10e1

10e2

10e3

10e4

10e5

10e6

-40

-20

0

20

40

60

O

O

X

X

X

X

DSON

Frequency (Hz)

10e1

10e2

10e3

10e4

10e5

10e6

-200

-150

-100

-50

0

O

O

X

X

Frequency (Hz)

Powerstage

Compensator

T

Bandwidth = 6.9kHz, Phase Margin = 78.4

, R

= 0.15

: , L = 10uH

xxx

xxx

xxx

Q = 0.5

1

25

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Q = 5

(b)

5

10

15

20

xxxx

xxxx

xxxx

xxxx

1

50

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Q = 0.5

Q = 5

(a)

45

40

35

30

25

20

15

10

5

LM2698

SNVS153E – MAY 2001 – REVISED APRIL 2013

www.ti.com

Figure 17. Minimum Inductance Requirements for

Figure 18. Minimum Inductance Requirements for

The goal of the compensation network is to provide the best static and dynamic performance while insuring

stability over line and load variations. The relationship of stability and performance can be best analyzed by

plotting the magnitude and phase of the open loop frequency response in the form of a bode plot. A typical bode

plot of the LM2698 open loop frequency response is shown in Figure 19.

Figure 19. Bode plot of the LM2698 Frequency Response using the Typical Application Circuit

zero. Right half plane zeros act like normal zeros to the magnitude (+20dB/decade slope influence) and like

poles to the phase (

−90° shift). Three curves are shown. The powerstage curve is the frequency response of the

powerstage, which includes the switch, diode, inductor, output capacitor, and load. The compensator curve is the

frequency response of the compensator, which is the error amp combined with the compensation network. T is

the product of the powerstage and the compensator and is the complete open loop frequency response. The

power stage response is fixed by line and load constraints, while the compensator is set by the external

compensation network at pin 1. The compensator can be designed in a few simple steps as follows.

12

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