k

8

.

18

159

M

3

R

outL

divider

OK

_

PFC

_

REF

L

I

V

R

=

Ω

=

µ

=

k

50

A

50

V

5

.

2

R

L

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

⋅

=

1

V

V

R

R

OK

_

PFC

_

REF

MAX

_

OUT

L

H

Ω

=

⎟

⎠

⎞

⎜

⎝

⎛

−

⋅

Ω

=

M

721

.

8

1

V

5

.

2

V

430

k

51

R

H