DEMO MANUAL DC1827A LT3795 110V LED Controller with Spread Spectrum Frequency Modulation Description Demonstration Circuit 1827A features a 110V LED con- common frequency for PWM dimming is 100Hz. The troller with spread spectrum frequency modulation - the high-side PMOS FET that is used for pwm dimming also LT 3795. Unlike most boosts, the circuit disconnects the disconnects the output to protect against shorts. output to protect against shorts or other fault conditions The soft-start pin (SS) is configured so the circuit will hiccup when the input voltage exceeds the output. The input volt- when a fault occurs and will not latch off. The demo circuit age range for normal operation is from 8V to 60V. OVLO also supports the adjustment capability of the LT3795 for becomes active for inputs above 63V and the maximum switching frequency and feedback loop compensation. input voltage is 110V. The LED current is 400mA and the The FB pin is programmed using a resistor divider to switching frequency is 250kHz. The efficiency is 92% when limit the output voltage in case there is no LED string on the input is 12V and the LED voltage is at 87V which is the output. When an open LED transient occurs either at the maximum LED voltage. start up or because the LED string opens, the peak output Spread spectrum switching is available to simplify conduct- voltage may overshoot to 100V but FB will regulate the ed emissions compliance. There is a 47nF capacitor from settled output voltage to 95V. the RAMP pin to ground to set the rate at which frequency The demo circuit uses ceramic input and output capacitors. modulation occurs, but resistor R20 shorting the RAMP pin An aluminum electrolytic capacitor can be easily added to capacitor must be removed to activate spread spectrum. the input if it is necessary for stability during conducted OVLO and EN/UVLO are both set using resistor dividers. emissions testing. The 120V switching MOSFET allows EN/UVLO is set so the circuit will UVLO when the input 110V on the input. FB programming inhibits switching at voltage falls below 6V and will turn on when the input high output voltages so the 100V rating of the rectifier is voltage rises above 7.5V. not exceeded. Current sense resistors program LED current and input The demo circuit is designed to be easily reconfigured regulation current and also determine the monitoring to buck mode, buck-boost mode and SEPIC topologies . voltages that indicate output and input current. The LED There are example schematics in the data sheet. Consult current is set by RS2. ISMON provides a 2.5V/1A voltage the factory for assistance. that is used to monitor the LED current. Input current Maximum input and output voltages of 110V, spread regulation occurs at 4A and is set by RS3. IVINCOMP spectrum switching, fault protection and full monitoring provides a 300mV/1A voltage that is used to monitor make the LT3795 attractive for high voltage and high the input current. Capacitor C11 on the IVINCOMP pin power LED circuits, battery chargers and voltage regula- provides compensation for the input current regulation tors that require an accurate current limit. DC1827A uses loop. CTRL1 and CTRL2 are analog dimming inputs that the LT3795EFE which is packaged in a thermally enhanced allow external voltages to reduce the LED current from the 28-lead TSSOP. The LT3795 data sheet must be read in programmed maximum. CTRL1 and CTRL2 are pulled up to conjunction with this demo manual to properly use or the V pin by 100k resistors. SHORTLED and OPENLED REF modify DC1827A. are open-collector status flag outputs that are pulled up to the INTV pin voltage. CC Design files for this circuit board are available at DEMO MANUAL DC1827A p s Specifications are at T = 25C A PARAMETER CONDITIONS MIN TYP MAX UNITS Minimum Input, V V = 87V, I 400mA 8 V PVIN LED LED Maximum Input Switching,V V = 87V, I 400mA 60 V PVIN LED LED Maximum Input Not Switching, V V 1.25V 110 V PVIN OVLO Maximum LED Voltage, V 87 V LED LED Current, ILED RS2 = 0.62 400 mA Input EN Voltage, V V Rising R1 = 499k, R2 = 115k, R3 = 12.4k 7.5 V PVIN(EN) PVIN Input UVLO Voltage, V V Falling R1 = 499k, R2 = 115k, R3 = 12.4k 6 V PVIN(UVLO) PVIN Input OVLO Turn-Off Voltage, V V Rising R1 = 499k, R2 = 115k, R3 = 12.4k 63.1 V PVIN(OVLO TURN-OFF) PVIN Input OVLO Turn-On Voltage, V ) V Falling R1 = 499k, R2 = 115k, R3 = 12.4k 62.1 V PVIN(OVLO TURN-ON PVIN Efficiency V =12V, V = 87V, I = 400mA 92 % PVIN LED LED V =24V, V = 87V, I = 400mA 91 % PVIN LED LED V =48V, V = 87V, I = 400mA 92 % PVIN LED LED Switching Frequency R5 = 31.6k 250 kHz Input Current Limit RS3 = 0.015 4 A Quick s p D It is easy to set up DC1827A to evaluate the performance 4. Connect an input to the PWM terminal. If PWM is not of the LT3795. Follow the procedure below: used, connect PWM to a 2V to 5V DC source or to V REF on the PCB using resistor R18. PWM must be pulled NOTE: PWM must be pulled high to work. If PWM is not high to work. For PWM dimming, connect a 100Hz or used, connect the PWM terminal to a 2V to 5V DC source higher PWM signal to the PWM terminal. or connect the PWM pin to V on the PCB using R18. REF 5. Turn the PVIN power supply on. 1. Connect a string of LEDs with a forward voltage of 87V or less, but greater than the PV voltage, to the LED+ 6. Release the EN/UVLO to GND connection. IN and GND terminals on the PCB as shown in Figure 1. 7. Observe the LED string running at the programmed 2. Connect the EN/UVLO terminal to GND. LED current or, observe the reduction of brightness in the LED string with PWM dimming. 3. With the power off, connect the input power supply to the PVIN and GND terminals within the voltage range specified on the PCB. Make sure that the input power supply voltage does not exceed the forward voltage of the LED string. OVLO becomes active to inhibit switch- ing for an input voltage greater than 63V. dc1827af 2 ure roce tart ummary erformance