1 Introduction
The LM3405A demo board is configured to drive a series string of high power, high brightness LEDs at a
forward current of 1A using the LM3405A constant current buck regulator. The board can accept a full
input operating range of 3V to 22V. The converter output voltage adjusts as needed to maintain a constant
current through the LED array. The LM3405A is a step-down regulator with an output voltage range
extending from a V
cycle (typically 94%). It can drive up to 5 LEDs in series at 1A forward current, with the single LED forward
voltage of approximately 3.7 V (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 VINthrough a shunt zener (D3). This will ensure that the gate drive voltage V
in the recommended range of 2.5 V to 5.5 V when VINvaries from 5 V to 22 V. When input voltage is in
the range of 3 V to 5 V, the anode of boost diode (D2) should be directly connected to VINby replacing R3
with a jumper and removing C4 and D3, to obtain sufficient gate drive voltage for best performance.
Table 1 lists the bill of materials (BOM) of this demo board. The measured performance characteristics
and layout of this board are also included below. Additionally, the Circuit Configuration Schematics section
illustrates other possible circuit configurations of this board to accommodate various input and output
requirements as discussed in the LM3405A 1.6MHz, 1A Constant Current Buck LED Driver with Internal
Compensation in Tiny SOT and MSOP PowerPAD Packages Data Sheet (SNVS508).
AN-1685 LM3405A Demo Board
of 205 mV (the reference voltage) to a V
O(MIN)
User's Guide
SNVA271B–October 2007–Revised April 2013
determined by the maximum duty
O(MAX)
- VSWfalls
BOOST
2 Connecting to LED Array
The LM3405A 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 IFdelivered to the LED array is 1.0A. To adjust this value the current setting
resistor R1 can be changed according to Equation 1:
IF= VFB/ R1 (1)
The feedback voltage VFBis typically regulated at 0.205 V. The resistor R1 should be rated to handle the
power dissipation of the LED current. R1 should be less than approximately 1 Ω, to ensure that the LED
current is kept above 200 mA. If average LED currents of less than 200 mA are desired, the EN/DIM pin
should be used for pulse width modulation (PWM) dimming.
4 PWM Dimming
The default connection of the PWM terminal is tied to VINthrough a 100 kΩ resistor (R2) to enable the
chip, which allows the set current to flow through the LEDs continuously. This PWM terminal can also be
connected 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 various dimming signal frequencies. Due to an approximately 100 µs delay
between the dimming signal and LED current, the dimming ratio reduces dramatically if the applied PWM
dimming frequency is greater than 5 kHz.
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LM3405A
VIN
VIN
EN/DIM
BOOST
SW
FB
GND
C3
D1
D2
D3
C4
R3
C1
PWM
L1
C2
R1
C5
V
OUT
1
2
3
4
5
6
P1
7126
1
R2
FB
2
3
4
5
D4 *
* Not installed
GND
ANODE of
First LED
CATHODE of
Last LED
IF
PWM Dimming
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Figure 1. LM3405A Demo Board Schematic
Figure 2. PWM Dimming of LEDs Figure .
2
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PWM Dimming
Figure 3. Average LED Current versus Duty Cycle of PWM Dimming Signal at PWM Terminal
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Bill of Materials (BOM)
5 Bill of Materials (BOM)
Part ID Part Value Part Number Manufacturer
U1 1A constant current buck regulator, SOT-6 LM3405A Texas Instruments
L1 10 µH, 1.3A, 53 mΩ, 6.0 x 6.0 x 2.8 mm SLF6028T-100M1R3-PF TDK
C1 10 µF, 25 V, X5R, 1206 GRM31CR61E106KA12L Murata
C2 1 µF, 35 V, X7R, 1206 GMK316BJ105KL-T Taiyo Yuden
C3 0.01 µF, 16 V, X7R, 0805 0805YC103KAT2A AVX
C4 0.1 µF, 16 V, X7R, 0805 GRM219R71C104KA01D Murata
C5 1 µF, 35 V, X7R, 0805 GMK212BJ105KG-T Taiyo Yuden
D1 Schottky, 40 V, 1A, SMA SS14-E3/61T Vishay
D2 Schottky, 30 V, 200 mA, SOD-323 BAT54WS-TP Micro Commercial Co.
D3 5.1 V, 0.35W, SOT23 MMBZ5231B-7-F Diodes
D4 Not installed
R1 0.5W, 0.2 Ω, 1%, 2010 WSL2010R2000FEA Vishay
R2 100 kΩ, 1/8W, 1%, 0805 CRCW0805100KFKEA Vishay
R3 1.0 kΩ, 1%, 1/8W, 0805 CRCW08051K00FKEA Vishay
P1 6-position connector 5535676-5 Tyco/AMP
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Table 1. Bill of Materials (BOM)
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6 Typical Performance Characteristics
Figure 4. Efficiency vs LED Current Figure 5. Efficiency vs LED Current
(VIN= 5 V, V
Derived from VIN) (VIN= 12 V)
BOOST
Typical Performance Characteristics
Figure 6. Efficiency vs Input Voltage Figure 7. Switching Waveforms
(IF= 1A)
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Typical Performance Characteristics
Figure 8. Startup During PWM Dimming Figure 9. Shutdown During PWM Dimming
(VIN= 12 V, IF= 1A) (VIN= 12 V, IF= 1A)
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7 Layout
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)
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Figure 13. Internal Plane 2 (VIN)
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LM3405A
V
IN
V
IN
EN/DIM
BOOST
SW
FB
GND
C3
D1
D2
D3
C4
R3
C1
PWM
L1
C2
R1
C5
V
OUT
1
2
3
4
5
6
P1
7126
1
R2
FB
2
3
4
5
D4
GND
LED
ANODE
LED
CATHODE
LM3405A
V
IN
V
IN
EN/DIM
BOOST
SW
FB
GND
C3
D1
D2
D3
C4
R3
C1
PWM
L1
C2
R1
C5
V
OUT
1
2
3
4
5
6
P1
7126
1
R2
FB
2
3
4
5
D4
GND
LED
ANODE
LED
CATHODE
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8 Additional Circuit Configuration Schematics
Additional Circuit Configuration Schematics
Figure 14. V
Figure 15. V
Derived from V
BOOST
Derived from V
BOOST
IN
OUT
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LM3405A
V
IN
V
IN
EN/DIM
BOOST
SW
FB
GND
C3
D1
D2
D3
C4
R3
C1
PWM
L1
C2
R1
C5
V
OUT
1
2
3
4
5
6
P1
7126
1
R2
FB
2
3
4
5
D4
GND
LED
ANODE
LED
CATHODE
LM3405A
V
IN
V
IN
EN/DIM
BOOST
SW
FB
GND
C3
D1
D2
D3
C4
R3*
C1
PWM
L1
C2
R1
C5
V
OUT
1
2
3
4
5
6
P1
7126
1
R2
FB
2
3
4
5
D4
GND
LED ANODE
LED
CATHODE
* a zener diode is connected in R3 spot
Additional Circuit Configuration Schematics
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Figure 16. V
Figure 17. V
10
AN-1685 LM3405A Demo Board SNVA271B–October 2007–Revised April 2013
Derived from VINthrough a Series Zener Diode
BOOST
Derived from V
BOOST
through a Series Zener Diode
OUT
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