L6566A View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566A' PDF : 52 Pages View PDF
Application information
L6566A
While the brownout protection is active the startup generator keeps on working but, there
being no PWM activity, the Vcc voltage continuously oscillates between the startup and the
HV generator restart thresholds, as shown in the timing diagram of Figure 25.
The brownout comparator is provided with current hysteresis in addition to voltage
hysteresis: an internal 15 µA current sink is ON as long as the voltage applied on the
AC_OK pin is such that the AC_FAIL signal is high. This approach provides an additional
degree of freedom: it is possible to set the ON threshold and the OFF threshold separately
by properly choosing the resistors of the external divider (see Equation 13 and 14 below).
With just voltage hysteresis, instead, fixing one threshold automatically fixes the other one
depending on the built-in hysteresis of the comparator.
With reference to Figure 25, the following relationships can be established for the ON
(VsenON) and OFF (VsenOFF) thresholds of the sensed voltage:
Equation 13
Vsen ON − 0.485 = 15 ⋅ 10 −6 + 0.485
RH
RL
Vsen OFF − 0.45 = 0.45
RH
RL
which, solved for RH and RL, yield:
Equation 14
RH
=
Vsen ON
− 1.078 ⋅ Vsen OFF
15 ⋅ 10 −6
;
RL
= RH
0.45
Vsen OFF − 0.45
Figure 26. AC voltage sensing with the L6566A
Rectified
input voltage
Q
Sensed
Vcc
voltage:
5
Vsen < 7V
L6561
L6562/A MULT
RH
L6563 3
RL
CF
AC_OK
L6566A
16
For minimum
temperature drift
Q
38/52
It is usually convenient to not use additional dividers connected to high-voltage rails
because this could make it difficult to meet no-load consumption targets envisaged by
energy-saving regulations. Figure 26 shows a simple voltage sensing technique that makes
use of the divider already used by the PFC control chip to sense the AC mains voltage with
just the addition of an extra tap.
The small-signal NPN Q and the capacitor CF create a peak detector, so that the information
of the RMS mains voltage can be found across CF. The tap position determines the DC
voltage to be sensed by the AC_OK pin. It is convenient to use a level as high as possible to
minimize the effect of VBE changes with temperature. However, it may be necessary to limit
the maximum sensed voltage below 7 V to prevent Q’s emitter reverse breakdown; it would
not be destructive because the reverse current would be quite small (the resistors seen by
Doc ID 13794 Rev 4
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