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Linear Technology Releases New Dual String 2MHz Boost 50mA LED Driver
The LT3909 offers PWM dimming ratios up to 40,000:1 and Requires Just a Single Set Resistor for LED Current
Linear Technology has released 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 2.9V to 40V
makes it ideal for applications ranging from single/multiple LiIon 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 Vf for optimum efficiency. Although
designed to drive LEDs in a boost topology, LED current is accurately regulated even when Vin is greater Vout.
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 3mm x 3mm DFN (or MSOP12E) package offers a very compact solution footprint for LED applications requiring
two strings of 50mA LEDs.
The LT3909 utilizes an internal 400mΩ, 40V, 1A switch with an integrated Schottky diode onchip,
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, softstart
and micropower shutdown.
The LT3909EDD is available in a 12-lead 3mm x 3mm DFN package and the LT3909EMSE is available in a
thermally enhanced 12lead MSOP package. Industrial temperature versions, the LT3909IDD and LT3909IMSE, are
tested and guaranteed to operate from a -40°C to 125°C operating junction temperature. High temperature versions,
the LT3909HDD and LT3909HMSE, are tested and guaranteed to operate from a 40°C to 150°C operating junction
Key Features of the LT3909
- Up to 36V of LEDs, 2-String x 50mA LED Driver
- Wide Input Range: 2.9V to 40V
- Up to 40,000:1 PWM Dimming
- ±1.8% LED Current Matching (Type ±0.3%)
- Integrated Schottky Diode
- Internal 400mΩ, 40V, 1A Switch
- Single Resistor Sets LED Current (10mA to 50mA) for Both High Side Current Sources
- Output Adapts to LED Vf for Optimum Efficiency
- LED Current Regulated Even for Vin > Vout
- Fault Flag+ Protection for Open-LED and LED-Short-to-GND (Other String Remains in Regulation)
- Programmable Maximum Vout
- Internal Compensation, Soft-Start and Thermal Regulation
- Accurate EN/UVLO Threshold
- Fixed 2MHz Switching Frequency
The company's Web site address is www.linear.com.
[Reprinted with kind permission from Linear Technology - Release Date: 20th October, 2015]