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 5.

Poles are marked with an ’X’, and zeros are marked with a

’O’. The bolded ’O’ labeled ’f

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 com-

pensator, which is the error amp combined with the compen-

sation network. T is the product of the powerstage and the

compensator and is the complete open loop frequency re-

sponse. 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.

Quick Compensator Design

Calculate:

where,

where R

Choose C

Choose

Where,

20012657

FIGURE 5. Bode plot of the LM2698 Frequency Response using the Typical Application Circuit

www.national.com

11