PM8800 STMicroelectronics, PM8800 Datasheet - Page 23

• Manufacturer Part Number: PM8800
• Description: Integrated IEEE 802.3af compliant PoE-PD interface and PWM controller with support of external source
• Manufacturer: STMicroelectronics

PM8800A
6
6.1
PWM section
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 kΩ 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.
3.3V
2.5k
3.3V
E/A
2.5kOhm
+
-
PM8800
E/A
1.25V
+
-
COMP
AGND
PM8800 A
GND
1.25V
VFB
A
COMP
AGND
GND
VFB
Z
COMP
R
Z
Vout
C
C
R
R
Vout
2
1
R
R
1
2
PWM section
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