LINEAR LTM8040 User Guide

DESIGN IDEAS L

µModule LED Driver Integrates All Circuitry, Including the Inductor, in a Surface Mount Package by David Ng Introduction

Once relegated to the hinterlands of VIN to refocus their time and effort on the

VIN LEDA 1A

14V TO 36V

low cost indicator lights, the LED is C1 details of lighting design critical to a

SHDN LPWR

again in the spotlight of the lighting LTM8040 product’s success. ADJ BIAS

world. LED lighting is now ubiquitous,

from car headlights to USB-powered A Superior LED Driver

RT GND

lava lamps. Car headlights exemplify The LTM8040 is a complete step-down applications that capitalize on the DC/DC switching converter system LED’s clear advantages—unwavering that can drive up to 1A through a

high quality light output, tough- Figure 1. Driving an LED string with the string of LEDs. Its 4V to 36V input LTM8040 is simple—just add the input

as-steel robustness, inherent high voltage range makes it suitable for a

capacitor and connect the LED string

efficiency—while a USB lava lamp wide range of power sources, includ- exemplifies applications where only able light bulb, but some new devices ing 2-cell lithium-ion battery packs, LEDs work. Despite these clear advan- are closing the gap. For instance, rectified 12VAC and industrial 24V. tages, their requirement for regulated the LTM®8040 µModule LED driver The LTM8040 features both analog and voltage and current make LED driver integrates all the driver circuitry into PWM dimming, allowing a 250:1 dim- circuits more complex than the vener- a single package, allowing designers ming range. The built-in 14V output

voltage clamp prevents damage in the case of an accidental open LED string.

LED The default switching frequency of the CURRENT

LTM8040 is 500k Hz, but switching

500m A/DIV 0 AMPS

ADJ PIN frequencies to 2MHz can be set with a

VOLTAGE

resistor from the RT pin to GND.

500m V/DIV

0 VOLTS

5ms/DIV Easy to Use

The high level of integration in the

Figure 2. Drive a 0V to 1.25V voltage into the ADJ pin to control the LED current amplitude

LTM8040 minimizes external compo- nents and simplifies board layout. As

VIN LEDA 1A shown in Figure 1, all that is necessary

VIN 4V TO 36V

VIN LEDA C1

4V TO 36V SHDN LPWR

to drive an LED string up to 1A is the

SHDN LPWR LTM8040

2.2µF LTM8040 and an input decoupling

LTM8040 ADJ BIAS

ADJ BIAS PWM capacitor. Even with all this built-in PWM

functionality, the LTM8040 itself is

GND small, measuring only 15mm × 9mm

× 4.32mm.

Rich Feature Set

Figure 3. Control the LED current with Figure 4. The LTM8040 can PWM its

The LTM8040 features an ADJ pin

a single resistor from ADJ to ground LED string with an external MOSFET.

for precise LED current amplitude control. The ADJ pin accepts a full- scale input voltage range of 0V to

LED 1.25V, linearly adjusting the output CURRENT

500m A/DIV LED current from 0A to 1A. Figure 2 shows the ratiometric response of

SIGNAL the output LED current versus the

5V/DIV

ADJ voltage. The ADJ pin is internally

DN445 F05

20ms/DIV

pulled up through a 5.11k precision resistor to an internal 1.25V refer-

Figure 5. The LTM8040 can PWM LED current with minimal distortion, even at frequencies as low as 10Hz. ence, so the output LED current can

Linear Technology Magazine • September 2009