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

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:

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

–

⎝⎠

⎛⎞

⋅

==