Powering and Dimming

High-Brightness LEDs with

the LM3405 Constant-

Current Buck Regulator

National Semiconductor

Application Note 1644

Vinit Jayaraj

January 17, 2008

The LM3405 is a 1A, constant-current buck regulator de-

signed to provide a simple, highly-efficient solution for pow-

ering LEDs with the highest power density. The low-feedback

reference voltage of 205 mV (typical) allows the use of LEDs

with large forward voltages. The LM3405 LED driver comes

with an integrated 1A high-side power switch, internal current

limit, over-voltage protection and thermal shutdown. The

LM3405 driver utilizes current mode control with internal com-

pensation offering ease-of-use and predictable, high-perfor-

mance regulation over a wide range of operating conditions.

This article describes a few of the many application solutions

for the LM3405 LED driver. The LM3405 device is offered in

a 6-pin Thin-SOT (TSOT) package and operates at an internal

switching frequency of 1.6 MHz, allowing the use of small-

value inductors, thereby saving board space.

Typical Application Circuit

Figure 1 shows the typical application circuit using the

LM3405 regulator driving a single LED. The boost voltage can

be derived from V

(VBOOST-VSW) should never exceed the operating limit of

5.5V, and must be greater than 2.5V for optimum efficiency.

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FIGURE 1. Typical Application Circuit

In the above circuit, the maximum V

boost voltage is derived from V

provides a detailed description of various circuits for boost-

voltage generation. The LM3405 driver tightly regulates the

FB-pin voltage to 205 mV (typical) and this allows the LED

current (I

R1 (min) is 200 m

Ω such that I

of an over current, the internal current limit will trigger and turn

off the internal power switch on a cycle-by-cycle basis. In case

of an over voltage (sensed at the FB pin), the internal power

switch is turned off. The LM3405 regulator also has a built-in

thermal shutdown (with hysteresis) which also turns off the

internal power switch if the die temperature exceeds a typical

value of 165°C.

PWM Dimming Using the EN/DIM Pin

The LEDs can be dimmed by applying a PWM-logic signal to

the EN/DIM pin of the LM3405 driver as shown in Figure 2.

© 2008 National Semiconductor Corporation

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