Features and Benefits
- 4-Switch Single Inductor Architecture Allows VIN Above, Below or Equal to VOUT
- Proprietary Peak-Buck Peak-Boost Current Mode
- 3V to 36V Input Voltage Range
- 0V to 36V Output Voltage Range
- ±1.5% Output Voltage Regulation
- ±3% LED Current Regulation
- 5000:1 External and 128:1 Internal PWM Dimming
- Rail-to-Rail LED Current Sense and Monitor Output
- Open and Short LED Protection with Fault Reporting
- 300kHz to 2MHz Fixed Switching Frequency with External Frequency Synchronization
- Flicker-Free Spread Spectrum for Low EMI
- Available in 28-Lead QFN (4mm × 5mm)
The LT3942 is a monolithic 4-switch synchronous buck-boost LED driver. The driver can regulate the LED string current up to 34V with input voltages above, below, or equal to the output voltage. The proprietary peak-buck peak-boost current mode control scheme allows adjustable and synchronizable 300kHz to 2MHz fixed frequency operation, or internal 25% triangle spread spectrum operation for low EMI. With 3V to 36V input, 0V to 36V output and seamless low-noise transitions between operation regions, the LT3942 is ideal for LED driver applications in automotive, industrial and battery-powered systems.
The LT3942 provides LED current regulation with up to 128:1 internal and 5000:1 external PWM dimming using an optional high-side PMOS switch. The CTRL pin provides flexible 20:1 analog dimming with ±2.5% LED current regulation at 100mV full scale. Robust fault protection detects an open or short LED condition, during which the LT3942 retries, latches off, or keeps running.
- General Purpose LED Driver
- Automotive and Industrial Lighting
- Voltage Regulator with Accurate Current Limit
Product Lifecycle Recommended for New Designs
This product has been released to the market. The data sheet contains all final specifications and operating conditions. For new designs, ADI recommends utilization of these products.
Evaluation Kits (1)
Demonstration circuit 2404A is a 36V synchronous buckboost LED driver featuring the LT®3942. It drives a single string of LEDs up to 14V at 1A when VIN is between 9V and 36V and will run down to 3VIN with reduced ILED. DC2404A runs at 2MHz switching frequency with spread spectrum frequency modulation (SSFM) disabled. When enabled, SSFM spreads the switching frequency of the LT3942 from fSW to fSW + 25% for reduced EMI performance.
The LT3942 has an operating input voltage range of 3V to 36V. It has internal, synchronous 40V switches for high efficiency and small size. It has an adjustable switching frequency between 300kHz and 2MHz. The LT3942 can by synchronized to an external source, programmed with SSFM enabled for low EMI, or set to normal operation.
The LT3942’s integrated PWMTG high-side PWM MOSFET driver assists with short-circuit protection of the output, as well as PWM dimming of the connected LEDs. The LED string can be PWM dimmed with an external PWM signal or can be dimmed using the internally generated PWM signal. DC2404A has jumpers that can be set to switch between internally and externally generated PWM signals. The external PWM setting defaults to 100% on with no PWM signal applied. The LT3942 can also be analog dimmed by placing a controllable DC voltage on the CTRL pin.
When running PWM dimming with SSFM enabled, the SSFM aligns itself with the PWM signal for flicker free operation. This applies to both internal and external PWM dimming.
Small ceramic input and output capacitors are used to save space and cost. The board is designed with tiny, high frequency capacitors placed near the PVIN and PVOUT pins for a reduction in radiated EMI. The open LED overvoltage protection uses the IC’s constant voltage regulation loop to regulate the output to approximately 17V peak if the LED string is opened. The output current can be monitored through the ISMON output pin.
Undervoltage lockout can be adjusted on DC2404A with a few simple resistor choices.
There is a small ferrite bead EMI filter on the input and output of DC2404A. These filters, combined with proper board layout and SSFM, are effective in reducing EMI. Please follow the recommended layout and four-layer PCB thickness of DC2404A for low EMI applications.
The LT3942 can operate as a constant current LED driver as well as a constant voltage regulator. Small changes to the circuit allow for constant output voltage regulation.
The LT3942 data sheet gives a complete description of the part, operation, and applications information. The data sheet must be read in conjunction with this demo manual for demonstration circuit 2404A. The LT3942EUFD is assembled in a 28-lead plastic QFN (UFD) package with a thermally enhanced exposed ground pad. Proper board layout is essential for maximum thermal performance. See the data sheet section Layout Considerations.
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:
LTpowerCAD® is a power supply design program which selects power stage components, provides detailed power efficiency, shows quick loop Bode plot stability and load transient analysis, and can export to LTspice for simulation.
Design tools for the following parts are available in LTpowerCAD:
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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