AN3112

Designing a fixed-off-time PFC

Doc ID 16820 Rev 3

25/36

In this way, in the case of a sudden line voltage rise, CFF is rapidly charged through the low

impedance of the internal diode; in case of line voltage drop, an internal mains drop detector

enables a low impedance switch which suddenly discharges CFF, avoiding long settling time

before reaching the new voltage level. Consequently an acceptably low steady-state ripple

and low current distortion can be achieved without any considerable undershoot or

overshoot on the pre-regulator's output, like in systems with no feed-forward compensation

This pin is internally connected to a comparator in order to provide the brownout (AC mains

undervoltage) protection. A voltage below 0.8 V shuts down (not latched) the IC and brings

its consumption to a considerably lower level. The IC restarts when the voltage at the pin

rises above 0.88 V. These details must be taken into account during the MULT divider

selection.

The suggested procedure to properly set the operating point of the multiplier is now

which occurs at maximum mains voltage, should be 3 V or thereabouts in wide range mains

and less in single mains. The sense resistor selected is Rs = 0.12

Ω and it is described in

the pin 4 section. According to the L6564 datasheet and to the linearity setting of the pin, the

maximum voltage on the multiplier input is:

From

(16) the maximum required divider ratio is calculated as:

Equation 49

Supposing a 60 µA current flowing into the multiplier divider the lower resistor value can be

calculated:

Equation 50

A commercial value of 51 k

Ω for the lower resistor is selected. The upper resistor value can

now be calculated:

Equation 51

In this example a RmultH = 6.9 M

Ω and a RmultL = 51 kΩ can be selected. For RmultH a

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

used.

The voltage on the multiplier pin with the selected component values re-calculated at

minimum line voltage is 0.93 V and at maximum line voltage is 2.74 V. So the multiplier

works correctly within its linear region.

(16)

V

3

3

max

max

MULT

p

10

8

Vac

265

2

V

00

.

3

VAC

2

V

k

−

⋅

=

⋅

=

⋅

=

Ω

=

µ

=

µ

=

k

50

A

60

V

00

.

3

A

60

V

R

max

MULT

multL

Ω

=

Ω

⋅

⋅

−

=

−

=

−

−

M

319

.

6

k

51

10

8

10

8

1

R

k

k

1

R

3

3

multL

p

p

multH