LM2642MTC National Semiconductor, LM2642MTC Datasheet - Page 19

• Manufacturer Part Number: LM2642MTC
• Description: Voltage Regulator IC
• Manufacturer: National Semiconductor

Specifications of LM2642MTC

• No. Of Pins: 28
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Package / Case: 28-TSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Loop Compensation
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
50Ω, R
Once the fp range is determined, R
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 KΩ, R2 = 60.4 KΩ:
FIGURE 11. Output-Control Transfer Function
omin
= 5V/3A = 1.7Ω:
e
= 20mΩ, C
o
= 100µF, R
c1
(Continued)
should be calculated
omax
= 5V/100mA =
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Bandwidth will vary proportional to the value of Rc1. Next,
Cc1 can be determined with the following equation:
Example: fpmin = 363 Hz, Rc1=20 KΩ:
The value of C
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 KΩ:
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:
FIGURE 12. Compensation Network
c1
should be within the range determined by
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