Download AN-2094 LM3481 SEPIC Evaluation Board (Rev

User's Guide
SNVA461A – September 2011 – Revised April 2013
AN-2094 LM3481 SEPIC Evaluation Board
1
Introduction
The LM3481 SEPIC evaluation board is designed to provide the design engineer with a fully functional
power converter solution with two output voltage options. It produces a selectable output voltage of 12 V
at 1.25A or 5V at 3A. The switching frequency for the converter is set to 500 kHz. The printed circuit board
consists of 2 layers of FR4 material with 2 ounce copper on both layers. This document contains the
evaluation board schematics, bill-of-materials (BOM) and a quick setup procedure. For complete circuit
design information, see the LM3481/LM3481Q High Efficiency Low-Side N-Channel Controller for
Switching Regulators Data Sheet (SNVS346).
The performance of the evaluation board is as follows:
5V Output Voltage Option:
Input Voltage Range
: 4.5V to 25V
Output Voltage
: 5V ± 2%
Output Current
: 0A to 3A
Switching Frequency
: 500 kHz
Load Regulation
: 0.1%
Board Size
: 2.4 x 1.8 inches
12V Output Voltage Option:
Input Voltage Range
: 4.5V to 18V
Output Voltage
: 12V ± 2%
Output Current
: 0A to 1.25A
Switching Frequency
: 500 kHz
Load Regulation
: 0.1%
Board Size
: 2.4 x 1.8 inches
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1
Powering and Loading Considerations
2
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Powering and Loading Considerations
Read this entire page prior to attempting to power the evaluation board.
2.1
Qucik Setup Procedure
1. Use an input power supply with at least 5A current capability. Connect the positive output of the power
supply to the VIN terminal P1. Connect the negative output of the power supply to the input GND
terminal P2.
2. Connect a load with 3A capability to the VOUT terminal P3 and GND terminal P4.
3. To set the output voltage to 5V, the J2 jumper should be out. If the desired output voltage is 12V, the
J2 jumper should be in.
4. Set VIN to 12V with no load being applied. Turn on the input power supply. Once the input voltage is
applied, the output voltage should be in regulation.
5. Adjust the input voltage and load current and set as desired while making sure that they are in range
as described above.
Note: Do not change jumper settings while the board is powered on.
3
Connection Diagram
DVM
VIN
P1
0-25V, 6A
DC Power Supply
GND
P2
LM3481
EVALUATION BOARD
VOUT
P3
Electronic Load
0A ± 3A
GND
P4
DVM
Figure 1. Basic Test Setup for the LM3481 SEPIC Evaluation Board
OSCILLOSCOPE
Bare Wire
COUT
VOUT
GND
Figure 2. Output Voltage Ripple Measurement Setup
2
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Board Configuration
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4
Board Configuration
4.1
Output Voltage Option
A jumper has been provided on the evaluation board in order to select the output voltage option of 12V or
5V. Shorting the jumper ses the output voltage to 12V. Leaving the jumper open will set the output voltage
to 5V.
4.2
Under-Voltage Lock-Out (UVLO)
The LM3481 evaluation board has a resistor divider connected to the UVLO pin to set the on and off
thresholds to 4.25V and 4.05V, respectively. There is also a jumper J1 on the board that provides the
‘Enable’ feature. If the jumper is open, the board will not produce an output voltage.
4.3
Output Voltage Ripple
Output voltage ripple measurement should be taken directly across the output capacitor C12 between
terminals P3 and P4. Care must be taken to minimize the loop area between the scope probe tip and the
ground lead in order to minimize noise in the measurement. This can be achieved by removing the probe’s
spring tip and ground lead and then wrap a bare wire around the scope probe shaft. The bare wire should
be in contact with the probe shaft since this is the ground lead for the probe. The measurement can be
taken by connecting the bare wire onto the ground side of the capacitor and the probe tip onto the positive
side of the capacitor. Figure 2 shows a diagram of this measurement technique.
4.4
External Clock Synchronization
A SYNC terminal has been provided on the evaluation board in order to synchronize the converter to an
external clock or other fixed frequency signal. For complete information on setting up the external clock,
see the LM3481/LM3481Q High Efficiency Low-Side N-Channel Controller for Switching Regulators Data
Sheet (SNVS346).
4.5
Active Loads
Many constant-current types of active loads can exhibit an initial short circuit, which is sustained well
beyond the normal soft-start cycle. To avoid loss of regulation of the output voltage during current limit
during startup, wait until the output voltage is up before turning on the load. Using an active load with a
constant-resistance mode will avoid this startup timing issue.
