DC2020A

DEMO MANUAL DC2020A LT3955EUHE 60V 80V LED Driver IN OUT Description Demonstration circuit 2020A is a 60V , 80V LED driver. by the capacitance on the PWM pin (C16 gives 320Hz for IN OUT It generates its own PWMOUT waveform from its internal DC2020A). 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 7.7V V DIM 33:1 brightness ratio. It accepts an input voltage from gives between 4% and 96% PWM duty cycle. Information 5V to 60V and drives up to 67V of LEDs at 300mA (when regarding PWM dimming ratios and performance can be PV is less than V ). DC2020A features both PWM and found in the LT3955 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. DC2020A features high efficiency at 350kHz switching Modifications can be made to DC2020A in order to convert frequency. At high LED string voltages up to 67V and the board to power different LED strings or from an LED 300mA of LED current, the single switch controller has driver to a constant voltage regulator or battery charger. It 92% efficiency. The LT3955 has an internal 80V, 3.5A can easily be changed from a boost topology to a SEPIC, switch that simplifies the schematic and layout. buck mode, or buck-boost mode LED driver. Please consult the factory or the LT3955 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 75V if the LED string is opened The LT3955 data sheet gives a complete description of although it may reach 79V 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 DC2020A. The LT3955EUHE 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 9V, 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 data in order to limit inductor and switch size. UVLO turns the sheet section Layout Considerations. LEDs off when V drops below 5V. IN Design files for this circuit board are available at DC2020A PWM dimming is simplified when compared DEMO MANUAL DC2020A p s Specifications are at T = 25C A PARAMETER CONDITIONS VALUE (TYPICAL) Input Voltage PV Range Operating 5V to V (up to 60V) IN LED Switching Frequency R1 = 28.7k 350kHz I RS1 = 0.82 9V < PVIN < V (60V) 300mA LED LED Low PV I (CTRL foldback) RS1 = 0.82 PVIN = 7.0V 240mA IN LED RS1 = 0.82 PVIN = 8.0V 278mA V Range R7 = 1M R8 = 16.9k PV < V < 67V LED IN LED Open LED Voltage R7 = 1M R8 = 16.9k 75V Typical Efficiency PV = 14V V = 67V I = 300mA PWM = INTV 92.1% IN LED LED CC PVIN Under Voltage Lockout (falling turn-off) R3 = 499k and R4 = 165k 5.0V PVIN Under Voltage Lockout (rising turn-on) R3 = 499k and R4 = 165k 6.1V INTV Operating 7.85V CC Peak Switch Current Limit 3.3A PWMOUT Dimming Duty Cycle VDIM = 7.7V 96% VDIM = 4.0V 50% VDIM = 1.5V 10% VDIM = 0.4V 4.3% Internal PWM Dimming Frequency C16 = 0.047F 0V < VDIM < 8V 300Hz Quick s Dp Demonstration circuit DC2020A is easy to set up to 5. Turn the input power supply on and make sure the evaluate the performance of the LT3955EUHE. Follow the voltage is between 6.1V and 60V (or V ) to start. LED procedure below: 6. Release the EN/UVLO-to-GND connection. 1. Connect a string of LEDs that will run with forward 7. Observe the LED string running at the programmed voltage less than or equal to 67V, but greater than the LED current and brightness related to the programmed + intended PVIN, to the LED and LED terminals on the PWMOUT duty cycle. PCB as shown in Figure 1. 8. To change the brightness with PWM dimming, simply 2. Connect the EN/UVLO terminal to GND. vary the VDIM voltage between 0V and 8V with the 3. With power off, connect the input power supply to the PWM terminal floating. PVIN and GND terminals. Make sure that the PVIN DC 9. To change the brightness with analog dimming, sim- input voltage will not exceed 60V (or V ). LED ply attach a voltage source on the CTRL terminal and 4. Connect the DIM terminal to a voltage between 0V reduce the voltage below 1.2V. and 8V to set the internal PWMOUT dimming duty 10. Observe the reduction of brightness in the LED string cycle. If this terminal is left floating the converter will when PWM or analog dimming. run with approximately 12% PWMOUT dimming duty cycle. Pull the PWM terminal high to INTV to set the CC converter at 100% duty cycle. dc2020afa 2 ure roce tart ummary erformance