LM25005 datasheet(17/21 Pages) NSC

17 / 21 page

Application Information (Continued)

Components R4 and C5 configure the error amplifier as a

type II configuration which has a pole at DC and a zero at f

=1/(2

πR4C5). The error amplifier zero cancels the modu-

lator pole leaving a single pole response at the crossover

frequency of the loop gain. A single pole response at the

crossover frequency yields a very stable loop with 90 de-

grees of phase margin.

For the design example, a target loop bandwidth (crossover

frequency) of 20 kHz was selected. The compensation net-

work zero (f

Z) should be selected at least an order of mag-

nitude less than the target crossover frequency. This con-

strains the product of R4 and C5 for a desired compensation

network zero1/(2

π R4 C5) to be less than 2kHz. Increasing

R4 while proportionally decreasing C5, increases the error

amp gain. Conversely, decreasing R4 while proportionally

increasing C5, decreases the error amp gain. For the design

example C5 was selected for 0.01µF and R4 was selected

for 49.9 k

Ω. These values configure the compensation net-

work zero at 320 Hz. The error amp gain at frequencies

greater than f

Z is: R4 / R5, which is approximately 10 (20dB).

The overall loop can be predicted as the sum (in dB) of the

modulator gain and the error amp gain.

If a network analyzer is available, the modulator gain can be

measured and the error amplifier gain can be configured for

the desired loop transfer function. If a network analyzer is not

available, the error amplifier compensation components can

be designed with the guidelines given. Step load transient

tests can be performed to verify acceptable performance.

The step load goal is minimum overshoot with a damped

response. C6 can be added to the compensation network to

decrease noise susceptibility of the error amplifier. The value

of C6 must be sufficiently small since the addition of this

capacitor adds a pole in the error amplifier transfer function.

This pole must be well beyond the loop crossover frequency.

A good approximation of the location of the pole added by C6

is: f

p2 =fzxC5/C6.

20170015

FIGURE 8. Gain and Phase of Modulator

LOAD = 5 Ohms and COUT = 177 µF

20170016

FIGURE 9. Error Amplifier Gain and Phase

20170017

FIGURE 10. Overall Loop Gain and Phase

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