LM26 datasheet(19/20 Pages) NSC

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Loop Compensation (Continued)

The control-output corner frequencies, and thus the desired

compensation corner frequencies, can be determined ap-

proximately by the following equations:

Since fp is determined by the output network, it will shift with

loading (Ro) and duty cycle. First determine the range of

frequencies (fpmin/max) of the pole across the expected

load range, then place the first compensation zero within that

range.

Example: R

e = 20m

Ω,C

o = 100µF, Romax = 5V/100mA =

Ω,R

omin = 5V/3A = 1.7

Ω:

Once the fp range is determined, R

c1 should be calculated

using:

Where B is the desired gain in V/V at fp (fz1), gm is the

transconductance of the error amplifier, and R1 and R2 are

the feedback resistors. A gain value around 10dB (3.3v/v) is

generally a good starting point.

Example: B = 3.3 v/v, gm=650 m, R1 = 20 K

Ω, R2 = 60.4 KΩ:

Bandwidth will vary proportional to the value of Rc1. Next,

Cc1 can be determined with the following equation:

Example: fpmin = 363 Hz, Rc1=20 K

Ω:

The value of C

c1 should be within the range determined by

Fpmin/max. A higher value will generally provide a more

stable loop, but too high a value will slow the transient

response time.

The compensation network (Figure 12) will also introduce a

low frequency pole which will be close to 0Hz.

A second pole should also be placed at fz. This pole can be

created with a single capacitor Cc2 and a shorted Rc2 (see

Figure 12). The minimum value for this capacitor can be

calculated by:

Cc2 may not be necessary, however it does create a more

stable control loop. This is especially important with high

load currents and in current sharing mode.

Example: fz = 80 kHz, Rc1 = 20 K

Ω:

A second zero can also be added with a resistor in series

with Cc2. If used, this zero should be placed at fn, where the

control to output gain rolls off at -40dB/dec. Generally, fn will

be well below the 0dB level and thus will have little effect on

stability. Rc2 can be calculated with the following equation:

20046212

FIGURE 11. Output-Control Transfer Function

20046274

FIGURE 12. Compensation Network

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