LM2648 datasheet(20/23 Pages) NSC

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Component Selection (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, L = 8 µH, Romax =

5V/100mA = 50

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, gm is the transcon-

ductance 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 = 3.3V/V, gm = 650 µmho, R1 = 20k

Ω,R2=

60.4k

Ω:

Bandwidth will vary proportional to the value of R

c1. Next,

c1 can be determined with the following equation:

Example: f

pmin = 363 Hz, Rc1 = 20k

Ω:

The value of C

c1 should be within the range determined by

pmin/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 7) 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 C

c2 and a shorted Rc2 (see

Figure 7). The minimum value for this capacitor can be

calculated by:

c2 may not be necessary, however it does create a more

stable control loop. Under high load current conditions, C

can also help reduce noise in the control loop. For this

purpose a typical value of approximately 220pF is recom-

mended, regardless of the calculated value below.

Example: fz = 80 kHz, R

c1 = 20k

Ω:

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

with C

c2. 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. R

c2 can be calculated with the following equation:

20059612

FIGURE 6. Output-Control Transfer Function

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