L6563 datasheet(20/37 Pages) STMICROELECTRONICS

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Application information

L6563 - L6563A

20/37

The twice-mains-frequency (2·fL) ripple appearing across CFF is triangular with a peak-to-

peak amplitude that, with good approximation, is given by:

Equation 4

where fL is the line frequency. The amount of 3

rd harmonic distortion introduced by this

ripple, related to the amplitude of its 2·fL component, will be:

Equation 5

Figure 35 shows a diagram that helps choose the time constant RFF·CFF based on the

amount of maximum desired 3rd harmonic distortion. Always connect RFF and CFF to the

pin, the IC will not work properly if the pin is either left floating or connected directly to

ground.

Figure 35. RFF·CFF as a function of 3

rd harmonic distortion introduced in the input

current

The dynamics of the voltage feedforward input is limited downwards at 0.5V (see Figure 34),

that is the output of the multiplier will not increase any more if the voltage on the VFF pin is

below 0.5V. This helps to prevent excessive power flow when the line voltage is lower than

the minimum specified value

∆

MULTpk

14f

LRFFCFF

---------------------------------------

100

πf

LRFFCFF

---------------------------------

D %

0.1

0.01

0.1

f =50Hz

f =60Hz

R · C

[s]

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