PM8800 STMicroelectronics, PM8800 Datasheet - Page 24

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

PWM section
6.2
24/35
When a shunt regulator is used for output voltage regulation, the output voltage is set by the
ratio of resistors R1 and R2, see
following equation:
where V
Loop compensation in typical isolated application is done by connecting an appropriate
compensation network around the external error amplifier.
A pull-up current source of few nA is internally connected to FB pin providing a safe switch-
off of the DC DC converter in case of feedback disconnection.
Oscillator and sync capability
The internal oscillator frequency can be programmed by connecting an external resistor
between the RT and AGND pins. The relation between the oscillator frequency f
R
Figure 15. PWM frequency vs R
The PWM switching frequency is equal to the programmed oscillator frequency.
V
f
osc
T
out
resistor is:
(
kHz
=
V
REF
)
REF
=
100
is the reference voltage of the shunt regulator chosen for the application.
⋅
⎛
⎝
1
1000
+
⋅
⎛
⎝
900
800
700
600
500
400
300
200
100
R
------ -
R
1
2
1
+
⎞
⎠
0
------------------------------------- -
3kΩ
0
+
125
R
T
(
kΩ
Figure 14
)
⎞
⎠
100
T
for details. The output voltage is given by the
RT [kOhm]
200
300
400
OSC
PM8800A
and the