Datasheet

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
input of the internal error amplifier having the reference voltage, V
REF_FB,
see Ta bl e 7 on
page 8.
The output of the error amplifier sources and sinks the current, I
COMP
, respectively to and
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,
V
REF_FB
, a single resistor has to be connected from the output to the FB pin. For higher
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
network. The V
COMP
dynamic range is between V
COMPL
and V
COMPH
shown in Figure 23 on
page 18.
When the voltage V
COMP
drops below the voltage threshold V
COMPL
, the converter enters
burst mode, see Section 13 on page 18.
When the voltage V
COMP
rises above the V
COMPH
threshold, the peak drain current, as well
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
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