The LT3909 offers PWM dimming ratios up to 40,000:1, providing an LED backlighting solution for automotive and avionics applications subject to a wide range of ambient lighting conditions. The LT3909 supplies up to 36V of LED current per branch, and its output voltage adapts to any variation in the LED voltage factor (VF) for optimal efficiency. Although designed to drive LEDs in a boost topology, the LED current is precisely regulated even when the input voltage (VIN) is greater than the output voltage (VOUT).
The LT3909 operates at a constant switching frequency of 2 MHz, allowing the use of very small external components while avoiding critical frequency bands such as AM radio in automotive applications. The combination of its tiny external components and 3 mm x 3 mm DFN (or MSOP-12E) package provides a compact solution for LED applications requiring two 50 mA LED strings.
The LT3909 uses an internal 1A, 400mΩ, 40V switch with an on-chip Schottky diode, eliminating both the added cost and space required for an external diode. A unique high-side current-sensing topology allows the LED cathode to be directly connected to ground, creating a current source through a single wire. A resistor adjusts the LED current from 10mA to 50mA for both high-side current sources. The LT3909 offers both open-circuit and short-circuit LED protection with a fault indicator that signals these conditions. Even if one LED branch experiences an open-circuit or short-to-ground fault, the other branch continues to operate normally. Other features of the device include internal compensation, soft-start, and micropower shutdown.
The LT3909EDD is available in a 12-pin 3mm x 3mm DFN package, and the LT3909EMSE is supplied in a thermally enhanced 12-pin MSOP package. The industrial-temperature versions, the LT3909IDD and LT3909IMSE, have been tested and are guaranteed to operate at junction temperatures between -40°C and 125°C. The high-temperature versions, the LT3909HDD and LT3909HMSE, have also been tested and are guaranteed to operate at junction temperatures between -40°C and 150°C.
Summary of features: LT3909
• Dual-branch LED driver at 50 mA and up to 36V per branch
• Wide input range: 2.9V to 40V
• PWM dimming up to 40,000:1
• Inter-branch LED current accuracy of ±1.8% (Typical ±0.3%)
• Integrated Schottky diode
• Internal 400mΩ, 40V, 1A switch
• A single resistor sets the LED current (10mA to 50mA) for both high-side current sources
• Output matches LED VF for optimal efficiency
• LED current is regulated even if VIN > VOUT
• Fault indicator plus protection for open and short-circuited LEDs (unaffected branch maintains regulation)
• Programmable and regulated maximum VOUT
• Internal compensation, soft-start, and thermal regulation
• Precise EN/UVLO threshold
• Fixed 2MHz frequency
• Low-profile DFN package 3mm x 3mm 12-pin or MSOP-12E package
