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LED driver integrates all circuitry in a SMT package

18 Sep 2015  | David Ng

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Before we begin, check out the previous instalments of this series. Click for Part 1, Part 2, Part 3, Part 4, Part 5 and Part 6.

Once relegated to the hinterlands of low cost indicator lights, the LED is again in the spotlight of the lighting world. LED lighting is now ubiquitous, from car headlights to USB-powered lava lamps. Car headlights exemplify applications that capitalise on the LED's clear advantages—unwavering high-quality light output, tough-as-steel robustness, inherent high efficiency—while a USB lava lamp exemplifies applications where only LEDs work.

Despite these clear advantages, their requirement for regulated voltage and current make LED driver circuits more complex than the venerable light bulb, but some new devices are closing the gap. For instance, the LTM8040µModule LED driver integrates all the driver circuitry into a single package, allowing designers to refocus their time and effort on the details of lighting design critical to a product's success.

LED driver
The LTM8040 is a complete step-down DC/DC switching converter system that can drive up to 1A through a string of LEDs. Its 4V to 36V input voltage range makes it suitable for a wide range of power sources, including 2-cell lithium-ion battery packs, rectified 12VAC and industrial 24V.

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Figure 1: Driving an LED string with the LTM8040 is simple—just add the input capacitor and connect the LED string.

The LTM8040 features both analogue and pulse-width modulation (PWM) dimming, allowing a 250:1 dimming range. The built-in 14V output voltage clamp prevents damage in the case of an accidental open LED string. The default switching frequency of the LTM8040 is 500kHz, but switching frequencies to 2MHz can be set with a resistor from the RT pin to GND.

Easy to use
The high level of integration in the LTM8040 minimises external components and simplifies board layout. As shown in figure 1, all that is necessary to drive an LED string up to 1A is the LTM8040 and an input decoupling capacitor. Even with all this built-in functionality, the LTM8040 itself is small, measuring only 15mm×9mm×4.32mm.

Feature set

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Figure 2: Drive a 0V to 1.25V voltage into the ADJ pin to control the LED's input current amplitude g.

The LTM8040 features an ADJ pin for precise LED current amplitude control. The ADJ pin accepts a full-scale input voltage range of 0V to 1.25V, linearly adjusting the output LED current from 0A to 1A. Figure 2 shows the ratiometric response of the output LED current versus the ADJ voltage.

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Figure 3: Control the LED current with a single resistor from ADJ to ground.

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