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VIPER12ADIP-E

Low power offline switched-mode power supply primary switcher

STMicroelectronics 

VIPER12A-E

Feedback pin principle of operation

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 Is 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:

R2 ⋅ (IS + IFB) = 0.23V

By extracting IS:

IS

=

0----.--2---3----V--

R2

–

IFB

Using the current sense ratio of the MOSFET GID:

ID

=

GID ⋅ IS

=

GID

⋅

⎛

⎝

0----.--2---3----V--

R2

–

IFB⎠⎞

Doc ID 11977 Rev 2

9/21

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