Application information

L6591

16/32

Figure 8.

Load-dependent operating modes: timing diagram

Figure 9.

Addition of an offset to the current sense lowers the burst-mode

operation threshold

To help the designer meet energy saving requirements even in power-factor-corrected

systems, where a PFC pre-regulator precedes the DC-DC converter, the L6591 allows that

the PFC pre-regulator can be turned off during burst-mode operation, hence eliminating the

no-load consumption of this stage (0.5

÷1 W). There is no compliance issue in that because

EMC regulations on low-frequency harmonic emissions refer to nominal load, no limit is

envisaged when the converter operates with light or no load.

To do so, the L6591 provides the PFC_STOP (#8) pin: it is an open collector output,

normally open, that is asserted low when the IC is idle during burst-mode operation. This

signal will be externally used for switching off the PFC controller and the pre-regulator as

shown in Figure 10. When the L6591 is in UVLO the pin is kept open, to let the PFC

controller starts first.

LVG, HVG

1.82V

t

t

t

COMP

Fix. Fr eq. M ode

Burst-mode

Fi x. Freq. Mode

PFC_STOP

PFC

GATE-DR IVE

1.75V

 

Rs

L6591

R

Vref

Rc

6

10

4

11

R

R + Rc

LVG

ISEN

GND

Rs

L6591

R

Vref

Rc

6

10

4

11

R

R + Rc

LVG

ISEN

GND