AN296 Silicon_Laboratories, AN296 Datasheet - Page 10

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

AN296
Where L is the inductance.
L = 33 µH, Vout = 3.3 V, V
F is the internally-set switch frequency of approximately 350 kHz.
I
This is the ripple current into the output filter. The peak-to-peak ripple current that must be handled by the input
filter is equal to the average current delivered to the output plus half of the ripple current in the inductor.
The rectifier diode in the non-isolated design must be rated for at least the input voltage. Generally, a 100 V diode
is chosen for margin. A Schottky diode is preferred to avoid the large voltage drop and excess power associated
with stored charge. Typical part numbers are PDS5100 from Diodes Incorporated or the equivalent UPS5100 from
Microsemi. Note that these 100 V diodes have a larger forward drop than the lower voltage diodes used for the
non-isolated design below.
Overall efficiency is determined by dividing the output power by the input power including conduction losses in the
inductor, rectifier, switching FET, input bridge, and hot swap switch, as well as bias and switching losses.
4.1.1. Output Voltage—Non-Isolated Design
The output voltage in the isolated case is determined by R3 and R4 according to the following equation:
For example, for a 5 V output, values of 8.66 kΩ for R6 and 2.87 kΩ for R5 are recommended.
4.1.2. Output Filter and Loop Stability—Non-Isolated Design
Generally, the current in the output inductor is continuous (does not return to zero). The current becomes
discontinuous for very light loads, but the continuous mode of operation is most difficult to stabilize due to the LC
filter resonance that occurs in this case.
The output filter section has a resonant frequency described by the following equation:
The circuit will be critically damped with a resistance of:
For a typical 33 µH inductor and 1000 µF filter cap, the resonant frequency is 876 Hz, and the resistance for critical
damping is 0.36 Ω.
The damping resistance is a combination of capacitor ESR, inductor series resistance, and switch and diode
resistance. It has been found that the combination of switcher FET resistance and Schottky diode effective series
resistance results in an effective 0.5–1 Ω in series with the inductance for the recommended applications circuit.
This damps the output resonance and allows for the use of low ESR filter capacitors without stability concerns.
The error amplifier is a transconductance type amplifier with gm = 50 mmho. Capacitor Cc provides roll-off of the
loop gain at
where 1.5 V is the voltage swing at ER
10
ripple
V
2
F
------------------------------ -
2 π
out
= 321 mA
×
×
=
=
--- -
C
L
------------
2πc
1
×
g
1.23
m
LC
c
×
×
------------- -
1.5 V
⎛
⎝
V
1
IN
+
R6
------- -
R5
in
= 55 V, V
⎞
⎠
f
OUT
= 0.7 V
that would vary the duty cycle from zero to 100%.
Rev. 0.8