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PM8800ATR Datasheet(PDF) 24 Page - STMicroelectronics

Part # PM8800ATR
Description  Integrated IEEE 802.3af compliant PoE-PD interface and PWM controller with support of external source
PDF  35 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

PM8800ATR Datasheet(HTML) 24 Page - STMicroelectronics

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PWM section
PM8800A
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 Figure 14 for details. The output voltage is given by the
following equation:
where VREF is the reference voltage of the shunt regulator chosen for the application.
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.
6.2
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 fOSC and the
RT resistor is:
Figure 15.
PWM frequency vs RT
The PWM switching frequency is equal to the programmed oscillator frequency.
V
out
V
REF
1
R
2
R
1
-------
+
⎝⎠
⎛⎞
⋅
=
f
osc kHz
()
100
1
125
3k
Ω R
+
T kΩ
()
--------------------------------------
+
⎝⎠
⎛⎞
⋅
=
0
100
200
300
400
500
600
700
800
900
1000
0
100
200
300
400
RT [kOhm ]



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