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User's Guide
SNVA254C–August 2007–Revised April 2013
AN-1657 LM3401 Demo Board
1 Introduction
The Texas Instruments LM3401 is a switching buck regulator derived current source designed to drive a
series string of high power, high brightness LEDs. The LM3401 controls an external P-FET, allowing the
user to select a power device for the desired level of forward LED current. The LM3401 demo board can
supply continuous current as high as 1.0A from an input voltage range of 4.5V to 35V. It is designed to
operate at ambient temperatures up to 75°C.
The LM3401 maintains constant current through any number of series-connected LEDs as long as the
input voltage is greater than the combined forward voltage of the LED string.
The LM3401 demo board includes an input terminal for a digital dimming signal, which is compatible with
PWM dimming frequencies up to 10 kHz.
The complete demo board schematic is shown in Figure 1. Typical waveforms and performance curves for
the LM3401 demo board are shown in Figure 3 through Figure 10.
The standard Bill of Materials is shown in Section 8, with an additional Bill of Materials for a high current
application shown in Table 3. The pcb layout is shown in Figure 11 and Figure 12.
2 Powering Up
First, referring to Table 1, verify that the current setting resistor, R1, matches the desired LED current, and
that the current limit resistor (R3) is set appropriately. The default values for R1 and R3 set the average
LED current at 340 mA with a typical peak current limit threshold of 1.5A.
Next, connect the LED string to the terminals as shown in Figure 2. Connect the input voltage supply to
the VIN and GND terminal posts. If dimming is being used, connect a digital PWM signal to the DIM post.
When all connections and polarities are verified, power can be applied. LED strings should not be
connected or disconnected from the output terminals while the demo board is powered up.
The LM3401 demo board can be easily modified to drive a variety of LEDs. Three current setting options
are described in Table 1. For more detailed design and device information, refer to LM3401 Hysteretic
PFET Controller for High Power LED Drive (SNVS516).
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SNVA254C–August 2007–Revised April 2013 AN-1657 LM3401 Demo Board
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