AN296 Silicon_Laboratories, AN296 Datasheet - Page 14

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

AN296
Increasing the turns ratio will increase snubber power. Therefore, there is an optimal turns ratio that compromises
between high peak current at a low turns ratio and high snubber power at a high turns ratio.
Silicon Laboratories, Inc. has partnered with Coilcraft to develop flyback transformers that are optimized for
maximum efficiency of the Si3400/01 at 3.3, 5, and 12 V output. Recommended part numbers are FA2924-AL for
3.3 V (40 µH and 1:0.3 turns ratio), FA2805-CL for 5 V (40 µH and 1:0.4 turns ratio), and FA2925-AL for 12 V
(40 µH and 1:1 turns ratio). Contact Silicon Laboratories for other output supply configurations and
recommendations.
The rectifier for 3.3 or 5 V output does not need as high a voltage rating because the transformer turns ratio limits
the reverse voltage to (1/N) x Vin. The PDS1040 from diodes incorporated or the equivalent UPS1040 from
Microsemi can be used, and these parts have much lower forward drop and overall loss due to their lower voltage
rating of 40 V. For 12 V output, the PS5100 is recommended.
4.2.1. Output Voltage—Isolated Design
In the isolated design, a TLV431 (U5 in Figure 4) is used as an isolated reference voltage. The TLV431 is available
from many suppliers and regulates at a reference voltage of 1.24 V; so, the output voltage is:
V
= 1.24 x (1+R5/R6)
out
An opto-isolator, such as PS2911 (U6 in Figure 4), which is also available from many suppliers, is used to couple
the error signal back to the Si3400/01.
4.2.2. Output Filter and Loop Stability—Isolated Design
In the flyback design, even if the transformer current always flows in the transformer primary or secondary (i.e. is
continuous), the secondary current does not flow during the time that primary current flows; thus, there is always a
large ripple current in the output that must be filtered. For the isolated design, it is recommended that a pi-section
filter be used with a 1.0 µH inductor, such as Coilcraft D01608-102ML.
The feedback compensation for the isolated case was chosen to be type 3 for improved load transient response.
The initial pole in the feedback path is determined by the Miller multiplied capacitance of C9 working against the R8
plus Thevin equivalent resistance of the voltage sense divider R5//R6.
Pole = 1/(2 x π x C9 x TLV431 gain x (R5 x R6)/(R6+R6) + R8)
A typical value of R8 in parallel with R6 is 9.16 kΩ, TLV431 gain = 700, and C41 = 15 nF. These values set this
pole at about 0.8 Hz. Variations in the TLV 431 gain increase open loop gain and decrease the pole value so that
the roll-off from this pole tends to be independent of TLV431 gain.
The overall design is that the C9-R8 zero compensates the R11-C21 pole so:
C9 x R8 approximately = R11 x C21
The double zero for phase margin is determined by C21-R12 and C8 against the Thevin equivalent impedance of
R5//R6. Recommended values of C8 = 560 pF and R12 = 100 Ω put the double zero at about 7 kHz. A final pole
determined by C8 and R9 at 95 kHz eliminates high-frequency gain peaking. Typical Bode plots are shown in
Figure 10 for the continuous case.
This compensation results in the load transient response of Figures 11 and 12 for a worst-case load transient that
starts at very light load (discontinuous case) and ends with a heavy load (continuous case). Further optimization of
this result is possible with larger or lower ESR output filter capacitors.
4.2.3. Soft Start Isolated Case
For the isolated case, a capacitor is connected between pin ISOSSFT and V
. A typical value is 1.0 µF. This
SS
feature is available beginning with Rev. E silicon. Silicon Labs does not recommend disabling isolated soft start.
For revisions before Rev. E, tie this pin to V
.
DD
A typical trace of isolated mode start-up is shown in Figure 13.
14
Rev. 0.8