lm2746mxax National Semiconductor Corporation, lm2746mxax Datasheet - Page 18

• Manufacturer Part Number: lm2746mxax
• Description: Low Voltage N-channel Mosfet Synchronous Buck Regulator Controller
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
as high as possible. Two zeroes f
double pole frequency to cancel the double pole phase lag.
Then, a pole, f
A final pole f
quency. The gain of the error amplifier transfer function is
selected to give the best bandwidth possible without violat-
ing the Nyquist stability criteria. In practice, a good crossover
point is one-fifth of the switching frequency, or 60kHz for this
example. The generic equation for the error amplifier transfer
function is:
In this equation the variable A
capacitance and resistance of the compensation compo-
nents, arranged as shown in Figure 12. A
provide the desired bandwidth. A starting value of 80,000 for
A
value will increase the bandwidth, but will also decrease
phase margin. Designs with 45-60˚ are usually best because
they represent a good tradeoff between bandwidth and
phase margin. In general, phase margin is lowest and gain
highest (worst-case) for maximum input voltage and mini-
mum output current. One method to select A
iterative process beginning with these worst-case conditions.
1. Increase A
2. Check overall bandwidth and phase margin
3. Change V
4. Change I
The process ends when both bandwidth and phase margin
are sufficiently high. For this example input voltage can vary
from 3.0 to 3.6V and output current can vary from 0 to 4A,
and after a few iterations a moderate gain factor of 101dB is
used.
The error amplifier of the LM2746 has a unity-gain band-
width of 9MHz. In order to model the effect of this limitation,
the open-loop gain can be calculated as:
The new error amplifier transfer function that takes into
account unity-gain bandwidth is:
The gain and phase of the error amplifier are shown in
Figure 14.
EA
and phase margin
and phase margin
should give a conservative bandwidth. Increasing the
P2
O
P1
IN
EA
to maximum and recheck overall bandwidth
is placed at one-half of the switching fre-
to minimum and recheck overall bandwidth
is placed at the frequency of the ESR zero.
EA
is a ratio of the values of the
Z1
and f
Z2
(Continued)
EA
are placed at the
EA
is selected to
is to use an
18
In VM regulators, the top feedback resistor R
of the compensation. Setting R
gives values for the other compensation resistors and ca-
pacitors that fall within a reasonable range. (Capacitances
1pF, resistances
selected to provide the poles and zeroes at the desired
frequencies, using the following equations:
FIGURE 14. Error Amp. Gain and Phase
<
1MΩ) C
C1
, C
FB2
C2
, C
to 10kΩ,
C3
, R
C1
FB2
, and R
±
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C2
are
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