User's Guide
SNVA066B – April 2003 – Revised April 2013
AN-1280 LM2731/LM2733 Evaluation Board
1
Introduction
The LM2731 and LM2733 are high frequency switching boost regulators that offer small size and high
power conversion efficiency. The "X" version of the part operates at 1.6MHz switching frequency and the
"Y" version at 600kHz. The primary difference between the LM2731 and LM2733 is that the LM2731 has a
higher current internal switch FET (with lower breakdown voltage), while the LM2733 has a higher voltage
FET that handles less current. The LM2733 targets applications with higher output voltages, while the
LM2731 is intended for applications requiring higher load currents at lower output voltages. This user's
guide will describe the evaluation board supplied to demonstrate the operation of these parts and give
information on its usage.
2
Basic Application Circuit
The basic application circuit shown in Figure 1 provides the component designators used on the
evaluation board.
L1
D1
U1
VIN
VIN
LM273X
R1
VOUT
SW
R2
FB
SHDN
SHDN
GND
C1
C3
R3
C2
Figure 1. Evaluation Board Basic Application Circuit
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SNVA066B – April 2003 – Revised April 2013
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AN-1280 LM2731/LM2733 Evaluation Board
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1
Component Layout
3
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Component Layout
OUT
C2
D1
R2
C3
R3
GND
U1
L1
R1
C1
S/D
IN
Figure 2. Evaluation Board Component Layout
2
AN-1280 LM2731/LM2733 Evaluation Board
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SNVA066B – April 2003 – Revised April 2013
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Basic Application Circuit - LM2733Y
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Basic Application Circuit - LM2733Y
Table 1. LM2733Y Bill of Materials (VIN = 5V, VOUT = 12V, IOUT = 250mA)
Designation
Description
Size
Manufacturer Part #
Vendor
C1
Cap 2.2µF 16V
1206
EMK316BJ225ML
Taiyo Yuden
C2
Cap 4.7µF 16V
1812
EMK432BJ475ML
Taiyo Yuden
C3
Cap 220pF 50V
0805
VJ0805A221JXACW1BC
Vishay
R1
RES, 51k Ohm, 5%, 0.1W
0805
CRCW080551K0JNEA
Vishay
R2
RES, 118k Ohm, 1%, 0.1W
0805
CRCW0805118KFKEA
Vishay
R3
RES, 13.3k Ohm, 1%, 0.1W
0805
CRCW080513K3FKEA
Vishay
L1
Shielded Inductor 10µH 4A
CDRH125-100MC
Sumida
D1
Diode 20V 0.5A
MBR0520
International
Rectifier
U1
IC LM2733YMF
VOUT
5V/Div
SOT23
Texas Instruments
VOUT
ripple
100 mV/Div
VIN
2V/Div
VSW
5V/Div
IO
100 mA/Div
TIME (10 ms/DIV)
Figure 3. Start Up
VIN = 5V, VOUT = 12V, IOUT = 250mA
TIME (1 µs/DIV)
Figure 4. Switching Waveform and
Output Voltage Ripple
VIN = 5V, VOUT = 12V, IOUT = 250mA
100
VOUT
ripple
100 mV/Div
IO
100 mA/Div
EFFICIENCY [%]
90
80
70
60
50
TIME (1 µs/DIV)
0 25 50 75 100 125 150 175 200 225 250
IOUT[mA]
Figure 5. Load Transient
VIN = 5V, VOUT = 12V, IOUT = 50mA to 200mA
SNVA066B – April 2003 – Revised April 2013
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Figure 6. Efficiency
VIN = 5V, VOUT = 12V
AN-1280 LM2731/LM2733 Evaluation Board
Copyright © 2003–2013, Texas Instruments Incorporated
3
Basic Application Circuit - LM2733X
5
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Basic Application Circuit - LM2733X
Table 2. LM2733X Bill of Materials (VIN = 5V, VOUT = 12V, IOUT = 250mA)
Designation
Description
Size
Manufacturer Part #
Vendor
C1
Cap 2.2µF 16V
1206
EMK316BJ225ML
Taiyo Yuden
C2
Cap 4.7µF 16V
1812
EMK432BJ475ML
Taiyo Yuden
C3
Cap 220pF 50V
0805
VJ0805A221JXACW1BC
Vishay
R1
RES, 51k Ohm, 5%, 0.1W
0805
CRCW080551K0JNEA
Vishay
R2
RES, 118k Ohm, 1%, 0.1W
0805
CRCW0805118KFKEA
Vishay
R3
RES, 13.3k Ohm, 1%, 0.1W
