DC1772A Analog Devices

DEMO MANUAL DC1772A LT3761EMSE High Voltage LED Controller with Internal PWM Dimming Description DC1772A is a high voltage LED controller with internal by the capacitance on the PWM pin (C8 gives 300Hz for PWM dimming. It generates its own PWMOUT waveform DC1772A). The PWMOUT duty cycle is determined by the for accurate PWM dimming with up to 25:1 brightness voltage on the DIM terminal. Between 0V and 7.7V VDIM ratio. It accepts an input voltage from 8V to 60V (8V UVLO gives between 4% and 96% PWM duty cycle. Information and 9.1V rising turn-on), and drives up to 60V of LEDs regarding PWM dimming ratios and performance can be at 1A (when PVIN is less than V ). DC1772A features found in the LT3761 data sheet in the Applications Infor- LED both PWM and analog dimming of the LED string. It has mation section. Analog dimming is also simple to use with an OPENLED flag that indicates when the LED string has a single voltage source on the CTRL terminal. been removed. Modifications can be made to DC1772A in order to convert DC1772A features high efficiency at 350kHz switching the board to higher or lower power or from an LED driver frequency. At high LED string voltages up to 60V and to a constant voltage regulator or battery charger. It can 1A of LED current, the single switch controller has 94% easily be changed from a boost topology to a SEPIC, buck efficiency. The open LED overvoltage protection uses mode, or buck-boost mode LED driver. Please consult the ICs constant voltage regulation loop to regulate the the factory or the LT3761 data sheet for details. It can be output to approximately 69.8V if the LED string is opened modified to provide LED+ to GND short-circuit protection although it may reach 74V peak during transient from as well. running LEDs to open. The maximum LED string voltage The LT3761 data sheet gives a complete description of can be raised from 60V to 65V (or higher) with a simple the part, operation and applications information. The data change of feedback resistors. sheet must be read in conjunction with this demo manual For low input voltage operation, the CTRL pin voltage is for demonstration circuit DC1772A. The LT3761EMSE is reduced as the input voltage drops below 10V, reducing assembled in a 16-lead plastic MSOP MSE package with LED brightness and restraining the peak switch currents a thermally enhanced ground pad. Proper board layout in order to limit inductor and switch size. UVLO turns the is essential for maximum thermal performance. See the LEDs off when PVIN drops below 8V. data sheet section Layout Considerations. DC1772A PWM dimming is simplified when compared Design files for this circuit board are available at with other LED drivers. The LT 3761 generates its own DEMO MANUAL DC1772A p s Specifications are at T = 25C A PARAMETER CONDITION VALUE (TYPICAL) Input Voltage PVIN Range Operating 8V to V (Up to 60V) LED Switching Frequency R6 = 29.4k 350kHz I RS2 = 0.25 10V < PVIN < V (60V) 1A LED LED Low PVIN I (CTRL Foldback) RS2 = 0.25 PVIN = 8.5V 930mA LED RS2 = 0.25 PVIN = 9V 975mA V Range R3 = 1M R4 = 18.2k PVIN < V < 60V LED LED Open LED Voltage R3 = 1M R4 = 18.2k 69.8V Typical Efficiency PVIN = 14V V = 60V I = 1A PWM = INTVCC 94% LED LED PVIN Under Voltage Lockout (Falling Turn-Off) R1 = 499k and R2 = 90.9k 8V PVIN Under Voltage Lockout (Rising Turn-On) R1 = 499k and R2 = 90.9k 9.1V INTVCC Operating 7.85V Peak Switch Current Limit RS1 = 0.008 12.5A PWMOUT Dimming Duty Cycle VDIM = 7.7V 96% VDIM = 4V 50% VDIM = 1.5V 10% VDIM = 0.4V 4.3% Internal PWM Dimming Frequency C8 = 0.047F 0V < VDIM < 7.7V 300Hz Quick s Dp Demonstration circuit 1772A is easy to set up to evaluate 5. Turn the input power supply on and make sure the the performance of the LT3761EMSE. Follow the proce- voltage is between 8V and 60V (or V ). LED dure below: 6. Release the EN/UVLO-to-GND connection. 1. Connect a string of LEDs that will run with forward volt- 7. Observe the LED string running at the programmed age less than 60V, but greater than PVIN, to the LED+ LED current and brightness related to the programmed and LED terminals on the PCB as shown in Figure 1. PWMOUT duty cycle. 2. Connect the EN/UVLO terminal to GND. 8. To change the brightness with PWM dimming, simply 3. With power off, connect the input power supply to the vary the VDIM voltage between 0V and 7.7V with the PVIN and GND terminals. Make sure that the PVIN DC PWM terminal floating. input voltage will not exceed 60V (or V ). LED 9. To change the brightness with analog dimming, simply 4. Connect the DIM terminal to a voltage between 0V and attach a voltage source on the CTRL terminal and reduce 7.7V to set the internal PWMOUT dimming duty cycle. the voltage below 1.2V. If this terminal is left floating the converter will run with 10. Observe the reduction of brightness in the LED string approximately 12% PWMOUT dimming duty cycle. Pull when PWM or analog dimming. the PWM terminal high to INTVCC to set the converter at 100% duty cycle. dc1772af 2 ure roce tart ummary erformance