DC1784A: 1A @ 50V Max, 2.5 ~ 40V in, Boost

DC1784A is a triple boost LED driver featuring the LT3797 triple boost LED controller. It accepts an input voltage from 2.5V to 40V (with higher transient) and drives three independent strings of up to 50V LEDs at 1A. DC1784A features independent PWM and analog dimming of each of the three LED strings. Each of the three channels has its own short-circuit protection, open LED protection, and FAULT flag output.

Each or every channel can be altered to run as a different topology such as buck-boost mode, buck mode, or SEPIC depending upon the relationship between input and output voltage. Although the boost is set up to power 50V of LEDs at 1A, the maximum LED string voltage can be changed to almost 100V and the LED current can be adjusted by merely changing a few resistors and external components.

DC1784A features high efficiency at 310kHz switching frequency. At high LED string voltages up to 50V and 1A of LED current, the triple LED boost controller has 93.5% efficiency. The switching frequency can be adjusted between 100kHz and 1MHz with a single resistor. All three channels run in-phase with each other and an external SYNC pin can be used to set the switching frequency and phasing, as well as to optimize PWM dimming.

For low input voltage operation, down to 2.5V, the CTRL pin voltage is reduced as the input voltage drops below 10.5V, reducing LED brightness and restraining the peak switch currents in order to limit thermal rise on this PCB. The LT3797 has a unique, internal buck-boost INTVCC supply that powers the gate drivers at 7.8V, regardless of the state of the input voltage, rather than using an LDO from the input. UVLO turns the LEDs off when VIN drops below 2.5V. Internal OVLO turns off the switching when the input exceeds 41V, but OVLO can be programmed externally to a lower voltage if desired. The LT3797 can withstand transients up to 60V. DC1784A can withstand transients up to 50V with C3 > 50V rating.

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 55V if the LED string is opened although it may reach 59V peak during transient from running LEDs to open. The unique FBH (feedback high) pins allow the overvoltage protection of non-grounded LED driver topologies such as buck-boost mode and buck mode to be accomplished with just two resistors.

Modifications can be made to DC1784A in order to convert the independent channels to higher or lower power or from LED drivers to constant voltage regulators. They can easily be changed from boost topology to SEPIC, buck mode, or buck-boost mode LED drivers. Please consult the factory or the LT3797 data sheet for details.

The LT3797 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 DC1784A. The LT3797EUKG is assembled in a 52-lead (7mm × 8mm) plastic QFN UKG package with a thermally enhanced ground pad. Proper board layout is essential for maximum thermal performance. See the data sheet section ‘Layout Considerations’.