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AN628 APPLICATION NOTE

CONTROLLER FUNCTION DESCRIPTION.

The L4981 I.C. controls the conversion process with a continuous mode average current method, using two con-

trol loops (current loop and voltage loop) see fig. 5. Moreover, several internal functions ensure high quality con-

version performance.

A description of the internal blocks will be detailed in the design criteria section and pin description. However,

referring to fig. 4, here below a brief description of the main functions is done:

Multiplier block.

This block produces an output current (programming current) as a product result of four different input signals

(see fig. 13 for details). The multiplier output current, through a resistor connected to the negative side of a

sense resistor, determines the error signal to the current loop.

Figure 5.

Figure 6.

Operational amplifier blocks.

to the multiplier block. The second (C/A), feeds back the line current and produces the reference for the PWM

section.

CURRENT

CONTROL LOOP

POWER

STAGE

VOLTAGE

CONTROL LOOP

LOAD

LINE

D94IN061

L

Raux

REF

Iaux

IPK

Ripk

I

11 8.5V

Iipk

-

+

OSC

S

R

Q

Ipk

1,10

GND

D94IN062A