PM8800A

PWM section

23/35

6

PWM section

6.1

Error amplifier and loop compensation

The PM8800A addresses both isolated and non-isolated configuration by embedding a wide

band high gain error amplifier.

In non-isolated topology, the voltage to be regulated is connected to the FB pin - the

inverting input of the EA - through a resistor divider. The non inverting input of the EA is set

to a fixed reference value of 1.25 V. The output of the error amplifier is connected to the

COMP pin which is pulled up internally with a 2.5 k

Ω resistor to a fixed reference of 3.3 V;

loop compensation can be done connecting an appropriate compensation network between

the FB pin and the COMP pin

Figure 13.

Non-isolated feedback

In typical isolated topology, the error amplifier is located outside the IC and the feedback

signal is taken on the collector of an opto coupler. The internal error amplifier is to be

bypassed connecting the FB pin to AGND. In order to minimize external components count

the opto coupler is directly connected to the COMP pin using the internal pull-up resistor as

bias for the opto coupler.

Figure 14.

Isolated opto coupler feedback.

VFB

GND

COMP

Vout

-

+

1.25V

3.3V

2.5kOhm

E/A

AGND

PM8800A

VFB

GND

COMP

Vout

-

+

1.25V

3.3V

2.5k

E/A

AGND

PM8800A