LINEAR µModule LED Driver with Integrated Circuitry, Including Inductor, in a SMT Package - Design Note 445 by David Ng. Owner's Manual and User Guide.

µModule LED Driver Integrates All Circuitry, Including the Inductor, in a Surface Mount Package – Design Note 445 David Ng Introduction the case of an accidental open LED string. The default

Once relegated to the hinterlands of low cost indicator switching frequency of the LTM8040 is 500k Hz, but lights, the LED is again in the spotlight of the lighting switching frequencies to 2MHz can be set with a resistor world. LED lighting is now ubiquitous, from car headlights from the RT pin to GND.

to USB-powered lava lamps. Car headlights exemplify

Easy to Use

applications that capitalize on the LED’s clear advan-

The high level of integration in the LTM8040 minimizes

tages—unwavering high quality light output, tough-as-

external components and simplifi es board layout. As

steel robustness, inherent high effi ciency—while a USB

shown in Figure 1, all that is necessary to drive an LED

lava lamp exemplifi es applications where only LEDs work.

string up to 1A is the LTM8040 and an input decoupling

Despite these clear advantages, their requirement for

capacitor. Even with all this built-in functionality, the

regulated voltage and current make LED driver circuits

LTM8040 itself is small, measuring only 15mm × 9mm

more complex than the venerable light bulb, but some new

× 4.32mm.

devices are closing the gap. For instance, the LTM®8040 μModule™ LED driver integrates all the driver circuitry Rich Feature Set into a single package, allowing designers to refocus their

The LTM8040 features an ADJ pin for precise LED cur-

time and effort on the details of lighting design critical

rent amplitude control. The ADJ pin accepts a full-scale

to a product’s success.

input voltage range of 0V to 1.25V, linearly adjusting the output LED current from 0A to 1A. Figure 2 shows the

A Superior LED Driver

ratiometric response of the output LED current versus the

The LTM8040 is a complete step-down DC/DC switch-

ADJ voltage. The ADJ pin is internally pulled up through

ing converter system that can drive up to 1A through a

a 5.11k precision resistor to an internal 1.25V reference,

string of LEDs. Its 4V to 36V input voltage range makes

so the output LED current can also be adjusted by ap-

it suitable for a wide range of power sources, including

plying a single resistor from ADJ to ground, as shown

2-cell lithium-ion battery packs, rectifi ed 12VAC and

in Figure 3.

industrial 24V. The LTM8040 features both analog and

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

PWM dimming, allowing a 250:1 dimming range. The

μModule is a trademark of Linear Technology Corporation.

built-in 14V output voltage clamp prevents damage in All other trademarks are the property of their respective owners.

VIN LEDA 1A

14V TO 36V

SHDN LPWR

2.2μF LED

LTM8040

CURRENT

ADJ BIAS

500m A/DIV 0 AMPS

ADJ PIN

RT VOLTAGE GND

500m V/DIV

0 VOLTS DN445 F02

5ms/DIV

DN445 F01

Figure 1. Driving an LED String with the LTM8040 Is Figure 2. Drive a 0V to 1.25V Voltage into the ADJ Pin to Simple—Just Add the Input Capacitor and Connect the Control the LED Current Amplitude LED String