VIPer22ADIP / VIPer22AS

9/15

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 9 presents the internal current

mode structure.

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:

The current limitation is obtained with the FB pin

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 limitation

In a real application, the FB pin is driven with an

optocoupler as shown on figure 9 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.

For low drain currents, the formula (1) is valid as

this threshold the device will stop switching. This is

specified in the 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.

It is then possible to build the total DC transfer

This figure also takes into account the internal

blanking time and its associated minimum turn on

time. This imposes a minimum drain current under

which the device is no more able to control it in a

linear way. This drain current depends on the

primary inductance value of the transformer and

the

input

voltage.

Two

cases

may

occur,

depending on the value of this current versus the

fixed 85 mA value, as described above.

START UP SEQUENCE

This device includes a high voltage start up current

source connected on the drain of the device. As

soon as a voltage is applied on the input of the

converter, this start up current source is activated

switched off and the device begins to operate by

turning on and off its main power MOSFET. As the

FB pin does not receive any current from the

optocoupler, the device operates at full current

capacity and the output voltage rises until reaching

Figure 9 : Internal Current Control Structure

60kHz

OSCILLATOR

PWM

LATCH

S

Q

R

0.23V

Id

DRAIN

SOURCE

FB

R1

R2

C

+Vdd

Secondary

feedback

I

FB

Is

1 k

Ω

230

Ω

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

Dl im

G

ID

0.23V

1

R

2

------

1

R

1

------

+





⋅⋅

=

t

ONmin

V

2

⋅

IN

L

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

t

ONmin

V

1

⋅

IN

L

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

85mA

0

Part masked by the