L8565L-S08-R datasheet(10/13 Pages) UTC

10 / 13 page

L8565

Preliminary

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

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www.unisonic.com.tw

QW-R114-004.d

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FUNCTIONAL DESCRIPTION(Cont.)

6.2 Current Loop Compensation

The compensation of the current loop is done at the ICOMP pin. This is the OTA2 output and a capacitor C3 has

to be installed at this node to ground (see Fig. 10). Under normal mode of operation, this pin gives a voltage which is

proportional to the averaged inductor current. This pin is internally shorted to 5V in the event of IC shuts down when

OLP and UVLO occur.

6.3 Pulse Width Modulation (PWM)

The IC employs an average current control scheme in continuous conduction mode (CCM) to achieve the power

factor correction.

Assuming the voltage loop is working and output voltage is kept constant, the off duty cycle DOFF for a CCM PFC

system is given as

OUT

OFF =

From the above equation, DOFF is proportional to VIN. The objective of the current loop is to regulate the average

inductor current such that it is proportional to the off duty cycle DOFF, and thus to the input voltage VIN. Fig. 11 shows

the scheme to achieve the objective.

The PWM is performed by the intersection of a ramp signal with the averaged inductor current at pin 3 (ICOMP). The

PWM cycle starts with the Gate turn off for a duration of TOFF(MIN) (250ns typ.) and the ramp is kept discharged. The

ramp is then allowed to rise after TOFF(MIN) expires. The off time of the boost transistor ends at the intersection of the

ramp signal and the averaged current waveform. This results in the proportional relationship between the average

current and the off duty cycle DOFF.

Fig. 12 shows the timing diagrams of TOFF(MIN) and the PWM waveforms.

ramp profile

ave(IIN) at ICOMP

GATE

drive

Fig. 11 Average Current Control in CCM

6.4 Nonlinear Gain Block

The nonlinear gain block controls the amplitude of the regulated inductor current. The input of this block is the

voltage at pin VCOMP. This block has been designed to support the wide input voltage range (85-265VAC).

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