VIPer22A-E, VIPer22ADIP-E, VIPer22AS-E

Operations

Doc ID 12050 Rev 2

9/21

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:

to a negative current sourced by this pin, and expressed by:

By reporting this expression in the previous one, it is possible to obtain the drain current

In a real application, the FB pin is driven with an optocoupler as shown on Figure 5. which

acts as a pull up. So, it is not possible to really short this pin to ground and the above drain

current value is not achievable. Nevertheless, the capacitor C is averaging the voltage on

the FB pin, and when the optocoupler is off (start up or short circuit), it can be assumed that

the corresponding voltage is very close to 0 V.

PWM COMPARATOR SECTION. Actually, as soon as the drain current is about 12 % of

Idlim, that is to say 85 mA, the device will enter a burst mode operation by missing switching

cycles. This is especially important when the converter is lightly loaded.

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

–

⎝⎠

⎛⎞

⋅

==

I

FB

0.23V

R

1

----------------

–

=

I

Dlim

G

ID

0.23V

1

R

2

-------

1

R

1

-------

+

⎝⎠

⎛⎞

⋅⋅

=