Linear Technology Corporation has introduced a new high-efficiency constant-current LED driver LT3909, which 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 a current matching degree of ± 1.8%. Its input voltage range of 2.9V to 40V makes it ideal for a wide range of applications from single / multiple lithium ion input to automotive and industrial applications.
The LT3909 provides a PWM dimming ratio of up to 40,000: 1, which provides an LED backlight solution for applications such as automotive and avionics displays under a variety of ambient lighting conditions. Each string of LT3909 can provide up to 36V LED, and its output voltage can adapt to any change of LED VF to achieve the best efficiency. Although designed to drive LEDs in a boost topology, LED current can be accurately adjusted even if VIN is greater than VOUT. The LT3909 operates at a constant switching frequency of 2MHz, allowing the use of very small external components while avoiding critical frequency bands such as AM radios in automotive applications. The combination of its slim external structure and 3mm x 3mm DFN (or MSOP-12E) package provides a very compact solution size for LED applications requiring two strings of 50mA LEDs.
The LT3909 utilizes an internal 400mΩ, 40V, 1A switch and an on-chip integrated Schottky diode, eliminating the added cost and space required for external diodes. The unique high-end LED current detection topology allows the LED cathode to be directly grounded, thereby forming a single-line current source. A single resistor sets the LED current of two high-side current sources from 10mA to 50mA. The LT3909 provides open and short LED protection with a fault flag to indicate these conditions. Even if one LED string has an open LED or short to GND, the other string will remain stable. Other features include internal compensation, soft-start and micro-power shutdown.
The LT3909EDD is available in a 12-lead 3mm x 3mm DFN package, while the LT3909EMSE is available in a thermally enhanced 12-lead MSOP package. Industrial temperature versions LT3909IDD and LT3909IMSE are tested and guaranteed to operate over a -40C to 125C operating junction temperature range. The high-temperature versions, LT3909HDD and LT3909HMSE, have been tested and guaranteed to operate over a -40C to 150C operating junction temperature range.
