L6563ATR datasheet(29/37 Pages) STMICROELECTRONICS

29 / 37 page

L6563 - L6563A

Application information

29/37

Figure 42. Interface circuits that let the L6563/A switch on or off a PWM controller

If this is not the case or it is not possible to achieve a start-up delay long enough (because

this prevents the DC-DC stage from starting up correctly) or, simply, the PWM controller is

devoid of soft start, the arrangement of Figure 43 lets the DC-DC converter start-up when

the voltage generated by the PFC stage reaches a preset value. The technique relies on the

UVLO thresholds of the PWM controller.

Figure 43. Interface circuits for actual power-up sequencing (master PFC)

Another possible use of the RUN and PWM_STOP pins (again, in systems where the PFC

stage is the master) is brownout protection, thanks to the hysteresis provided.

Brownout protection is basically a not-latched device shutdown function that must be

activated when a condition of mains undervoltage is detected. This condition may cause

overheating of the primary power section due to an excess of RMS current. Brownout can

also cause the PFC pre-regulator to work open loop and this could be dangerous to the PFC

stage itself and the downstream converter, should the input voltage return abruptly to its

rated value. Another problem is the spurious restarts that may occur during converter power

down and that cause the output voltage of the converter not to decay to zero monotonically.

For these reasons it is usually preferable to shutdown the unit in case of brownout.

L6563ATR Datasheet(PDF) 29 Page - STMicroelectronics