LM2744EVAL National Semiconductor, LM2744EVAL Datasheet - Page 17

• Manufacturer Part Number: LM2744EVAL
• Description: BOARD EVALUATION LM2744
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM2744EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2V
• Current - Output: 3.5A
• Voltage - Input: 1.8 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 1MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2744
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

current. One method to select A
cess beginning with these worst-case conditions.
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
110,000 is used.
The error amplifier of the LM2744 has a unity-gain bandwidth
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 ac-
count unity-gain bandwidth is:
The gain and phase of the error amplifier are shown in Figure
13.
Increase A
Check overall bandwidth and phase margin
Change V
and phase margin
Change I
and phase margin
O
IN
EA
to maximum and recheck overall bandwidth
to minimum and recheck overall bandwidth
EA
is to use an iterative pro-
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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:
In practice, a good trade off between phase margin and band-
width can be obtained by selecting the closest ±10% capac-
itor values above what are suggested for C
closest ±10% capacitor value below the suggestion for C
and the closest ±1% resistor values below the suggestions
for R
than 100Ω, it should be replaced by a short circuit. Following
this guideline, the compensation components will be:
C
C
R
The transfer function of the compensation block can be de-
rived by considering the compensation components as
impedance blocks Z
C1
C3
C2
= 27pF ±10%, C
= 2.7nF ±10%, R
= 2.55kΩ ±1%
C1
, R
FIGURE 13. Error Amp Gain and Phase
C2
. Note that if the suggested value for R
F
C1
C2
C1
and Z
, C
= 820pF ±10%
= 39.2kΩ ±1%
C2
, C
I
around an inverting op-amp:
FB2
C3
, R
to 10kΩ, ±1% usually gives
C1
, and R
C1
FB2
C2
and C
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are selected
forms a part
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C2
C2
is less
, the
C3
,