4.6
SEPIC Operation and Advantages
The Single Ended Primary Inductance Converter (SEPIC) is a DC-DC converter that allows the output
voltage to be either higher than or lower than the input voltage. The SEPIC capacitor is charged to a
voltage potential of approximately VIN. This capacitor acts as an AC short placing the two coils of the
inductor in parallel. The duty cycle relationship and operation is similar to that of the Buck-Boost. During
the on-time of the MOSFET a voltage of VIN is applied across the mutual inductor. During the off-time of
MOSFET, the mutual inductor voltage flies to VOUT refreshing the charge on the output capacitor. Since the
SEPIC capacitor is charged to VIN, the voltage across the MOSFET is VIN + VOUT. Additionally, the current
through the MOSFET is IIN + IOUT.
The SEPIC has quite a few advantages over the inverting Buck-Boost and the Flyback topology. The
power MOSFET and the diode voltages in the Flyback topology are unclamped and are largely functions
of the transformer leakage inductance and the stray capacitance. This causes large voltage spikes at the
switching intervals. Compared to this for the SEPIC topology the MOSFET and the diode are clamped by
the output and blocking capacitors and thus there is little circuit ringing. The SEPIC also has a common
ground, unlike the Buck-Boost topology, which makes the input and output voltage sensing very easy.
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Board Configuration
4.7
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Momentary Overload or Short Circuit Protection
In case of an overload or a short circuit event, the voltage across the sense resistor would increase
beyond 220 mV. This would then activate the short circuit current limit and limit the switching frequency by
a factor of 8 while the event exists. In case of a prolonged overload/short circuit event, the large currents
would cause the junction temperatures of the MOSFET and the diode to increase significantly and
potentially cause thermal failure.
4
AN-2094 LM3481 SEPIC Evaluation Board
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Typical Performance Characteristics
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5
Typical Performance Characteristics
Efficiency at VOUT = 12V
100
80
90
70
80
EFFICIENCY (%)
EFFICIENCY (%)
Efficiency at VOUT = 5V
90
60
50
40
30
Vin = 4.5V
Vin = 6V
Vin = 12V
Vin = 18V
Vin = 25V
20
10
0
0.0
0.5
1.0
70
60
50
40
30
Vin = 4.5V
Vin = 6V
Vin = 12V
Vin = 18V
20
10
0
1.5
2.0
ILOAD(A)
2.5
3.0
0.0
Startup at VOUT = 5V and VIN = 4.5V
0.5
1.0
1.5
2.0
ILOAD(A)
2.5
3.0
Startup at VOUT = 5V and VIN = 25V
6
27
6
6
4
4
VIN
3
3
VOUT
2
5
21
VIN
18
4
15
3
12
9
2
VOUT(V)
5
VIN(V)
5
VOUT(V)
VIN(V)
24
2
6
1
1
0
0
2
4
6
0
0
8 10 12 14 16 18 20
TIME (ms)
Startup at VOUT = 12V and VIN = 4.5V
5
0
2
4
6
Startup at VOUT = 12V and VIN = 18V
16
25
15
VOUT
14
4
2
VOUT
6
VIN(V)
8
VOUT(V)
VIN(V)
10
3
VIN
20
12
VIN
8 10 12 14 16 18 20
TIME (ms)
12
15
9
10
6
5
3
0
0
VOUT(V)
0
1
VOUT
3
4
1
2
0
0
0
2
4
6
8 10 12 14 16 18 20
TIME (ms)
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0
2
4
6
8 10 12 14 16 18 20
TIME (ms)
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Copyright © 2011–2013, Texas Instruments Incorporated
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Typical Performance Characteristics
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Load Transient at VIN = 5V
VIN= 5V
VOUT
5.2
Load Transient at VIN = 12V
6
13.0
5
