LM2743MTCX/NOPB National Semiconductor, LM2743MTCX/NOPB Datasheet - Page 18

• Manufacturer Part Number: LM2743MTCX/NOPB
• Description: IC REG CTLR BUCK N-CH 14-TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck)r

Specifications of LM2743MTCX/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 ~ 16 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP
• For Use With: LM2743EVAL - BOARD EVALUATION LM2743LM2743-19AEVAL - BOARD EVALUATION LM2743-19A
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM2743MTCX

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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
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° to 60° are usually best be-
cause 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
minimum output current. One method to select A
an iterative process beginning with these worst-case condi-
tions.
1.
2.
3.
4.
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 LM2743 has a unity-gain bandwidth
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 ac-
count unity-gain bandwidth is:
The gain and phase of the error amplifier are shown in
15.
EA
Increase A
Check overall bandwidth and phase margin
Change V
and phase margin
Change I
and phase margin
should give a conservative bandwidth. Increasing the
O
IN
EA
to maximum and recheck overall bandwidth
to minimum and recheck overall bandwidth
EA
Figure
is a ratio of the values of the
13. A
EA
is selected to
EA
is to use
Figure
18
In VM regulators, the top feedback resistor R
of the compensation. Setting R
values for the other compensation resistors and capacitors
that fall within a reasonable range. (Capacitances > 1pF, re-
sistances < 1MΩ) C
to provide the poles and zeroes at the desired frequencies,
using the following equations:
FIGURE 15. Error Amp. Gain and Phase
C1
, C
C2
, C
FB2
C3
, R
to 10 kΩ, ±1% usually gives
C1
, and R
FB2
C2
are selected
20095275
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forms a part