0805
CRCW080513K3FKEA
Vishay
L1
Shielded Inductor 10µH 4A
CDRH125-100MC
Sumida
D1
Diode 20V 0.5A
MBR0520
International
Rectifier
U1
IC LM2733XMF
SOT23
Texas Instruments
VOUT
ripple
VOUT
5V/Div
20 mV/Div
VIN
2V/Div
VSW
5V/Div
IO
50 mA/Div
TIME (5 ms/DIV)
TIME (200 ns/DIV)
Figure 7. Start Up
VIN = 5V, VOUT = 12V, IOUT = 250mA
Figure 8. Switching Waveform and
Output Voltage Ripple
VIN = 5V, VOUT = 12V, IOUT = 250mA
100
VOUT
ripple
100 mV/Div
EFFICIENCY [%]
90
IO
100 mA/Div
80
70
60
50
TIME (200 µs/DIV)
0 25 50 75 100 125 150 175 200 225 250
IOUT[mA]
Figure 9. Load Transient
VIN = 5V, VOUT = 12V, IOUT = 50mA to 200mA
4
AN-1280 LM2731/LM2733 Evaluation Board
Copyright © 2003–2013, Texas Instruments Incorporated
Figure 10. Efficiency
VIN = 5V, VOUT = 12V
SNVA066B – April 2003 – Revised April 2013
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Basic Application Circuit - LM2731Y
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6
Basic Application Circuit - LM2731Y
Table 3. LM2731Y Bill of Materials (VIN = 3.3V, VOUT = 5V, IOUT = 500mA
Designation
Description
Size
Manufacturer Part #
Vendor
C1
Cap 2.2µF 16V
1206
EMK316BJ225ML
Taiyo Yuden
C2
Cap 22µF 16V
1812
EMK432BJ226ML
Taiyo Yuden
C3
Cap 470pF 50V
0805
VJ0805A471JXAMX
Vishay
R1
RES, 51k Ohm, 5%, 0.1W
0805
CRCW080551K0JNEA
Vishay
R2
RES, 40.2k Ohm, 1%, 0.1W
0805
CRCW08054022F
Vishay
R3
RES, 13.3k Ohm, 1%, 0.1W
0805
CRCW080513K3FKEA
Vishay
L1
Shielded Inductor 4.7µH 1.68A
NRS6012T6R8MMGJ
Taiyo Yuden
D1
Diode 20V 0.5A
MBR0520
International
Rectifier
U1
IC LM2731YMF
SOT23
Texas Instruments
VOUT
ripple
50 mV/Div
VOUT
2V/Div
VIN
1V/Div
VSW
2V/Div
IO
200 mA/Div
TIME (2 ms/DIV)
TIME (500 ns/DIV)
Figure 11. Start Up
VIN = 3.3V, VOUT = 5V, IOUT = 500mA
Figure 12. Switching Waveform and
Output Voltage Ripple
VIN = 3.3V, VOUT = 5V, IOUT = 500mA
100
VOUT
ripple
100 mV/Div
EFFICIENCY [%]
90
IO
100 mA/Div
80
70
60
50
TIME (200 µs/DIV)
0 50 100 150 200 250 300 350 400 450 500
IOUT[mA]
Figure 13. Load Transient
VIN = 3.3V, VOUT = 5V, IOUT = 200mA to 400mA
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Figure 14. Efficiency
VIN = 3.3V, VOUT = 5V
AN-1280 LM2731/LM2733 Evaluation Board
Copyright © 2003–2013, Texas Instruments Incorporated
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Basic Application Circuit - LM2731X
7
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Basic Application Circuit - LM2731X
Table 4. LM2731XBill of Materials (VIN = 5V, VOUT = 12V, IOUT = 500mA
Designation
Description
Size
Manufacturer Part #
Vendor
C1
Cap 2.2µF 16V
1206
EMK316BJ225ML
Taiyo Yuden
C2
Cap 4.7µF 16V
1812
EMK432BJ475ML
Taiyo Yuden
C3
Cap 220pF 50V
0805
VJ0805A221JXACW1BC
Vishay
R1
RES, 51k Ohm, 5%, 0.1W
0805
CRCW080551K0JNEA
Vishay
R2
RES, 118k Ohm, 1%, 0.1W
0805
CRCW0805118KFKEA
Vishay
R3
RES, 13.3k Ohm, 1%, 0.1W
0805
CRCW080513K3FKEA
Vishay
L1
Shielded Inductor 10µH 4A
CDH53-4R7MC
Sumida
D1
Diode 20V 0.5A
MBR0520
International
Rectifier
U1
IC LM2731XMF
SOT23
Texas Instruments
VOUT
ripple 100 mV/Div
VOUT
5V/Div
VIN
2V/Div
VSW
5V/Div
IO
200 mA/Div
TIME (5 ms/DIV)
TIME (200 ns/DIV)
Figure 15. Start Up
VIN=5V, VOUT=12V, IOUT=500mA
Figure 16. Switching Waveform and
Output Voltage Ripple
VIN=5V, VOUT=12V, IOUT=500mA
100
VOUT
ripple 200 mV/Div
IO
200 mA/Div
EFFICIENCY [%]
90
80
70
60
50
TIME (200 µs/DIV)
0 50 100 150 200 250 300 350 400 450 500
IOUT[mA]
Figure 17. Load Transient
VIN=5V, VOUT=12V, IOUT= 200mA to 400mA
6
AN-1280 LM2731/LM2733 Evaluation Board
Copyright © 2003–2013, Texas Instruments Incorporated
Figure 18. Efficiency
VIN=5V, VOUT=12V
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Adjusting the Output Voltage
