LED drivers integrate all circuitry, including the inductor, into a single surface mount package

Introduction

Once relegated to low-cost indicator lamps, LEDs are once again in the spotlight in lighting. LED lighting is now ubiquitous, from automotive headlamps to USB-powered lava lamps. While automotive headlamps showcase applications that can take advantage of the clear benefits of LEDs—consistent, high-quality light output, steel-like ruggedness, and inherent efficiency—USB lava lamps highlight applications where LEDs are uniquely suited. Despite the obvious benefits of LEDs, their requirements for regulated voltage and current make LED driver circuits more complex than the bulbs of the past. However, new devices are gradually closing the gap. For example, the LTM®8040 µModule LED driver integrates all driver circuitry into a single package, allowing designers to refocus their time and energy on the details of lighting design that are critical to the success of their products.

An excellent LED driver

LTM8040 is a complete step-down DC/DC switching converter system that can drive up to 1A of current through an LED string. The device's 4V to 36V input voltage range makes it suitable for a variety of power applications, including two lithium-ion battery packs, rectified 12VAC and industrial 24V. LTM8040 has both analog and PWM dimming functions, providing a 250:1 dimming range. The built-in 14V output voltage clamp circuit is used to avoid damage from accidental LED string open circuit. The default switching frequency of the LTM8040 is 500kHz, but the switching frequency can be set to 2MHz using a resistor connected between the RT pin and GND.

The high level of integration of the LTM8040 minimizes

external component count and simplifies board layout. As shown in Figure 1, all that is needed to drive an LED string at up to 1A is the LTM8040 and an input decoupling capacitor. Even with all of these features built in, the LTM8040 itself is still very small, measuring only 15mm x 9mm x 4.32mm.

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

Rich Feature Set

The LTM8040 has an ADJ pin for precise LED current amplitude control. The ADJ pin accepts a full-scale input voltage range of 0V to 1.25V and linearly adjusts the output LED current (from 0A to 1A). Figure 2 shows the ratiometric response of the output LED current versus the ADJ voltage. The ADJ pin is pulled up to an internal 1.25V reference through a 5.11k precision resistor internally, so the output LED current can also be adjusted by placing a single resistor between the ADJ pin and ground, as shown in Figure 3.

Figure 2: Driving a 0V ~ 1.25V voltage to the ADJ pin to control the LED current amplitude

Figure 3: LED current is controlled using a single resistor connected between ADJ and ground.

The PWM control pin provides high dimming ratios. Using an external MOSFET in series with the LED string (as shown in Figure 4), the LTM8040 can achieve dimming ratios in excess of 250:1. As shown in Figure 5, distortion of the PWM LED current is minimal, even at frequencies as low as 10Hz. The 10Hz performance shown is intended to illustrate the capabilities of the LTM8040—this frequency is too low for practical pulse width modulation and is well within the range of human eye discrimination.

Figure 4: The LTM8040 can pulse-width modulate (PWM) its LED string using an external MOSFET

Figure 5: The LTM8040 can pulse-width modulate (PWM) the LED current with minimal distortion,
even at frequencies as low as 10Hz.

In addition, the LTM8040 features a low power shutdown state. When the /SHDN pin is active low, the input quiescent current is less than 1µA.

Figure 6: The LTM8040 LED driver has a footprint of only
9mm x 15mm x 4.32mm and is a complete system in an LGA package.

Conclusion

The LTM8040 µModule LED driver makes driving LEDs easy. Its high level of integration and rich feature set (including open LED protection, analog and PWM dimming) can save a lot of design time and board space.