DC2020A: 300mA @ 67V Max, 5 ~ 60V in, Boost

Demonstration circuit 2020A is a 60VIN, 80VOUT LED driver. It generates its own PWMOUT waveform from its internal PWM generator for accurate PWM dimming with up to 33:1 brightness ratio. It accepts an input voltage from 5V to 60V and drives up to 67V of LEDs at 300mA (when PVIN is less than VLED). DC2020A features both PWM and analog dimming of the LED string. It has an OPENLED flag that indicates when the LED string has been removed.

DC2020A features high efficiency at 350 kHz switching frequency. At high LED string voltages up to 67V and 300mA of LED current, the single switch controller has 92% efficiency. The LT3955 has an internal 80V, 3.5A switch that simplifies the schematic and layout.

Small ceramic input and output capacitors are used to save space and cost. The open LED overvoltage protection uses the IC’s constant voltage regulation loop to regulate the output to approximately 75V if the LED string is opened although it may reach 79V peak during transient from running LEDs to open string.

For low input voltage operation, the CTRL pin voltage is reduced as the input voltage drops below 9V, reducing LED brightness and restraining the peak switch currents in order to limit inductor and switch size. UVLO turns the LEDs off when VIN drops below 5V.

DC2020A PWM dimming is simplified when compared with other LED drivers. The LT3955 generates its own PWMOUT dimming waveform at a frequency determined by the capacitance on the PWM pin (C16 gives 320Hz for DC2020A). The PWMOUT duty cycle is determined by the voltage on the DIM terminal. Between 0V and 7.7V VDIM gives between 4% and 96% PWM duty cycle. Information regarding PWM dimming ratios and performance can be found in the LT3955 data sheet in the applications section.
Analog dimming is also simple to use with a single voltage source on the CTRL terminal.

Modifications can be made to DC2020A in order to convert the board to power different LED strings or from an LED driver to a constant voltage regulator or battery charger. It can easily be changed from a boost topology to a SEPIC, buck mode, or buck-boost mode LED driver. Please consult the factory or the LT3955 data sheet for details. It can be modified to provide LED+ to GND short-circuit protection as well.

The LT3955 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 DC2020A. The LT3955EUHE is assembled in a 36-lead plastic QFN package with two thermal pads underneath the IC. Proper board layout is essential for maximum thermal performance. See the data sheet section Layout Considerations.