Features and Benefits
- True Color PWM™ Delivers Constant Color with 400:1 Dimming Range
- Wide Input Range: 4V to 36V
- Up to 1A LED Current
- Adjustable 200kHz–2MHz Switching Frequency
- Adjustable Control of LED Current
- Integrated Boost Diode
- High Output Current Accuracy is Maintained Over a Wide Range from 35mA to 1A
- Open LED (LT3474) and Short-Circuit Protection
- High Side Sense Allows Grounded Cathode Connection
- Uses Small Inductors and Ceramic Capacitors
- LT3474-1 Drives LED Strings Up to 26V
- Compact 16-Lead TSSOP Thermally Enhanced Surface Mount Package
The LT3474/LT3474-1 are fixed frequency step-down DC/DC converters designed to operate as constant-current sources. An internal sense resistor monitors the output current allowing accurate current regulation, ideal for driving high current LEDs. High output current accuracy is maintained over a wide current range, from 35mA to 1A, allowing a wide dimming range.
Unique PWM circuitry allows a dimming range of 400:1, avoiding the color shift normally associated with LED current dimming.
The high switching frequency offers several advantages, permitting the use of small inductors and ceramic capacitors. Small inductors combined with the 16 lead TSSOP surface mount package save space and cost versus alternative solutions. The constant switching frequency combined with low-impedance ceramic capacitors result in low, predictable output ripple.
With their wide input range of 4V to 36V, the LT3474/ LT3474-1 regulate a broad array of power sources, from 5V logic rails to unregulated wall transformers, lead acid batteries and distributed power supplies. A current mode PWM architecture provides fast transient response and cycle-by-cycle current limiting. Frequency foldback and thermal shutdown provide additional protection.
- Automotive and Avionic Lighting
- Architectural Detail Lighting
- Display Backlighting
- Constant Current Sources
Product Lifecycle Production
At least one model within this product family is in production and available for purchase. The product is appropriate for new designs but newer alternatives may exist.
Evaluation Kits (2)
Demonstration circuit 842 features the LT3474 constant frequency 1A step-down LED driver in a compact 16-lead TSSOP thermally enhanced package. The demonstration circuit is designed to drive a single Lumileds Luxeon III star white LED (included on the board) from a wide 6V to 36V input voltage range. The typical LED forward voltage at 1A is 3.9V.
The high switching frequency scheme permits the use of small inductors and ceramic capacitors to save space and cost. The current mode PWM scheme provides fast transient response and cycle-by-cycle current limiting. Frequency foldback and thermal shutdown provide additional protection.
Demonstration circuit 1309 features the LT3474-1 36V step-down 1A LED driver. The demonstration circuit is designed to drive a string of 1A LEDs from a high input voltage. The LT3474-1 operates with a 4V to 36V input voltage range and maintains high LED current accuracy from 35mA to 1A. The LT3474-1 can be dimmed with its analog CTRL pin or with a PWM dimming input signal. A few simple modifications can be made to DC1309 in order to add PWM dimming to the circuit.
An external overvoltage protection clamp provides open LED protection and is set to about 24V on DC1309A. The level of the clamp can be changed by replacing the zener diode (D4) with a different zener diode. The LT3474-1 differs from the LT3474 in its output voltage capability. The LT3474 has a 13V clamp that does not allow the use of an LED string of greater than 13V on the output. However, LT3474-1 does not have this clamp and it can power LEDs up to 26V, as long as the external clamp is adjusted correctly.
Tools & Simulations
LTspice® is a powerful, fast and free simulation software, schematic capture and waveform viewer with enhancements and models for improving the simulation of analog circuits.
To launch ready-to-run LTspice demonstration circuits for this part:
Step 1: Download and install LTspice on your computer.
Step 2: Click on the link in the section below to download a demonstration circuit.
Step 3: If LTspice does not automatically open after clicking the link below, you can instead run the simulation by right clicking on the link and selecting “Save Target As.” After saving the file to your computer, start LTspice and open the demonstration circuit by selecting ‘Open’ from the ‘File’ menu.
Models for the following parts are available in LTspice:
ADI has always placed the highest emphasis on delivering products that meet the maximum levels of quality and reliability. We achieve this by incorporating quality and reliability checks in every scope of product and process design, and in the manufacturing process as well. "Zero defects" for shipped products is always our goal.
Sample & Buy
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