Datasheet

LC
OUT OUT
L C
1
¦ =
2p
Z
R C
1
1
¦ =
2p 3 6
Z
R C
2
1
¦ =
2p 1 8
P
R C
1
1
¦ =
2p 5 8
P
R C
2
1
¦ =
2p 3 7
INT
R C
1
¦ =
2p 1 6
-
CO
INT
0.74
10 ´ ¦
¦ =
2
INT
C
R
1
6 =
2p 1¦
LC
R
C
1
1=
2p 6¦
TPS54110
www.ti.com SLVS500C –DECEMBER 2003– REVISED FEBRUARY 2011
There are a number of different ways to design a compensation network. This procedure outlines a relatively
simple procedure that produces good results with most output filter combinations. Use the SWIFT Designer
Software for designs with unusually high closed-loop crossover frequencies; with low-value, low-ESR output
capacitors such as ceramics; or if you are unsure about the design procedure.
A number of considerations apply when designing compensation networks for the TPS54110. The compensated
error-amplifier gain must not be limited by the open-loop amplifier gain characteristics and must not produce
excessive gain at the switching frequency. Also, the closed-loop crossover frequency must be set less than one
fifth of the switching frequency, and the phase margin at crossover must be greater than 45 degrees. The
general procedure outlined here meets these requirements without going into great detail about the theory of
loop compensation.
First, calculate the output filter LC corner frequency using Equation 10:
(10)
For the design example, ƒ
LC
= 6103 Hz.
Choose a closed-loop crossover frequency greater than f
LC
and less than one fifth of the switching frequency.
Also, keep the crossover frequency below 100 kHz, as the error amplifier may not provide the desired gain at
higher frequencies. The 60-kHz crossover frequency chosen for this design provides comparatively wide loop
bandwidth while still allowing adequate phase boost to ensure stability.
Next, the values for the compensation components that set the poles and zeros of the compensation network are
calculated. Assuming an R1 value > than R5 and a C6 value > C7, the pole and zero locations are given by
Equation 11 through Equation 14:
(11)
(12)
(13)
(14)
Additionally there is a pole at the origin, which has unity gain at a frequency:
(15)
This pole is used to set the overall gain of the compensated error amplifier and determines the closed loop
crossover frequency. Since R1 is given as 10 kΩ and the crossover frequency is selected as 60 kHz, the desired
f
INT
is calculated from Equation 16:
(16)
And the value for C6 is given by Equation 17:
(17)
Since C6 is calculated to be 2900 pF, and the location of the integrator crossover frequency is important in
setting the overall loop crossover, adjust the value of R1 so that C6 is a standard value of 2700 pF, using
Equation 18:
(18)
The value for R1 is 10.7 KΩ
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