L6591

Application information

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Figure 13.

Oscillator waveforms and their relationship with gate-driving signals

5.7

Adaptive UVLO

A major problem when optimizing a converter for minimum no-load consumption is that the

voltage generated by the self-supply system under these conditions falls considerably as

compared even to a few mA load. This very often causes the IC's supply voltage Vcc to drop

and go below the UVLO threshold of the controller so that the operation becomes

intermittent, which is undesired. A low UVLO threshold would be helpful but it could be an

issue to drive the MOSFETS with a sufficient gate-drive voltage at heavy load during power

off.

To help the designer overcome this problem, the L6591, besides reducing its own

consumption during burst-mode operation, also features a proprietary adaptive UVLO

function. It consists of shifting the UVLO threshold downwards at light load, namely when

more headroom. To prevent any malfunctioning during transients from minimum to

maximum load the normal (higher) UVLO threshold is re-established when the voltage at pin

UVLO threshold (see Figure 16). The normal UVLO threshold ensures that under heavy

load conditions the MOSFETS will be driven with an appropriate gate voltage.

Figure 14.

Adaptive UVLO block

HVG

t

t

LVG

t

Clock

t

 

UVLO

Vcc

L6591

COMP

R

S

Q

SW

9

7

PFC_ ST OP

logic

PFC_STOP

8

VCOMP

Vcc

Q

t

t

t