User's Guide
SNVA271B – October 2007 – Revised April 2013
AN-1685 LM3405A Demo Board
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
O(MIN)
of 205 mV (the reference voltage) to a V
O(MAX)
determined by the maximum duty
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 V
IN
through a shunt zener (D3). This will ensure that the gate drive voltage V
BOOST
- V
SW
falls
in the recommended range of 2.5 V to 5.5 V when V
IN
varies 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 V
IN
by 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
).
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 I
F
delivered to the LED array is 1.0A. To adjust this value the current setting
resistor R1 can be changed according to
Equation 1
:
I
F
= V
FB
/ R1
(1)
The feedback voltage V
FB
is 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 V
IN
through a 100 k
Ω
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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1
SNVA271B – October 2007 – Revised April 2013
AN-1685 LM3405A Demo Board
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