Dual string, 2 MHz boost 50 mA LED driver

October 21, 2015 // By Graham Prophet
LT3909 is a high efficiency, constant current LED driver that can drive two strings of up to 10 LEDs at 50 mA. To ensure constant brightness, each LED string contains an accurate high side current source with ±1.8% current matching. Its input voltage range of 2.9V to 40V suits it for applications ranging from single/multiple Li-Ion inputs to automotive and industrial applications.

The LT3909 offers PWM dimming ratios up to 40,000:1, supporting LED backlighting in applications such as automotive and avionic displays subject to a wide range of ambient lighting conditions. The LT3909 delivers up to 36V of LED drive per string, and its output voltage adapts to any variation in LED VF for optimum efficiency. Although designed to drive LEDs in a boost topology, LED current is accurately regulated even when VIN is greater than VOUT. The LT3909 operates at a constant switching frequency of 2 MHz, enabling the use of very small externals while avoiding critical frequency bands such as AM radio in automotive applications. The combination of its tiny externals and 3 x 3mm DFN (or MSOP-12E) package offers a very compact solution footprint for LED applications requiring two strings of 50 mA LEDs.

The LT3909 has an internal 400 mΩ, 40V, 1A switch with an integrated Schottky diode on-chip. A high side LED current sense topology enables the LED cathode to be connected directly to ground, creating a single wire current source. A single resistor sets LED current from 10 mA to 50 mA for both high side current sources. The LT3909 offers both open and shorted LED protection with a fault flag to indicate these conditions. Even if one LED string experiences an open-LED or short-to-GND failure, the other string will remain in regulation. Other features include internal compensation, soft-start and micropower shutdown.

The LT3909EDD is available in a 12-lead 3 x 3mm DFN package and the LT3909EMSE is available in a thermally enhanced 12-lead MSOP package. There are industrial temperature versions, and high temperature versions. Pricing starts at $2.50 (1,000).

Linear Technology; www.linear.com/product/LT3909