User's Guide
SNVA222A – March 2007 – Revised April 2013
AN-1590 LM3405 Demo Board
1
Introduction
The LM3405 demo board is configured to drive a series string of high power, high brightness LEDs at a
forward current of 1A using the LM3405 constant current buck regulator. The board can accept a full input
operating range of 3V to 15V. The converter output voltage adjusts as needed to maintain a constant
current through the LED array. The LM3405 is a step-down regulator with an output voltage range
extending from a VO(MIN) of 205mV (the reference voltage) to a VO(MAX) determined by the maximum duty
cycle (typically 94%). It can drive up to 3 LEDs in series at 1A forward current, with the single LED forward
voltage of approximately 3.7V (Typical of white, blue, and green LEDs using InGaN technology).
As shown in the demo board schematic circuit in Figure 1, the board is configured with the boost voltage
derived from VIN through a shunt zener (D3). This will ensure that the gate drive voltage VBOOST - VSW falls
in the recommended range of 2.5V to 5.5V when VIN varies from 5V to 15V. In cases of low input voltages
(3V to 5V) being used, the boost diode (D2) can be directly connected to VIN (R3 short, C4 and D3 not
installed) to obtain sufficient gate drive voltage for best performance.
Table 1 lists the bill of materials of this demo board. The measured performance characteristics and layout
of this board are also included below. Additional Circuit Configuration Schematics section illustrates other
possible circuit configurations of this board to accommodate various input and output requirements as
discussed in the LM3405 datasheet.
2
Connecting to LED Array
The LM3405 Demo Board includes a female 6-position SIP connector P1 as well as two standard 72mil
turret connectors for the cathode and anode connections of the LED array. Solid 18 or 20 gauge wire with
about 1cm of insulation stripped away makes a convenient, solderless connection to P1.
3
Setting the LED Current
The default forward current IF delivered to the LED array is 1.0A. To adjust this value the current setting
resistor R1 can be changed according to the following equation:
IF = VFB / R1
(1)
The feedback voltage VFB is regulated at 0.205V typically. The resistor R1 should be rated to handle the
power dissipation of the LED current. R1 should be less than approximately 1Ω, to ensure that the LED
current is kept above 200mA. If average LED currents of less than 200mA are desired, the EN/DIM pin
should be used for PWM dimming.
4
PWM Dimming
The default connection of the PWM terminal is tied to VIN through a 100kΩ resistor (R2) to enable the chip,
which allows the set current to flow through the LEDs continuously. This PWM terminal can also be
connected separately to a periodic pulse signal at different frequencies and/or duty cycle for PWM
dimming. A typical LED current waveform in PWM dimming mode is shown in Figure 2. Figure 3 shows
the average LED current versus duty cycle of PWM dimming signal for various frequencies. Due to an
approximately 100µs delay between the dimming signal and LED current, the dimming ratio reduces
dramatically if the applied PWM dimming frequency is greater than 5kHz.
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1
PWM Dimming
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C4
D4 *
D3
R3
D2
5
VIN
BOOST
VIN
1
C3
C1
SW
R2
6
LM3405
4
PWM
VOUT
L1
P1
12
2
D1
C2
FB
3
7
GND
ANODE of
First LED
3
4
C5
EN/DIM
GND
1
5
6
IF
CATHODE of
Last LED
FB
2
R1
* Not installed
Figure 1. LM3405 Demo Board Schematic
Figure 2. PWM Dimming of LEDs
2
Figure 3. Average LED Current versus Duty Cycle of
PWM Dimming Signal at PWM Terminal
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Bill of Materials
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5
Bill of Materials
Table 1. Bill of Materials
6
Part ID
Part Value
Part Number
Manufacturer
U1
L1
1A constant current buck regulator, SOT-6
LM3405
Texas Instruments
6.8µH, 1.5A, 35mΩ, 6.0 x 6.0 x 2.8mm
SLF6028T-6R8M1R5-PF
TDK
C1
10µF, 25V, X5R, 1206
GRM31CR61E106KA12L
Murata
C2
1µF, 25V, X7R, 1206
C1206C105K3RACTU
Kemet
C3
0.01µF, 16V, X7R, 0805
0805YC103KAT2A
AVX
C4
0.1µF, 16V, X7R, 0805
GRM219R71C104KA01D
Murata
C5
1µF, 25V, X5R, 0805
GRM216R61E105KA12D
Murata
D1
Schottky, 30V, 1A, SMA
MBRA130LT3G
ON Semiconductor
D2
Schottky, 30V, 200mA, SOD-323
BAT54WS-TP
Micro Commercial Co.
