DEMO MANUAL DC2345A LT8391EFE 60V Synchronous 4-Switch Buck-Boost Led Controller with Spread Spectrum DESCRIPTION Demonstration circuit 2345A is a 60V synchronous both sides of the synchronous switches for a reduction 4-switch buck-boost LED controller with spread spectrum in radiated EMI. The open LED overvoltage protection featuring the LT 8391 LED driver. It accepts an input voltage uses the ICs constant voltage regulation loop to regulate from 4V to 60V and drives a single string of LEDs up to the output to approximately 29.7V if the LED string is 25V at 2A. DC2345A runs at 400kHz switching frequency opened although it may reach 32V peak during transient without spread spectrum, but spread spectrum frequency from running LEDs to open. There is a protection diode modulation (SSFM) can be enabled with a simple jumper. from LED+ to GND to prevent negative ringing during a SSFM spreads the switching frequency to fsw 15% for short-circuit with long wires. reduced EMI. Undervoltage lockout can be adjusted on the circuit with The LT8391 has a wide input voltage range down to 4V and a few simple resistor choices. up to 60V. It has adjustable switching frequency between There is an EMI filter on the input of DC2345A. This filter 150kHz and 600kHz. There is a simple jumper option for has a 3.2H inductor and two 4.7F capacitors. It is ef- external frequency synchronization, spread spectrum fective in reducing the conducted EMI in the AM band to frequency modulation, or neither. pass CISPR25 class 5 regulations. When run with SSFM, The LT8391 can be PWM dimmed with an external PWM DC2345A has low conducted EMI. signal and an internally-generated PWM signal. DC2345A The LT8391 data sheet gives a complete description of has a jumper that can e set to switch between internally- the part, operation and applications information. The data generated PWM signal, externally-generated PWM signal, sheet must be read in conjunction with this Demo Manual and no PWM signal (100% on). It can be analog dimmed for demonstration circuit DC2345A. The LT8391EFE is with a control voltage on either of its two control pins. assembled in a 28-lead plastic TSSOP (FE) package with LT8391 features both open LED and short LED (LED+ to a thermally enhanced ground pad. Proper board layout GND) protection as well as a fault output flag. is essential for maximum thermal performance. See the When run with both PWM dimming and spread spectrum, data sheet section Layout Considerations. the spread spectrum aligns itself with the PWM signal for Design files for this circuit board are available at flicker-free operation. DEMO MANUAL DC2345A PERFORMANCE SUMMARY Specifications are at T = 25C A PARAMETER CONDITIONS MIN TYP MAX Input Voltage EMIVIN Range Operating V = 25V 4V 60V LED Switching Frequency R3 = 100k 400kHz I R1 = 0.05 7.0V < PV < 60V V = 25V 2.0A LED IN LED V range R5 = 1M R6 = 34.8k 26V LED Open LED Voltage V R5 = 1M R6 = 34.8k 29.7V OUT Typical Efficiency (100% PWM DC) PV = 14V V = 25V I = 2.0A 97% IN LED LED Internally-Generated PWM Dimming Range Operating JP3 = INT JP1 = INT 1/128 100% Internally-Generated PWM Dimming Frequency Operating JP3 = INT JP1 = INT R16 = 200k 200Hz Peak Switch Current Limit Boost Region R2 = 0.004 12.5A Peak Switch Current Limit Buck Region R2 = 0.004 12.5A QUICK START PROCEDURE Demonstration circuit DC2345A is easy to set up to evaluate 7. Observe the LED string running at the programmed the performance of the LT8391 Follow the procedure 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 25V (at 2A) attach a voltage source to either the CTRL1 or CTRL2 to the LED+ and GND banana jacks on the PCB as terminal and set the voltage between 0V and 1.5V. See shown in Figure 1. data sheet for details. 2. Connect the EN/UVLO terminal to GND. 9. To change brightness with external PWM dimming, set JP1 to EXT/ON and JP3 to EXT. Attach a 3V rectangular 3. Set JP1 to EXT/ON and JP3 to ON for 100% always-on waveform with varying duty cycle to the PWM terminal. LED operation. Set JP2 to NO SPREAD/SYNC to run without SSFM or external synchronization. 10. To change brightness with internally-generated PWM dimming, set JP1 to INT and JP3 to INT. Adjust the 4. With power off, connect the input power supply to the setting of the VR1 variable resistor with a small flat- EMIVIN and GND banana jacks. Make sure that the DC head screwdriver to toggle between 0% and 100% input voltage will not exceed 60V. PWM dimming duty cycle in 1/128 steps. 5. Turn the input power supply on and make sure the 11. To enable spread spectrum frequency modulation, set voltage is between 4V and 60V for proper operation. JP2 to SPREAD ON. 6. Release the EN/UVLO-to-GND connection. dc2345af 2