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8
Adjusting the Output Voltage
The output voltage is set using R2 and R3 as given by the formula:
VOUT = 1.23 (R2/R3) + 1.23
(1)
Solved for R2:
R2 = (VOUT - 1.23) / 1.23 × R3
(2)
The evaluation board as shipped has a 13.3k resistor installed at R3. The appropriate value for R2 for any
output may be calculated from the above formula.
9
Feedforward Compensation
The feedforward capacitor C3 should be selected to set the compensation zero at approximately 8 kHz.
The value of C3 is calculated using:
C3 = 1 / (2 × π × 8k × R2)
(3)
The value of C3 is calculated after R2 is selected for the output voltage needed for the specific
application.
10
Guidelines for Component Selection
Since it is assumed that some of the eval boards will be modified to be used in different voltage and
current configurations, some guidelines are given to help select components which are likely to be
changed.
INDUCTOR L1: The amount of inductance required depends on switching frequency, duty cycle and
amount of allowable ripple current. 10 µH is a good choice for most applications. At low boost ratios such
as 3.3V to 5V, the LM2731 loop stability requires that the inductance not exceed 6.8 µH. Smaller inductors
may be used in applications with less output current. Higher ripple current resulting from a smaller inductor
means the maximum average current (and power) will be less. Duty cycle also affects ripple current, since
the time the switch is ON determines the length of time that the current has to ramp up. Any design must
be verified for maximum load current over the full temperature range of the application to make sure the
inductance is sufficient.
Smaller inductors can be used (and make more sense economically) if the load current is fairly light. The
part may operate in discontinuous mode (where inductor current drops to zero during each switching
cycle) using less inductance, but this is harmless and actually increases stability (phase margin) compared
to continuous operation.
DIODE D1: Because of the fast switching speeds, a Schottky diode must be used for D1. The voltage
rating (minimum) should be at least 5V higher than the output voltage for safe design margin. The average
current rating of the diode should be at least 50% more than the maximum output load current of the
application.
OUTPUT CAPACITOR C2: The output capacitor(s) used on the LM273X must be good quality ceramics
of the X7R or X5R type. Z5U or Z5F types will not give sufficient capacitance because of the applied
voltage reducing effective capacitance.
The output capacitor is also critical for stability. As a basic guideline, it is recommended for the LM2733:
4.7 µF minimum, at output voltages of 10V or above. At lower output voltages, use 10-22 µF. In general,
the higher the load current, the more output capacitance is required for stability. For the LM2731: use at
least 10 µF in 5V to 12V applications, and use 22 µF at lower boost ratios (such as 3.3 to 5V).
Stability of the specific application should be verified over the full operating temperature range by load
step testing, where the load current is increased from no load to full load abruptly. This can be done
simply by tapping the lead from the load box onto the output terminal. The amount of ringing seen on the
output voltage waveform will define the stability of the design.
SNVA066B – April 2003 – Revised April 2013
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AN-1280 LM2731/LM2733 Evaluation Board
Copyright © 2003–2013, Texas Instruments Incorporated
7
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