ICE3BRXX65J(Z)(G)
Application Note
16
2010-06-20
6.6.2
Blanking Time for over load protection
The IC controller provides a blanking window before entering into the auto restart mode due to output
overload/short circuit. The purpose is to ensure that the system will not enter protection mode unintentionally.
There are 2 kinds of the blanking time; basic and the extendable. The basic one is a built-in feature which is
set at 20ms. The extendable one is to extend the basic one with a user defined additional blanking time. The
extendable blanking time can be achieved by adding a capacitor, C
BK
to the BA pin. When there is over load
occurred ( V
FB
> 4V ), the C
BK
capacitor will be charged up by a constant current source, I
BK
( 13uA ) from
0.9V to 4.0V. Then the auto restart protection will be activated. The charging time from 0.9V to 4.0V to the
C
BK
capacitor is the extended blanking time. The total blanking time is the addition of the basic and the
extended blanking time.
BK
BK
BK
blanking
C
ms
I
C
ms
Extended
Basic
t
*
5
.
238461
20
*
)
9
.
0
0
.
4
(
20
+
=
−
+
=
+
=
(9)
The measured total blanking time showing in Figure 17 is 45ms using C
BK
=0.1uF.
In case of output overload or short circuit, the transferred power during the blanking period is limited to the
maximum power defined by the value of the sense resistor R
sense
.
The noise level in BA pin can be quite high particularly in some high power application. In order to avoid mis-
triggering of other protection features, it is recommended to add a minimum 100pF filter capacitor at BA pin.
The maximum capacitor added at BA is restricted to be less than 0.65uF. Otherwise, the IC cannot be
startup properly.
Channel 1; C1 : Output Voltage ( V
o
)
Channel 2; C2 : Output current ( I
o
)
Channel 3; C3 : Feedback Voltage ( V
FB
)
Channel 4; C4 : BA voltage ( V
BA
)
Blanking time to enter auto-restart mode : 19ms
Over load protection without extended blanking
time;Cba=100pF @ Vin=85Vac and output power
step from 2.4A to 4A load
Figure 16 Blanking window for output over load protection ( basic blanking time )
Channel 1; C1 : Output Voltage ( V
o
)
Channel 2; C2 : Output current ( I
o
)
Channel 3; C3 : Feedback Voltage ( V
FB
)
Channel 4; C4 : BA voltage ( V
BA
)
Blanking time to enter auto-restart mode : 71ms
Over load protection with extended blanking
time;C7 = 0.22µF @ Vin=85Vac and output power
step from 2.4A to 4A load
Figure 17 Blanking window for output overload protection ( extended blanking time=24ms with
C
BK
=0.1uF )
19ms
52ms
19ms
Vo
Io
V
FB
V
BA
Vo
Io
V
FB
V
BA