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L6563
20/25
eration of the DC-DC stage.
This function is quite flexible and can be used in different ways. In systems comprising an auxiliary con-
verter and a main converter (e.g. desktop PC's silver box or hi-end LCD-TV), where the auxiliary converter
also powers the controllers of the main converter, the pin RUN can be used to start and stop the main
converter. In the simplest case, to enable/disable the PWM controller the PWM_STOP pin can be con-
nected to either the output of the error amplifier (
Figure 43 a
) or, if the chip is provided with it, to its soft-
start pin (
Figure 43
b
). The use of the soft-start pin allows the designer to delay the start-up of the DC-DC
stage with respect to that of the PFC stage, which is often desired. An underlying assumption in order for
that to work properly is that the UVLO thresholds of the PWM controller are certainly higher than those of
the L6563.
Figure 43. Interface circuits that let the L6563 switch on or off a PWM controller (master PFC)
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 ar-
rangement of
Figure 44
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 44. 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 monotoni-
cally. For these reasons it is usually preferable to shutdown the unit in case of brownout.
a)
b)
L6563
10
9
OFF
ON
PWM
controller
X
L5991/A: X=6
UC284x: X=1
UC384x: X=1
L6565:
X=2
RUN
PWM_STOP
C
SS
L6563
OFF
ON
L5991/A
7 SS
10
9
RUN
PWM_STOP
Supply rail
10 k
BC558C
BC337
VZ1
L6563
10
OFF
ON
14
9
7
PWM
controller
L5991/A:
X=8+9
UC284x: X=7
UC384x: X=7
L6565:
X=8
L6598: X=12
X
RUN
Vcc
PWM_STOP
P
HV bus
VZ2
VZ1 < UVLO, VZ2 < 9 V
VZ1 + VZ2 < Vccmax