參數(shù)資料
型號： L6566BTR
廠商： 意法半導(dǎo)體
英文描述： Multi-mode controller for SMPS
中文描述： 多模式開關(guān)電源控制器
文件頁數(shù)： 39/51頁
文件大?。?/td> 401K
代理商： L6566BTR
L6566B
Application information
39/51
Figure 26. Voltage sensing techniques to implement brownout protection with the
L6566B
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 R
H
and R
L
have been found as suggested above,
and k
opt
, either calculated from (6) or (8) or experimentally found, R
L
will be split as:
Equation 18
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, Vsen
ON
must be lower
than the peak voltage at minimum mains and Vsen
OFF
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 V
HVstart
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 C
F
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. C
F
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 R
L
·C
F
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 C
F
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 k
).
HV Input bus
AC_OK
16
R
H
R
L
VFF
15
Optionalfor
OVPsettings
R
L1
R
L2
L6566B
a)
b)
AC mains (L/N)
16
R
H
R
L
15
AC_OK
VFF
R
L1
R
L2
L6566B
Optionalfor
OVP settings
AC mains (N/L)
R
H
C
F
(
)
2
L
L
1
L
H
L
opt
2
L
R
R
R
;
R
R
k
R
=
+
=
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