VIPER26

FB pin and COMP pin

Doc ID 17736 Rev 2

15/25

12

FB pin and COMP pin

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

isolated topology, the feedback signal from the output voltage is applied directly to the FB

see the

Table 8 on page 7.

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; the power MOSFET is

switched off when the two values are the same on cycle by cycle basis. See the

Figure 2 on

page 3 and the Figure 23 on page 15.

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 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 reported on

Figure 23 on page 15, in case of 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 the

Figure 23 on

page 15. The current loop has to be closed on the COMP pin through the opto-transistor in

burst mode, see

Section 13 on page 16.

reach its limit, as well as the deliverable output power

Figure 23.

Feedback circuit

FB

COMP

Without Isolation:

switch open & E/A enabled

With Isolation:

switch closed & E/A disabled

No

Isolation

V

OUT

+

-

PWM stop

from R

SENSE

R

Isolation

R

L

nR

SW

V

REF

R

COMP

E/A

BUS

+

-

to PWM

V

COMPL

R

H

V

REF_FB