DEMO MANUAL DC2857A LT3922-1 36V 2A Boost LED Driver with 25,000:1 PWM Dimming DESCRIPTION Demonstration circuit DC2857A is a 36V 2A synchronous When run with both PWM dimming and spread spectrum, 2MHz boost LED driver featuring the LT 3922-1. It drives the spread spectrum aligns itself with the PWM signal for a single string of LEDs at 330mA up to 34V when V is flicker-free operation. IN between 7V and 28V. It runs down to 4V with reduced IN Small ceramic input and output capacitors are used to I and can withstand V as high as 36V, but has over- LED IN save space and cost. The board is designed with tiny, high voltage lockout (OVLO) set for 28V for this application. frequency capacitors on both sides of the VOUT pins for DC2857A runs at 2MHz switching frequency. Spread a reduction in radiated EMI. The open LED overvoltage spectrum frequency modulation (SSFM) can be turned protection uses the ICs constant voltage regulation loop on with a simple jumper, reducing EMI. DC2857A comes to regulate the output to approximately 37.5V if the LED with low EMI features including optimized layout, SSFM string is opened, although it may reach almost 40V peak and input EMI filter. It passes CISPR 25 class 5 conducted during transient from running LEDs to open. There is a and radiated EMI. It is protected against both open and + protection diode from LED to GND to prevent negative short LED conditions and reports the faults. ringing during a short-circuit. The output current can be The LT3922-1 has an input voltage range from 2.8V to monitored through the ISMON output pin. 36V. Its internal synchronous 2A 40V switches allow up Undervoltage and overvoltage lockout can be adjusted on to 34V of LEDs on the output with room for over voltage the circuit with a few simple resistor choices. protection and overshoot during an open LED event. It There is an EMI filter on the input of DC2857A. Please has adjustable switching frequency between 200kHz and follow the recommend layout and four-layer thickness of 2.5MHz. It can be synchronized to an external source or DC2857A for low EMI applications. programmed with SSFM for low EMI. The PWMTG high- side PWM MOSFET driver assists with short-circuit pro- The LT3922-1 data sheet gives a complete description of tection and versatility. LT3922-1 can be configured as a the part, operation and applications information. The data boost, boost-buck, or buck mode LED driver and maintain sheet must be read in conjunction with this demo manual all of its low EMI, PWM dimming, and fault diagnostic for demonstration circuit DC2857A. The LT3922EUFD-1 is features. assembled in a 28-lead plastic 4mm x 5mm QFN package with a thermally enhanced ground pad. Proper board lay- The LT3922-1 can be PWM dimmed with an external PWM out is essential for maximum thermal performance. See signal or an internally-generated PWM signal. DC2857A the data sheet section Layout Considerations. has a jumper that can be set to switch between internally- generated PWM signal, externally-generated PWM signal, Design files for this circuit board are available at and no PWM signal (100% on). It can be analog dimmed DEMO MANUAL DC2857A PERFORMANCE SUMMARY Specifications are at T = 25C A PARAMETER CONDITIONS MIN TYP MAX UNITS Input Voltage Low EMI V Range Operating I > 0mA 4 28 V IN LED Input Voltage Low EMI V Range Operating V = 34V, I = 330mA 7 28 V IN LED LED Safe Input Voltage Low EMI V Range 0 36 V IN Switching Frequency R2 = 45.3k, SSFM = OFF 2 MHz Switching Frequency R2 = 45.3k, SSFM = ON 2 to 2.5 MHz I R1 = 0.3, 7.0V < Low EMI, V < 28V, V = 34V 330 mA LED IN LED V range R4 = 1M, R5 = 33.2k V 34 V LED IN Open LED Voltage V R4 = 1M, R5 = 33.2k, OPEN LOAD 37.5 V OUT Efficiency (100% PWM DC) PVIN = 12V, V = 34V, I = 330mA 90 % LED LED Efficiency with EMI Filters Removed PVIN = 12V, V = 34V, I = 330mA 91 % LED LED Internally-Generated PWM Dimming Range Operating JP1 = INT, JP2 = INT 1/128 100% Internally-Generated PWM Dimming Frequency Operating JP1 = INT, JP2 = INT, R10 = 332k 122 Hz QUICK START PROCEDURE Demonstration circuit DC2857A is easy to set up to evalu- 6. Release the EN/UVLO-to-GND connection. ate the performance of the LT3922-1 Follow the proce- 7. Observe the LED string running at the programmed dure below: LED current. 1. With power off, connect a string of LEDs that will run 8. To change the brightness with analog dimming, simply with forward voltage less than or equal to 34V (at attach a voltage source the CTRL terminal and set the + 330mA) to the LED and LED terminals on the PCB voltage between 0V and 1.5V. See data sheet for details. as shown in Figure1. 9. To change brightness with external PWM dimming, set 2. Connect the EN/UVLO terminal to GND. JP1 to EXT and JP2 to EXT/ON. Attach a 3V rectangular 3. Always-on LED operation. Set JP3 to NO SSFM to run waveform with varying duty cycle to the PWM terminal. without SSFM or external synchronization. 10. To change brightness with internally-generated PWM 4. With power off, connect the input power supply to the dimming, set JP1 to INT and JP2 to INT. Adjust the LOW EMI VIN and GND terminals. Make sure that the setting of the VR1 variable resistor with a small flat- input voltage will not exceed 36V. head screwdriver to toggle between 0% and 100% PWM dimming duty cycle in 1/128 steps. 5. Turn the input power supply on and make sure the voltage is between 5V and 36V to start operation. 11. To enable spread spectrum frequency modulation, set JP3 to SSFM ON. UG-1406 Rev 0 2