AN3009

Designing a transition mode PFC

Doc ID 16032 Rev 4

21/36

Equation 38

Equation 39

suitable voltage rating (>400 V) is needed, or else additional in-series resistors must be

used.

Also note that the maximum value of the resistor divider is limited by the L6564’s INV pin

input bias current given in the datasheet. To guarantee correct output voltage regulation, the

current flowing in the resistor divider must be significantly higher than the current flowing

into the pin.

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

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

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

flowing into the divider, we obtain:

Equation 40

By selecting a commercial value of 51 k

Ω, we then get:

Equation 41

By connecting in series two 3.3 M

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

obtained.

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

(PFC_OK) can be selected without any constraints. The unique criterion is that both dividers

have to sink a current from the output bus, which needs to be significantly higher than the

current biasing the error amplifier and PFC_OK comparator.

The OVP function described above can handle “normal” overvoltage conditions, that is,

those resulting from an abrupt load/line change or occurring at start-up. If the overvoltage is

generated by a feedback disconnection for instance, when one of the upper resistors of the

output divider fails to open, an additional circuitry detects the voltage drop of pin INV. If the

voltage on pin INV is lower than 1.66 V (typ.) and at same time the OVP is active, a feedback

failure is assumed.

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

OVP

L

H

Ω

=

⎟⎟⎠

⎞

⎜⎜⎝

⎛

−

⋅

Ω

=

M

721

.

8

1

V

5

.

2

V

430

k

51