TPS54550PWPR datasheet(21/33 Pages) TI2

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Z1 +

2pR3C6

(19)

Z2 +

2pR1C8

(20)

P1 +

2pR5C8

(21)

P2 +

2pR3C7

(22)

INT +

2pR1C6

(23)

LC +

2p LOUTCOUT

(17)

R2 +

0.891

OUT *

0.891

(18)

TPS54550

SLVS623A–MARCH 2006–REVISED APRIL 2006

Use

the

SWIFT

Designer

Software

recommended for designs with unusually high closed

loop crossover frequencies, low value, low ESR

output capacitors such as ceramics, or if the

designer is unsure about the design procedure.

When designing compensation networks for the

TPS54550,

number

factors

need

considered. The gain of the compensated error

amplifier should not be limited by the open-loop

Additionally, there is a pole at the origin, which has

amplifier gain characteristics and should not produce

unity gain with the following frequency:

excessive gain at the switching frequency. Also, the

closed loop crossover frequency should be set less

than 1/5 of the switching frequency, and the phase

margin at crossover must be greater than 45

This pole is used to set the overall gain of the

degrees.

The

general

procedure

outlined

here

compensated error amplifier and determines the

produces results consistent with these requirements

closed loop crossover frequency.

without going into great detail about the theory of

There are a number of popular ways to design Type

loop compensation.

3 compensation networks. The theory behind these

First, calculate the output filter LC corner frequency

calculations is beyond the scope of this document. It

using Equation 17:

is always best to to use any calculated compensation

values as the basis for an initial design, and then

verify the actual closed loop response. The initial

values may then be adjusted to suit the individual

design requirements. The SWIFT software design

For the design example, fLC = 4315 Hz.

tool can also be used to provide an intial circuit

design.

The closed loop crossover frequency should be

greater than fLC and less than 1/5 of the switching

In this circuit, the first compensation zero was set at

frequency. Also, the crossover frequency should not

approximately 1/2 the LC corner frequency, with the

exceed 50 kHz, as the error amplifier may not

second zero slightly below that to increase the phase

provide the desired gain. For this design, a crossover

gain prior to the double pole of the LC output filter. At

frequency of 13 kHz was chosen. This value is

the LC corner frequency, the overall phase response

chosen for comparatively wide loop bandwidth while

rapidly drops by 180 degrees, so it is imprtant to

still allowing for adequate phase boost to insure

increase the initial phase of 90 degrees prior to the

stability.

LC corner.

Next, calculate the R2 resistor value for the output

The two compensation poles are set high enough to

voltage of 3.3 V using Equation 18:

to not cause loss of phase margin at the closed loop

cross over and low enough to not cause the error

amplifier gain to exceed the unity gain bandwidth

limit

the

internal

operational

amplifier.

The

For any TPS54550 design, start with an R1 value of

integrator frequency is then chosen to set the overall

1.0 k

Ω. R2 is then 374 Ω.

gain and crossover frequency.

Now the values for the compensation components

This

results

the

following

pole

and

zero

that set the poles and zeroes of the compensation

frequencies:

network can be calculated. Assuming that R1 > R5

fZ1 = 2340 Hz

and C6 > C7, the pole and zero locations are given

by Equation 19 through Equation 22:

fZ2 = 1591 Hz

blank

fP1 = 120 kHz

blank

fP2 = 159 kHz

blank

fINT = 234 Hz

blank

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