D3
5.1V, 0.35W, SOT23
BZX84C5V1
Fairchild Semiconductor
D4
Not installed
R1
0.5W, 0.2Ω, 1%, 2010
WSL2010R2000FEA
Vishay
R2
100kΩ, 1/8W, 1%, 0805
CRCW0805100KFKEA
Vishay
R3
1.0kΩ, 1%, 1/8W, 0805
CRCW08051K00FKEA
Vishay
P1
6-position connector
5535676-5
Tyco/AMP
Typical Performance Characteristics
Figure 4. Efficiency vs. LED Current
(VIN = 5V, VBOOST Derived from VIN)
Figure 5. Efficiency vs. LED Current
(VIN = 12V)
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Typical Performance Characteristics
4
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Figure 6. Efficiency vs. Input Voltage
(IF = 1A)
Figure 7. Switching Waveforms
Figure 8. Startup during PWM Dimming
(VIN = 12V, IF = 1A)
Figure 9. Shutdown during PWM Dimming
(VIN = 12V, IF = 1A)
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Layout
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7
Layout
Figure 10. Top Layer and Top Overlay
Figure 11. Bottom Layer and Bottom Overlay
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Layout
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Figure 12. Internal Plane 1 (GND)
Figure 13. Internal Plane 2 (VIN)
6
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Additional Circuit Configuration Schematics
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8
Additional Circuit Configuration Schematics
D4
D3
C4
R3
D2
5
VIN
BOOST
VIN
1
C3
C1
SW
R2
6
LM3405
4
PWM
VOUT
L1
D1
C2
EN/DIM
FB
GND
C5
3
GND
2
P1
12 1
2
3
4
5
7
6
LED
ANODE
LED
CATHODE
FB
R1
Figure 14. VBOOST Derived from VIN
D4
D3
C4
R3
D2
5
VIN
BOOST
VIN
1
C3
C1
SW
R2
6
LM3405
PWM
4
D1
VOUT
L1
C2
EN/DIM
GND
FB
GND
2
C5
3
P1
12 1
2
3
4
5
7
6
LED
ANODE
LED
CATHODE
FB
R1
Figure 15. VBOOST Derived from VOUT
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Additional Circuit Configuration Schematics
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D4
D3
C4
R3*
D2
5
VIN
BOOST
VIN
1
C3
C1
SW
R2
6
LM3405
4
PWM
VOUT
L1
P1
12
1
LED ANODE
2
D1
C2
3
4
C5
EN/DIM
FB
GND
3
7
GND
5
6
LED
CATHODE
FB
2
R1
* a zener diode is connected in R3 spot
Figure 16. VBOOST Derived from VIN through a Series Zener Diode
D4
D3
C4
R3
D2
5
VIN
BOOST
VIN
1
C3
C1
SW
R2
6
LM3405
4
PWM
D1
VOUT
L1
C2
EN/DIM
FB
GND
3
GND
2
C5
P1
12 1
2
3
4
5
7
6
LED
ANODE
LED
CATHODE
FB
R1
Figure 17. VBOOST Derived from VOUT through a Series Zener Diode
8
AN-1590 LM3405 Demo Board
SNVA222A – March 2007 – Revised April 2013
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Copyright © 2007–2013, Texas Instruments Incorporated
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