VIPER27

Operation descriptions

Doc ID 15133 Rev 5

21/31

off the converter itself.

To avoid this event, the appropriate feedback network has to be selected according to the

output load. More the network feedback fixes the compensation loop stability. The

Figure 28

on page 22 and Figure 29 show the two different feedback networks.

formula:

Equation 5

In the

as well as it needs to activate the over load protection (see equation 5).

capacitor could not be at its nominal value and the controller interprets this situation as an

over load condition. In this case, the OLP delay helps to avoid an incorrect device shut down

during the start up phase.

Owing to the above considerations, the OLP delay time must be long enough to by-pass the

initial output voltage transient and check the over load condition only when the output

voltage is in steady state. The output transient time depends from the value of the output

capacitor and from the load.

ensure enough OLP delay, an alternative compensation network can be used and it is

showed in

Figure 29 on page 22.

is usually used to compensate the high frequency zero due to the ESR (equivalent series

resistor) of the output capacitance of the fly-back converter.

The mathematical expressions of these poles and zero frequency, considering the scheme

in

Figure 29 are reported by the equations below:

Equation 6

T

OLP

delay

–

C

FB

V

FBolp

V

FBlin

–

3

μA

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

×

=

1

FB

1

FB

ZFB

R

C

2

1

f

⋅

⋅

π

⋅

=