lm2633mtd National Semiconductor Corporation, lm2633mtd Datasheet - Page 34

• Manufacturer Part Number: lm2633mtd
• Description: Advanced Two-phase Synchronous Triple Regulator Controller For Notebook Cpus
• Manufacturer: National Semiconductor Corporation

www.national.com
Control Loop Design
Back to the previous example. Let B = K, f
The corresponding Bode plots of the compensation network
and the loop transfer function are shown in Figure 12 and
Figure 13 respectively.
It can be seen from Figure 13 that the crossover frequency is
20kHz, and the phase margin is about 84 degrees.
One thing that should be pointed out is this Bode plot is only
for the 0.4
current is lower than 4A, the portion of the gain plot from the
corresponding f
is higher than 4A, then the portion of the gain plot from
310Hz to fp will be flat. However, this usually does not have
much effect on the cross-over frequency and phase margin
because it happens at low frequencies.
= f
FIGURE 12. Example Compensation Transfer Function
n
, then:
load. That is, when load current is 4A. If load
f
p
c_o
to 310Hz will be −40dB/dec. If load current
= 5.1 x 310Hz = 1581Hz
(Continued)
z1
= f
p
, f
20000876
p2
= f
z
(45)
(46)
, f
z2
34
If a shorter recovery time is desired during a load transient,
f
function becomes higher. However, try not to let f
than the desired cross-over frequency, otherwise phase mar-
gin can be too low. Figure 14 shows a situation where f
placed at a higher frequency than the f
−40 dB/dec section before the cross-over frequency. Notice
the phase margin is lower.
Sometimes the slow transient response is caused by the
current source and sink capability of the error amplifier.
Reducing the value of the compensation capacitor helps, but
make sure the small-signal loop is stable.
The power stage component selection can be significantly
different from the example values. Figure 15 shows how the
two high frequency poles of a current-mode-control buck
regulator change with the Q value.
z1
can be increased so that the gain of the loop transfer
FIGURE 13. Example Loop Transfer Function
FIGURE 14. Higher Low Frequency Gain
p
, which results in a
z1
20000877
be higher
200008D6
z1
is