Datasheet

Operation description VIPER37
30/35 Doc ID 022218 Rev 1
When the V
INon
and the V
INoff
levels are fixed, with reference to Figure 33 , the following
relationships can be established for the calculation of the resistors R
H
and R
L
:
Equation 9
Equation 10
For a proper operation of this function, V
IN on
must be less than the peak voltage at
minimum mains and V
IN off
less than the minimum voltage on the input bulk capacitor at
minimum mains and maximum load.
The BR pin is a high impedance input connected to high value resistors, it is therefore prone
to pick up noise, which might alter the OFF threshold when the converter operates or
creates an 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.
If the brownout function is not used, the BR pin must be connected to GND, ensuring that
the voltage is lower than the minimum V
DIS
threshold (50 mV), see Tabl e 8 .
Figure 33. Brownout protection: BR external setting and timing diagram


'LVDEOH
&
%5
9
',6


9
LQB2.
9
%5WK
5
+
,
%5K\VW
5
/
9
,1B'&
9''

6
/54
6
).
6
).ON
6
).OFF
6
$2!).?34!24
6
"2
6
IN?/+
)
"2
6
"2TH
)
"2HYST
6
$$
6
$$ON
6
$$OFF
6
$3
6
$$2%34!24
BRhyst
BRth
BRthINoff
BRhystINoffINon
BRhyst
BRhyst
L
I
V
VV
VVV
I
V
R ×
−
−
−
+−=
BRhyst
BRhyst
L
L
BRhyst
BRhystINoffINon
H
I
V
R
R
I
VVV
R
+
×
−−
=