AN628 APPLICATION NOTE

34/35

APPENDIX B

Disable

Sometimes it is useful to disable the controller. For example, in a complete system in which a PWM regulator

follows the PFC stage, at low output power it is advantageous to shutdown the PFC section to improve the over-

all system efficiency (stand-by / sleep mode). Likewise most of controllers, one way to do this (using L4981A/

B), is pulling down either the Soft-Start or the E/A output pin . In addition the L4981A/B can be disabled ground-

ing the P-UVLO (pin 15) see fig 33.The P-UVLO function has been designed to program the supply thresholds

by means of an external divider (see application note for details) but it can be effectively used for this purpose

forcing a voltage below the internal reference (1.28V).Besides turning off the driver output stage this method

puts the controller in "before start-up" condition and gives the advantage of minimizing the supply consumption

of the IC.

Figure 33.

+

-

UVLO

1.28V

19

P-UVLO

15

10

D95IN281B

I

≥ 1mA

DISABLE