1

20 x 5 x Ro x Co

fp =

+

0.5

2 x 5 x L x fsw x Co

1

25 x Re x Co

fz =

2

fsw

fn =

B

fzc

fpc1

FREQUENCY

fpc

(0Hz)

-2

0dB

/dec

-20

dB

/dec

0dB/dec

-60

-40

-20

0

20

0.01

0.1

1

10

100

FREQUENCY (kHz)

1000

0

-45

-90

-135

-180

fp

fz

fn

LM26003, LM26003-Q1

www.ti.com

SNVS576F – AUGUST 2008 – REVISED FEBRUARY 2015

Figure 17. Control-Output Transfer Function

The control-output transfer function consists of one pole (fp), one zero (fz), and a double pole at fn (half the

switching frequency).

Referring to Figure 17, the following should be done to create a –20dB/decade roll-off of the loop gain:

1. Place a pole at 0Hz (fpc)

2. Place a zero at fp (fzc)

3. Place a second pole at fz (fpc1)

The resulting feedback (compensation) bode plot is shown below in Figure 18. Adding the control-output

response to the feedback response will then result in a nearly continuous -20db/decade slope.

Figure 18. Feedback Transfer Function

The control-output corner frequencies can be determined approximately by the following equations:

where

•

Co is the output capacitance

•

Ro is the load resistance

•

Re is the output capacitor ESR

•

fsw is the switching frequency.

(17)

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