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.