LM2747MTC/NOPB National Semiconductor, LM2747MTC/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM2747MTC/NOPB
• Description: IC CTRLR SYNC BUCK 14-TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck)r

Specifications of LM2747MTC/NOPB

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 13.5 V
• Current - Output: 20A
• Frequency - Switching: 50kHz ~ 1MHz
• Voltage - Input: 1 ~ 17 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP
• For Use With: LM2747EVAL - BOARD EVALUATION LM2747LM2747-19AEVAL - BOARD EVALUATION LM2747-19A
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: *LM2747MTC; *LM2747MTC/NOPB; LM2747MTC

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 60 kHz 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 13. 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 trade-off 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 the both bandwidth and the 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 LM2747 has a unity-gain band-
width of 9 MHz. 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 15.
EA
and phase margin
and phase margin
should give a conservative bandwidth. Increasing the
P2
O
IN
P1
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
17
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
1 pF, resistances
selected to provide the poles and zeroes at the desired
frequencies, using the following equations:
FIGURE 15. Error Amp. Gain and Phase
<
1 MΩ) C
C1
, C
FB2
C2
, C
to 10 kΩ
C3
, R
C1
FB2
, and R
±
20150974
20150975
forms a part
1%, usually
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C2
are
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