12.5
6
VIN= 12V
5
ILOAD
4.0
3.6
3.2
0.0
0.5
1.0
TIME (ms)
1.5
1
10.5
1
0
10.0
0
5.5
21
5.0
20
4.5
19
4.0
VIN
20
3.5
3.0
16
12
8
4
0
1
2
3
4 5 6 7
TIME (ms)
8
Input Transient at Iload = 3A and VOUT = 12V
22
13.0
12.5
12.0
11.5
VOUT
18
11.0
17
10.5
VIN
16
10.0
15
9.5
2.0
14
9.0
1.5
13
8.5
1.0
12
9 10
8.0
0
1
2
3
4 5 6 7
TIME (ms)
8
9 10
Steady State Operation at VIN = 25V and Iload = 3A
5.5
42
VIN= 25V
5.0
35
4.5
28
4.0
21
16
11.0
12
10.5
8
3.5
14
10.0
4
3.0
7
9.5
0
2.5
0
0
2
4
6
TIME ( s)
AN-2094 LM3481 SEPIC Evaluation Board
8
10
VOUT(V)
11.5
VSW(V)
VOUT(V)
ILOAD= 3A
2.5
Steady State Operation at VIN = 4.5V and Iload = 1.25A
12.5
24
VIN= 4.5V
12.0
20
6
VIN(V)
VOUT
24
2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
TIME (ms)
VOUT(V)
VIN(V)
28
ILOAD
11.0
2.0
ILOAD= 3A
3
2
Input Transient at Iload = 3A and VOUT = 5V
36
6.0
32
11.5
ILOAD(A)
3
4
VOUT(V)
4.4
12.0
0
1
2
3
4 5 6
TIME ( s)
7
8
VSW(V)
4
VOUT(V)
4.8
ILOAD(A)
VOUT(V)
VOUT
9 10
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Evaluation Board Schematic
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Evaluation Board Schematic
Figure 3. LM3481 Evaluation Board Schematic
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Evaluation Board Schematic
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Table 1. LM3481 Bill of Materials (BOM)
8
Qty
Designator
Value
1
C1
100 µF
2
C2, C3
4.7 µF
1
C4
0.027 µF
1
C5
1
1
Package
Description
Manufacturer
Part Number
Polarized Capacitor, 63V,
10%
Sanyo
63CV100KX
1812
Ceramic, X7R, 50V, 10%
MuRata
C4532X7R1H475M
0603
Ceramic, X7R, 100V, 10%
Kemet
C0603C273K1RACTU
100 pF
0603
Ceramic, X7R, 100V, 10%
AVX
06031C331KAT2A
C6
4700 pF
0603
Ceramic, X7R, 100V, 10%
AVX
06031C222KAT2A
C7
2200 pF
0603
Ceramic, X7R, 16V, 10%
TDK
C1608X7R1C105K
1
C8
1000 pF
1206
Ceramic, X7R, 50V, 10%
TDK
C3216X7R1H105K
2
C9, C12
22 µF
1812
Ceramic, X7R, 20V, 20%
TDK
C4532X7R1C226M
2
C10, C11
180 µF
Electrolytic capacitor
Sanyo
16SVP180M
1
D1
Schottky Diode, 40V
Central Semi
CMSH5-40
2
J1, J2
Through Hole Jumpers
Sullins Electronics
PBC02SAAN
Coupled Inductor, 8.25A
Cooper
DRQ127-4R7-R
Terminal, Turret, TH
Keystone
Electronics
1598-2
SMC
1
L1
4
P1, P2, P3, P4
4.7 µH
1
Q1
PowerPAK
N Channel MOSFET, 30V
Vishay
Si7386DP
1
R1
100 ohm
0603
RES, 1%, 0.1W
Vishay-Dale
CRCW0603100RFKEA
1
R2
40.2k ohm
0603
RES, 1%, 0.1W
Vishay-Dale
CRCW060340K2FKEA
1
R3
20.5k ohm
0603
RES, 1%, 0.1W
Vishay-Dale
CRCW060320K5FKEA
1
R4
10.7k ohm
0603
RES, 1%, 0.1W
Vishay-Dale
CRCW060310K7FKEA
1
R5
20.0k ohm
0603
RES, 1%, 0.1W
Vishay-Dale
CRCW060320K0FKEA
1
R6
59.0k ohm
0603
RES, 1%, 0.1W
Vishay-Dale
CRCW060359K0FKEA
1
R7
DNP
Resistor
1
R8
20m ohm
RES, 1%, 1W
Vishay-Dale
WSL2512R0200FEA
1
R9
6.8k ohm
0603
RES, 5%, 0.1W
Vishay-Dale
CRCW06036K80JNEA
1
R10
38.3k ohm
0603
3
TP1, TP2, TP3
1
U1
VSSOP
AN-2094 LM3481 SEPIC Evaluation Board
DNP Resistor
RES, 1%, 0.1W
Vishay-Dale
CRCW060338K3FKEA
Through Hole Test Point,
Miniature, White
Keystone
Electronics
5002
Low-Side N-Channel
Controller
Texas Instruments
LM3481
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PCB Layout
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PCB Layout
Figure 4. Top Layer Components and Overlay
Figure 5. Top Layer Copper
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PCB Layout
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Figure 6. Bottom Overlay
Figure 7. Bottom Layer Copper
10
AN-2094 LM3481 SEPIC Evaluation Board
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