DEMO MANUAL DC2079A LT3954 40V 40V LED Driver IN OUT Description Demonstration circuit 2079A is a 40V , 40V LED driver. by the capacitance on the PWM pin (C16 gives 300Hz for IN OUT It generates its own PWMOUT waveform from its internal DC2079A). The PWMOUT duty cycle is determined by the PWM generator for accurate PWM dimming with up to voltage on the DIM terminal. Between 0V and 8V VDIM 33:1 brightness ratio. It accepts an input voltage from gives between 3% and 97% PWM duty cycle. Information 5V to 30V and drives up to 32V of LEDs at 650mA (when regarding PWM dimming ratios and performance can be PV is less than V ). DC2079A features both PWM and found in the LT3954 data sheet in the applications sec- IN LED analog dimming of the LED string. It has an OPENLED flag tion. Analog dimming is also simple to use with a single that indicates when the LED string has been removed. voltage source on the CTRL terminal. DC2079A features high efficiency at 350kHz switching Modifications can be made to DC2079A in order to convert frequency. At high LED string voltages up to 32V and the board to power different LED strings or from an LED 650mA of LED current, the single switch controller has driver to a constant voltage regulator or battery charger. It 94.6% efficiency (14V input). The LT3954 has an internal can easily be changed from a boost topology to a SEPIC, 40V, 5.4A switch that simplifies the schematic and layout. buck mode, or buck-boost mode LED driver. Please consult the factory or the LT3954 data sheet for details. Small ceramic input and output capacitors are used to save It can be modified to provide LED+ to GND short-circuit space and cost. The open LED overvoltage protection uses protection as well. the ICs constant voltage regulation loop to regulate the output to approximately 35V if the LED string is opened The LT3954 data sheet gives a complete description of although it may reach 38V peak during transient from the part, operation and applications information. The data running LEDs to open string. sheet must be read in conjunction with this demo manual for demonstration circuit DC2079A. The LT3954EUHE For low input voltage operation, the CTRL pin voltage is is assembled in a 36-lead plastic QFN package with two reduced as the input voltage drops below 8.5V, reducing thermal pads underneath the IC. Proper board layout is LED brightness and restraining the peak switch currents essential for maximum thermal performance. See the in order to limit inductor and switch size. UVLO turns the Layout Considerations section in the data sheet. LEDs off when V drops below 5.3V. IN Design files for this circuit board are available at DC2079A PWM dimming is simplified when compared DEMO MANUAL DC2079A p s Specifications are at T = 25C A PARAMETER CONDITION VALUE (TYPICAL) Input Voltage PVIN Range Operating 5V to V (Up to 40V) LED Switching Frequency R1 = 28.7k 350kHz I RS1 = 0.39 8.4V < PVIN < V (40V) 645mA LED LED Low PVIN I (CTRL Foldback) RS1 = 0.39, PVIN = 6.0V 486mA LED RS1 = 0.39 , PVIN = 7.0V 577mA RS1 = 0.39, PVIN = 8.0V 640mA V Range R7 = 1M, R8 = 37.4k PV < V < 32V LED IN LED Open LED Voltage R7 = 1M, R8 = 37.4k 34.8V Typical Efficiency PV =14V, V = 32V, I = 645mA PWM = INTV 94.6% IN LED LED CC PVIN Undervoltage Lockout (Falling Turn-Off) R3 = 499k and R4 = 147k 5.3V PVIN Undervoltage Lockout (Rising Turn-On) R3 = 499k and R4 = 147k 6.5V INTV Operating 7.85V CC Peak Switch Current Limit 5.4A PWMOUT Dimming Duty Cycle V = 8.0V 97.5% DIM V = 4.0V 50% DIM V = 1.5V 10% DIM V = 0V 3.1% DIM Internal PWM Dimming Frequency C16 = 0.047F, 0V < V < 8V 300Hz DIM dc2079af 2 ummary erformance