Datasheet

VIPER12A-E Feedback pin principle of operation
Doc ID 11977 Rev 2 9/21
6 Feedback pin principle of operation
A feedback pin controls the operation of the device. Unlike conventional PWM control
circuits which use a voltage input (the inverted input of an operational amplifier), the FB pin
is sensitive to current. Figure 5. presents the internal current mode structure.
Figure 5. Internal current control structure
The power MOSFET delivers a sense current I
s
which is proportional to the main current Id.
R2 receives this current and the current coming from the FB pin. The voltage across R2 is
then compared to a fixed reference voltage of about 0.23 V. The MOSFET is switched off
when the following equation is reached:
By extracting I
S
:
Using the current sense ratio of the MOSFET G
ID
:
R
2
I
S
I
FB
+()⋅ 0.23V=
I
S
0.23V
R
2
--------------- - I
FB
–=
I
D
G
ID
I
S
⋅ G
ID
0.23V
R
2
--------------- - I
FB
–
⎝⎠
⎛⎞
⋅==