AN296 Silicon_Laboratories, AN296 Datasheet - Page 11

• Manufacturer Part Number: AN296
• Description: Using THE Si3400 AND Si3401 POE PD Controllers IN Isolated AND Non-isolated Designs
• Manufacturer: Silicon_Laboratories

A typical V
The network of Rc and Cc stabilizes the feedback loop by introducing a zero in the feedback loop. It has been
found that values of 3.3 nF (C7 in Figure 3) and 30.1 kΩ (R7 in Figure 3) work well. This translates to a zero at
1.6 kHz. Finally, the 150 pF capacitor (C19 in Figure 3) gives a pole at 35 kHz for the final gain rolloff. With these
values, a typical Bode plot is shown in Figure 8.
While this is a very conservative design, there is quite a bit of variation in the output capacitor value and ESR,
particularly with temperature. For designs that must operate below 0 °C, it has been found that a low ESR 560 µF
capacitor such as the Panasonic EEU-FM0J561 gives better results with the same stability network.
4.1.3. Soft Start Non-Isolated Case
In the non-isolated case, capacitor CSS (C9 in Figure 3) acts to allow the duty cycle of the switcher FET to
gradually increase. There is an internal impedance of 50 kΩ in the Si3400/01 that works in combination with CSS
to slowly ramp the reference voltage to the error amplifier. A typical CSS of 0.33 nF gives a soft-start time constant
of 10 ms, which is a good value for a 1000 µF output filter capacitor. A typical startup waveform with a 2.5 Ω load is
shown in Figure 5 on page 5.
4.2. Isolated Flyback Design
For the isolated design, a flyback transformer approach is used. In a flyback transformer, the primary inductance is
"charged" when the main switcher FET turns on, and the energy stored in this inductance is delivered to the
secondary when the switcher FET turns off. This type of circuit may be designed to operate in either the continuous
or discontinuous mode. In the continuous mode, current always flows in either the transformer primary or
secondary. In the discontinuous mode, the secondary current drops to zero before the next cycle of primary
current. Typical waveforms are shown in Figure 9.
IN
dB
-20
-40
-60
-80
60
40
20
of 50 V and Cc of 3.3 nF gives a crossover at 8 kHz, ignoring the output filter.
0
magnitude(Gain) in dB
200
500
Figure 8. Typical Bode Plot
1000
2000
Rev. 0.8
phase(Gain) in °
5000
10000
20000
f/Hz
AN296
°
-100
-150
-200
150
100
-50
50
0
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