User's Guide
SNVA151A – September 2006 – Revised May 2013
AN-1450 LM3743 Evaluation Board
1
Introduction
The LM3743 is a voltage mode PWM buck controller that implements synchronous rectification. It provides
a low cost, fault tolerant, and efficient point of load solution. To reduce component count several
parameters are fixed, such as switching frequency and the short circuit protection level. In steady state
operation the LM3743 is always synchronous, even at no load, thus simplifying the compensation design.
The LM3743 ensures a smooth and controlled start-up when the output is pre-biased. Two levels of
current limit protection enhance the robustness of the power supply and requires no current sense resistor
in the power path. The primary level of protection is the low side current limit and is achieved by sensing
the voltage VDS across the low side MOSFET. The second level of protection is the high side current limit,
which protects power components from extremely high currents caused by switch node short-circuit to
ground.
2
Specifics Of The Board
The input voltage to the power components (VIN) and the input voltage on the control section (VCC) must
be in the range of 3.0V to 5.5V. The 300 kHz demo board design regulates to an output voltage of 1.2V
and up to 13A. The 1 MHz demo board design regulates to an output voltage of 1.5V and up to 9A. For
additional design modifications, see the Design Consideration section of the LM3743 High-Performance
Synchronous Buck Controller with Comprehensive Fault Protection Features Data Sheet (SNVS427). The
PCB is designed on four layers, the top and bottom layers are 2oz. copper and the two inner layers are
1oz. copper. The board measures 2.4 in. x 1.36 in. (6.1 cm x 3.44 cm) and is 62mil (.062”) thick on a FR4
laminate.
3
Additional Footprints
When the tracking feature of the LM3743 is used, remove the soft-start capacitor C4 and use a resistor
divider on designators R7 and C4. The ground and track post have been provided for your connecting
convenience.
4
MOSFET Footprints
The LM3743 demo board accommodates footprints for single N-MOSFETs with standard SOIC-8
packages and with exposed drain pads. The MOSFET footprint has the pinout as shown in Figure 1.
Some examples of MOSFET packages with exposed drain pads are also illustrated.
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SNVA151A – September 2006 – Revised May 2013
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AN-1450 LM3743 Evaluation Board
1
Performance Characteristics
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S
D
S
D
SO-8
S
D
G
D
S
D
S
D
S
D
G
D
S
D
S
D
S
D
G
D
Figure 1. SOIC-8 Footprint
5
Performance Characteristics
Efficiency
Figure 2. Efficiency vs. Load Current
VOUT = 1.2V, fSW = 300 kHz
Figure 3. Efficiency vs. Load Current
VOUT = 1.5V, fSW = 1MHz
Switch Node Voltage and Output Voltage Ripple
2
AN-1450 LM3743 Evaluation Board
SNVA151A – September 2006 – Revised May 2013
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Performance Characteristics
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20 mV/DIV
20 mV/DIV
2V/DIV
2V/DIV
200 ns/DIV
200 ns/DIV
Figure 4. VIN = 5V, VOUT = 1.5V,
ILOAD = 0A, fSW = 1MHz
20 MHz Bandwidth Limit
Figure 5. VIN = 5V, VOUT = 1.5V,
ILOAD = 9A, fSW = 1MHz
20 MHz Bandwidth Limit
20 mV/DIV
20 mV/DIV
2V/DIV
2V/DIV
1 Ps/DIV
1 Ps/DIV
Figure 6. VIN = 5V, VOUT = 1.2V,
ILOAD = 0A, fSW = 300kHz
20 MHz Bandwidth Limit
Figure 7. VIN = 5V, VOUT = 1.2V,
ILOAD = 13A, fSW = 300kHz
20 MHz Bandwidth Limit
SNVA151A – September 2006 – Revised May 2013
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AN-1450 LM3743 Evaluation Board
3
Performance Characteristics
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Table 1. 300 kHz Bill of Materials (BOM)
Designator
