L6564T datasheet(21/33 Pages) STMICROELECTRONICS

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L6564T

Application information

Doc ID 022671 Rev 1

21/33

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:

where fL is the line frequency. The amount of 3rd harmonic distortion introduced by this

ripple, related to the amplitude of its 2•fL component, is:

Figure 29 shows a diagram that helps choose the time constant RFF·CFF based on the

amount of maximum desired 3rd harmonic distortion. Note that there is a minimum value for

the time constant RFF · CFF below which improper activation of the VFF fast discharge may

occur. In fact, the twice-mains-frequency ripple across CFF under steady-state conditions

must be lower than the minimum line drop detection threshold (

∆VFF_min = 40 mV).

Therefore:

Always connect RFF and CFF to the pin, the IC does not work properly if the pin is either left

floating or connected directly to ground.

Figure 29.

RFF·CFF as a function of 3rd harmonic distortion introduced in the input

current

MULTpk

∆

100

min

VFF

max

MULTpk

−

∆

⋅

D %

0.1

0.01

0.1

f = 50 Hz

f = 60 Hz

· C

[s]

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