AN331
C
OMPENSATING THE
F
EEDBACK
L
OOP FOR THE
Si3400
A N D
Si3 4 01
1. Introduction
The Si3400 and Si3401 reference designs are available for many output voltages (e.g., 3.3, 5, 9, 12 V) and output
capacitor types. In general, Silicon Laboratories strongly recommends using these standard designs to minimize
risk and ensure robust performance. Refer to the design databases posted on the Si3400/01 documentation page
on the Silicon Labs website for more information:
EVB Data Sheets
EVB Reference Design Databases (Schematics and Layout)
However, some designers may want to consider other cases of output filtering, input filtering, inductors, etc. for a
variety of reasons (cost, footprint, availability, etc.).
While it would be desirable to use circuit simulation to optimize the feedback loop, it is very difficult to get reliable
information about important factors such as capacitor ESR. Also, the stabilizing effect of the input side hot-swap
switch and input filter ESR must be taken into account, which is not straightforward for commonly available SPICE
implementations. For these reasons, the feedback loop must be experimentally optimized if a known reference
design is not used.
The application note outlines the general process for compensating the feedback loop experimentally. In case a
predefined compensation and output filter is not used, it is strongly recommended that this procedure be followed
to ensure robust performance.
1.1. Breaking the feedback loop
The feedback loop is broken and a transformer is used to inject an ac signal across the break. Using a transformer
allows the loop stability to be measured in a closed loop system with whatever load a filtering is present. The loop
is broken at the output and at the point sensing the output voltage. The transformer ac and dc impedance must be
small compared to the impedance sensing the output voltage.
Figures 1 and 2 show the recommended transformer placement for the non-isolated and isolated reference
designs.
Rev. 0.1 9/07
Copyright © 2007 by Silicon Laboratories
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