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AN628 APPLICATION NOTE

APPENDIX A

LFF (pin 6) Function.

Since in Power Factor applications the Error Amp. compensation network has to filter the mains frequency con-

tents, in order to reduce harmonic distortions, the crossover frequency of the loop gain must be low. This in-

volves a poor load transient response.

An additional function (LFF) is available in L4981A/B devices. It is expecially suitable to modify the multiplier

output current, proportionally to the load, in order to improve the system response bypassing the E/A.The control

is working with VLFF voltage between 2V and 5.1V.

In fig. 32 is shown an application example to explain this function. An external OP-AMP has been used to get

the suitable signal voltage avoiding sense resistor (R1) power dissipation.

In the real application the sense resistor is often replaced by sense transformer.

Figure 32. Application example

Design criteria:

It is avisable to ensure a minimum VLFF

≅ 2V at the minimum output current.Since the OP-AMP (LM258) Vol =

0.7V (@ 1mA), to get the minimum voltage at VLFF, 1.3V has to be added. A resistor devider tied to the refer-

ence voltage (pin 11 of the controller) shifts the output of the OP-AMP.

therefore

Ω - 1W R2 is roughly 160KΩ.

L4981

L

Q

D

~

~

VREF=5.1V

pin6

(VLFF)

LOAD

R1

10K

10K

LM258

+

-

R2

3.6K

R3

D95IN206B

1.3V

5.1V

0.7V

–

()

R3

3.6k

Ω

+

------------------------------------ R3

R3

⇒

⋅

1.3k

Ω

==

V

O

R1 I

Omax

1

R2

10k

----------

+





⋅⋅

5.1V

==