LM2648 National Semiconductor, LM2648 Datasheet - Page 20

• Manufacturer Part Number: LM2648
• Description: Synchronous Step-Down 3-Channel Switching Regulator Controller
• Manufacturer: National Semiconductor

www.national.com
Component Selection
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
5V/100mA = 50Ω, R
Once the fp range is determined, R
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.
FIGURE 6. Output-Control Transfer Function
e
= 20mΩ, C
omin
= 5V/3A = 1.7Ω:
o
= 100µF, L = 8 µH, R
c1
(Continued)
should be calculated
omax
20059612
=
20
Example: B = 3.3V/V, gm = 650 µmho, R1 = 20kΩ, R2 =
60.4kΩ:
Bandwidth will vary proportional to the value of R
C
Example: f
The value of C
f
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
Figure 7). The minimum value for this capacitor can be
calculated by:
C
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
A second zero can also be added with a resistor in series
with C
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
p
c1
min/max. A higher value will generally provide a more
c2
can be determined with the following equation:
may not be necessary, however it does create a more
c2
. If used, this zero should be placed at fn, where the
c2
p
min = 363 Hz, R
can be calculated with the following equation:
c1
should be within the range determined by
c1
= 20kΩ:
c1
= 20kΩ:
c2
and a shorted R
c1
c2
. Next,
(see
c2