L6566BHTR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566BHTR' PDF : 51 Pages View PDF
Application information
L6566BH
Figure 26. Voltage sensing techniques to implement brownout protection with the
L6566BH
38/51
HV In p ut bus
RH
AC_OK
16
RL1
VFF
RL
15
RL2
L6566BH
Optional for
OVP settings
a)
AC mains (L/N)
AC mains (N/L) RH
AC_OK
16
RH
RL1
VFF
RL
RL2
15
L6566BH
CF
b)
Optionalfor
OVP settings
AM11502v1
It is usually 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 is split as:
Equation 18
( ) RL2 = k opt RL + RH ; RL1 = RL − RL2
Circuit a) senses the input voltage bus (across the bulk capacitor, downstream of 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, if necessary,
holdup requirements during mains missing cycles as well. Brownout level is 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 up to several seconds.
Circuit b) senses the mains voltage directly, upstream of 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 is equal to 1/π the peak mains voltage, while in the second case it is
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 does not depend on the load.
When the input source is removed, CF is discharged after some ten ms then this circuit is
suitable for a quick restart after a latched shutdown.
The AC_OK pin is a high impedance input connected to high value resistors, therefore it is
prone to pick-up noise, which might alter the OFF threshold when the converter is running or
lead 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.
Doc ID 16610 Rev 2
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