Constant current IC drives two 10-LED strings at 50mA

26th October 2015

Barney Scott

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Datasheet

Linear Technology announces the LT3909, a high efficiency, constant current LED driver that can drive two strings of up to 10 LEDs at 50mA. To ensure constant brightness, each LED string contains an accurate high side current source with ±1.8% current matching. Its input voltage range of 2.9 to 40V makes it suitable 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, providing an LED backlighting solution in applications such as automotive and avionic displays subject to a wide range of ambient lighting conditions. The LT3909 delivers up to 36V of LEDs per string, and its output voltage adapts to any variation in LED V_F for optimum efficiency. Although designed to drive LEDs in a boost topology, LED current is accurately regulated even when V_IN is greater than V_OUT.

The LT3909 operates at a constant switching frequency of 2MHz, 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 3mm² DFN (or MSOP-12E) package offers a very compact solution footprint for LED applications requiring two strings of 50mA LEDs.

The LT3909 utilises an internal 400mΩ, 40V, 1A switch with an integrated Schottky diode on-chip, eliminating both the added cost and space required of an external diode. A unique 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 10mA to 50mA 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 3mm² DFN package and the LT3909EMSE is available in a thermally enhanced 12-lead MSOP package. Industrial temperature versions, the LT3909IDD and LT3909IMSE, are tested and guaranteed to operate from a -40 to +125°C operating junction temperature. High temperature versions, the LT3909HDD and LT3909HMSE, are tested and guaranteed to operate from a -40 to +150°C operating junction temperature. Pricing starts at $2.50 each for 1,000-piece quantities and all versions are available from stock.