L6566B View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566B' PDF : 51 Pages View PDF
L6566B
Application information
Figure 26. Voltage sensing techniques to implement brownout protection with the
L6566B
HV Input bus
AC mains (L/N)
RH
AC_OK
16
RL1
VFF
RL
15
RL2
L6566B
Optionalfor
OVPsettings
a)
AC mains (N/L)
RH
AC_OK
16
RH
RL1
VFF
RL
RL2
15
L6566B
CF
b)
Optionalfor
OVP settings
It is typically convenient to use a single divider to bias both the AC_OK and the VFF pins, as
shown in Figure 26: this is possible because in all practical cases the voltage on the VFF pin
is lower than that on the AC_OK pin. Once RH and RL have been found as suggested above,
and kopt, either calculated from (6) or (8) or experimentally found, RL will be split as:
Equation 18
( ) RL2 = k opt RL + RH ; RL1 = RL − RL2
Circuit a) senses the input voltage bus (across the bulk capacitor, downstream the bridge
rectifier); in this case, for a proper operation of the brownout function, VsenON must be lower
than the peak voltage at minimum mains and VsenOFF lower than the minimum voltage on
the input bulk capacitor at minimum mains and maximum load considering, in case, holdup
requirements during mains missing cycles as well. Brownout level will be load-dependent. In
case of latched shutdown, when the input source is removed it is necessary to wait until the
bulk capacitor voltage falls below the start voltage of the HV generator VHVstart in order for
the unit to restart, which may take even several seconds.
Circuit b) senses the mains voltage directly, upstream the bridge rectifier. It can be
configured either for half-wave sensing (only the line/neutral wire is sensed) or full-wave
sensing (both neutral and line are sensed); in the first case, assuming CF is large enough,
the sensed voltage will be equal to 1/π the peak mains voltage, while in the second case it
will be equal to 2/π the peak mains voltage. CF needs to be quite a big capacitor (in the uF)
to have small residual ripple superimposed on the dc level; as a rule-of-thumb, use a time
constant RL ·CF at least 4-5 times the maximum line cycle period in case of half-wave
sensing, 2-3 times in case of full-wave sensing. Then fine tune if needed, considering also
transient conditions such as mains missing cycles. Brownout level will not depend on the
load. When the input source is removed CF will be discharged after some ten ms then this
circuit is suitable to have a quick restart after a latched shutdown.
The AC_OK pin is a high impedance input connected to high value resistors, thus it is prone
to pick up noise, which might alter the OFF threshold when the converter is running or give
origin to undesired switch-off of the device during ESD tests. It is possible to bypass the pin
to ground with a small film capacitor (e.g. 1-10 nF) to prevent any malfunctioning of this kind.
The voltage on the pin is clamped upwards at about 3.15 V; then, if the function is not used
the pin has to be connected to Vcc through a resistor (220 to 680 kΩ).
39/51
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