Demonstration circuit 2987A is a 36V, 4A synchronous 2MHz buck LED driver featuring the LT3935. This demonstration circuit powers one or two LEDs at 4A. DC2987A runs from an input voltage of 8V to 36V as built and can run down to 3.6VIN if UVLO is adjusted. It runs at 2MHz switching frequency. With Spread Spectrum Frequency Modulation (SSFM) turned on, it runs from 2MHz to 2.5MHz. An optional low-side NMOS PWM dimming MOSFET can be used for a high PWM dimming ratio. DC2987A features undervoltage lockout (UVLO) set at 7V with 0.7V hysteresis for turn-on. This Silent Switcher® demo circuit features low emissions.
The LT3935 has an input voltage range from 3.6V to 36V. It has internal, synchronous 5.8A, 40V switches for high power, and high efficiency with a single IC. It has an adjustable switching frequency between 200kHz and 2MHz. It can be synchronized (SYNC) to an external source or run with SSFM for low EMI.
The LT3935 can be PWM dimmed for accurate brightness control. An optional low-side MOSFET can be driven from the PWM input source for high and accurate dimming ratio. However, LED– can be connected directly to GND for the least components. In this arrangement, PWM dimming has less range. Analog dimming is accomplished by driving the CTRL pin with a voltage below 1.5V to lower the LED sense voltage.
Small ceramic input and output capacitors save space and cost. The open LED overvoltage protection uses the IC’s constant-voltage regulation loop to regulate the output to approximately 9V if the LED string is opened. Then a fault flag is asserted. The output current can be monitored through the ISMON output pin.
The UVLO voltage, LED current, output voltage range, switching frequency, brightness control, and SSFM can all be adjusted with simple modifications to the demo circuit.
The LT3935 data sheet gives a complete description of the device, operation and applications information. The data sheet must be read in conjunction with this demo manual for DC2987A. The LT3935JV is assembled in a 28-lead 5mm × 4mm LQFN package with a thermally enhanced GND.
