VIPER06

FB pin and COMP pin

Doc ID 022794 Rev 1

17/28

12

FB pin and COMP pin

The device can be used both in non-isolated and isolated topology. In non-isolated topology,

the feedback signal from the output voltage is applied directly to the FB pin as the inverting

page 8.

from the compensation network connected on the COMP pin. This signal is then compared

in the PWM comparator with the signal coming from the SenseFET in order to switch off the

power MOSFET on a cycle-by-cycle basis. See the Figure 2 on page 4 and the Figure 22.

When the power supply output voltage is equal to the error amplifier reference voltage,

output voltages the external resistor divider is needed. If the voltage on the FB pin is

accidentally left floating, an internal pull-up protects the controller.

The output of the error amplifier is externally accessible through the COMP pin and it’s used

for the loop compensation, usually an RC network.

As shown in Figure 22, in case of an isolated power supply, the internal error amplifier has to

be disabled (FB pin shorted to GND). In this case an internal resistor is connected between

an internal reference voltage and the COMP pin, see Figure 22. The current loop has to be

closed on the COMP pin through the opto-transistor in parallel with the compensation

page 18.

burst mode, see Section 13 on page 18.

as the deliverable output power, will reach its limit.

Figure 22.

Feedback circuit

FB

COMP

Without Isolation:

switch open & E/A enabled

With Isolation:

switch closed & E/A disabled

No

Isolation

+

-

PWM stop

from R

SENSE

R

Isolation

R

L

nR

SW

V

REF

R

COMP

E/A

BUS

+

-

to PWM

V

COMPL

V

REF_FB