Function
Description
Part Number
Vendor
U1
Controller
Buck Controller
LM3743
Texas Instruments
C1
Input Filter
22 µF,10 V, X7R, 1812
GRM43ER71A226KE01B
Murata
C2
Input Filter
Al-elec, 10 V, 470 µF
10ME470WX
Sanyo
C3
Decoupling
1 µF, 10 V, X7R, 0805
0805ZC105KAT
AVX
C4
Soft Start Cap
180 nF, 25 V, X7R
VJ0805Y184KXXA
Vishay
C5
Output Filter
470 µF, 4 V, 15mΩ
4TPE470MFL
Sanyo
C6
Output Filter
10 µF, 6.3 V, 0805
0805D106MAT
AVX
C7
Comp Cap
56 pF, 25 V, 0805
VJ0805A560KXAA
Vishay
C8
Comp Cap
2.2 nF, 25 V, 0805
VJ0805A222KXAA
Vishay
C9
Comp Cap
1.5 nF, 25 V, 0805
VJ0805Y152KXXA
Vishay
C10
Bootstrap Cap
.47 µF, 25 V, 0805
VJ0805Y474KXXA
Vishay
R1
Current Limit Res
1.78 kΩ, 0805
CRCW08051781F
Vishay
R2
Top FB Resistor
10 kΩ, 0805
CRCW08051002F
Vishay
R3
Bottom FB Resistor
20 kΩ, 0805
CRCW08052002F
Vishay
R4
Comp Resistor
22.6 kΩ, 0805
CRCW08052262F
Vishay
R5
Comp Resistor
3.57 kΩ, 0805
CRCW08053571F
Vishay
R6
VCC Filter Resistor
2 Ω, 0805
CRCW08052R00F
Vishay
L1
Output Filter
1.5 µH, 13.8A, 2.9 mΩ
DR125-1R5
Cooper
D1
Bootstrap Diode
500 mA, 20 V, VF =.3 V
MBR0520LT1
Onsemi
Q1
Top FET
4.5 mΩ @VGS = 4.5 V, 21 nC
Si7882DP
Vishay
Q2
Bottom FET
4.5 mΩ @VGS = 4.5 V, 21 nC
Si7882DP
Vishay
VIN = 3 to 5.5V
D1
R6
BOOT
C10
VCC
HGATE
C3
C1
Q1
+
+
C2
SW
LM3743
L1
R1
Vo = 1.2V, 13A
ILIM
R7
SS/TRACK
TRACK
GND
LGATE
C4
Q2
+
C5
+
C6
GND
COMP/EN
FB
R2
R5
C8
C7
C9
R4
R3
Figure 8. 300 kHz Demo Board Schematic
4
AN-1450 LM3743 Evaluation Board
SNVA151A – September 2006 – Revised May 2013
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Performance Characteristics
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Table 2. 1 MHz Bill of Materials
Designator
Function
Description
Part Number
Vendor
U1
Controller
Buck Controller
LM3743
Texas Instruments
C1
Input Filter
22 µF, 10 V, X7R, 1812
GRM43ER71A226KE01B
Murata
Sanyo
C2
Input Filter
Al-elec, 10 V, 470 µF
10ME470WX
C3
Decoupling
1 µF, 10 V, X7R, 0805
0805ZC105KAT
AVX
C4
Soft Start Cap
180 nF, 25 V, X7R
VJ0805Y184KXXA
Vishay
C5
Output Filter
470 µF, 4 V, 15mΩ
4TPE470MFL
Sanyo
C6
Output Filter
10 µF, 6.3 V, 0805
0805D106MAT
AVX
C7
Comp Cap
8.2 pF, 25 V, 0805
VJ0805A8R2KXAA
Vishay
C8
Comp Cap
820 pF, 25 V, 0805
VJ0805A821KXAA
Vishay
C9
Comp Cap
470 pF, 25 V, 0805
VJ0805Y471KXXA
Vishay
C10
Bootstrap Cap
.1 µF, 25 V, 0805
VJ0805Y104KXXA
Vishay
R1
Current Limit Res
1.1 kΩ, 0805
CRCW08051101F
Vishay
R2
Top FB Resistor
10 kΩ, 0805
CRCW08051002F
Vishay
R3
Bottom FB Resistor
11.3 kΩ, 0805
CRCW08057871F
Vishay
R4
Comp Resistor
35.7 kΩ, 0805
CRCW08053572F
Vishay
R5
Comp Resistor
8.87 kΩ, 0805
CRCW08058871F
Vishay
R6
VCC Filter Resistor
2 Ω, 0805
CRCW08052R00F
Vishay
L1
Output Filter
.47 µH, 11.6A, 3.6 mΩ
FP3-R47
Cooper
D1
Bootstrap Diode
500 mA, 20 V, VF =.3 V
MBR0520LT1
Onsemi
Q1
Top FET
4.5 mΩ @ VGS = 4.5 V, 21 nC
Si7882DP
Vishay
Q2
Bottom FET
4.5 mΩ @ VGS = 4.5 V, 21 nC
Si7882DP
Vishay
VIN = 3 to 5.5V
D1
R6
BOOT
C10
VCC
HGATE
C3
C1
Q1
+
+
C2
SW
LM3743
R1
Vo = 1.5V, 9A
L1
ILIM
R7
SS/TRACK
TRACK
GND
LGATE
C4
Q2
+
C5
+
C6
GND
COMP/EN
FB
R2
R5
C8
C7
C9
R4
R3
Figure 9. 1 MHz Demo Board Schematic
SNVA151A – September 2006 – Revised May 2013
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AN-1450 LM3743 Evaluation Board
5
PCB Layout Diagram(s)
6
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PCB Layout Diagram(s)
Figure 10. Top Layer
Figure 11. Internal Layer I (Ground)
6
AN-1450 LM3743 Evaluation Board
SNVA151A – September 2006 – Revised May 2013
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PCB Layout Diagram(s)
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Figure 12. Internal Layer II (VIN and VOUT) Heat Sinking Layer
Figure 13. Bottom Layer Heat Sinking Layer and Signal Traces
SNVA151A – September 2006 – Revised May 2013
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