H2

(x) =

1

π

⋅







∫

0

1

+ x ⋅ sin(θ)







d

θ ≈

0.25

1

+ 1.074 ⋅ x

(19),

can be expressed as:

H2

F2

.

The absolute value in (19) is needed since the integral results negative, because the harmonic is 180°

out of phase. Finally, the peak-to-peak amplitude of the low frequency output ripple is:

1

π

⋅

H2

F2

⋅

.

In most cases, once a capacitor is selected so as to meet the requirement on the low frequency ripple,

the ESR will be low enough to make the high frequency ripple negligible.

Multiplier Bias and Sense Resistor Selection

A resistor divider feeds a portion of the input voltage into pin 3 (MULT) to build the sinusoidal reference

for the peak primary current. To set properly the operating point of the multiplier the following procedure

is recommended.

mum mains voltage, should be 2.5 to 3V in wide range mains applications and 1 to 1.5V in case of single

mains. The minimum peak value, occurring at minimum mains voltage will be:

This value, multiplied by the minimum guaranteed

age of the multiplier:

In this way, the divider ratio will be:

=

and the individual resistor values can be chosen by setting the current through them, in the hundreds

µA

or less, to minimise power dissipation.

The value of the sense resistor, connected between the source of the MOSFET and ground, across

which the L6561 reads the primary current, is calculated as follows:

≤

.

The resistor will be rated for a power dissipation equal to:

2

⋅

F2

3

AN1059 APPLICATION NOTE

12/20