LM3405_08 Datasheet, PDF (1/6 Pages) National Semiconductor (TI) – Powering and Dimming High-Brightness LEDs with the LM3405 Constant-Current Buck Regulator

Powering and Dimming
High-Brightness LEDs with
the LM3405 Constant-
Current Buck Regulator
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
National Semiconductor
Application Note 1644
Vinit Jayaraj
January 17, 2008
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_IN or V_OUT. The BOOST to SW voltage
(VBOOST-VSW) should never exceed the operating limit of
5.5V, and must be greater than 2.5V for optimum efficiency.
FIGURE 1. Typical Application Circuit
30022901
In the above circuit, the maximum V_IN is 5V and therefore the
boost voltage is derived from V_IN. The LM3405 data sheet
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_F) to be set by the following equation:
R1 (min) is 200 mΩ such that I_F (max) is limited to 1A. In case
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 300229
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