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LM3401 Demo Board
National Semiconductor
Application Note 1657
Allan Fisher
July 29, 2008
Introduction
The 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 dig-
ital 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 Table 2, with an
additional Bill of Materials for a high current application shown
in Table 4. The pcb layout is shown in Figure 11 and Figure
12.
Powering Up
First, referring to Table 1, verify that the current setting resis-
tor, 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 dig-
ital 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 ter-
minals 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 the LM3401 datasheet.
30025401
FIGURE 1. Complete LM3401 Demo Board Schematic
TABLE 1. LED current setting resistors (* = default value)
LED
average current
R1 R3
(typical current limit)
R1 power dissipation
340 mA *
0.59Ω* 24.9 kΩ (1.5A)*
68 mW
700 mA
0.29Ω 47 kΩ (2.8A)
142 mW
1.0A
0.20Ω 68 kΩ (3.9A)
200 mW
© 2008 National Semiconductor Corporation 300254 www.national.com
LM3401 Demo Board AN-1657
