DC1827A: 400mA @ 87V Max, 8 ~ 60V in, Boost Driver

Demonstration Circuit 1827A features a 110V LED controller with spread spectrum frequency modulation – the LT®3795. Unlike most boosts, the circuit disconnects the output to protect against shorts or other fault conditions when the input voltage exceeds the output. The input voltage range for normal operation is from 8V to 60V. OVLO becomes active for inputs above 63V and the maximum input voltage is 110V. The LED current is 400mA and the switching frequency is 250 kHz. The efficiency is 92% when the input is 12V and the LED voltage is at 87V which is the maximum LED voltage.

Spread spectrum switching is available to simplify conducted emissions compliance. There is a 47nF capacitor from the RAMP pin to ground to set the rate at which frequency modulation occurs, but resistor R20 shorting the RAMP pin capacitor must be removed to activate spread spectrum.

OVLO and EN/UVLO are both set using resistor dividers. EN/UVLO is set so the circuit will UVLO when the input voltage falls below 6V and will turn on when the input voltage rises above 7.5V.

Current sense resistors program LED current and input regulation current and also determine the monitoring voltages that indicate output and input current. The LED current is set by RS2. ISMON provides a 2.5V/1A voltage that is used to monitor the LED current. Input current regulation occurs at 4A and is set by RS3. IVINCOMP provides a 300mV/1A voltage that is used to monitor the input current. Capacitor C11 on the IVINCOMP pin provides compensation for the input current regulation loop. CTRL1 and CTRL2 are analog dimming inputs that allow external voltages to reduce the LED current from the programmed maximum. CTRL1 and CTRL2 are pulled up to the VREF pin by 100k resistors. SHORTLED and OPENLED are open-collector status flag outputs that are pulled up to the INTVCC pin voltage.

The circuit requires application of an external voltage to the PWM terminal for operation. The external voltage can be a DC level or an appropriate PWM dimming signal. A common frequency for PWM dimming is 100 Hz. The high-side PMOS FET that is used for PWM dimming also disconnects the output to protect against shorts.

The soft-start pin (SS) is configured so the circuit will hiccup when a fault occurs and will not latch off. The demo circuit also supports the adjustment capability of the LT3795 for switching frequency and feedback loop compensation. The FB pin is programmed using a resistor divider to limit the output voltage in case there is no LED string on the output. When an open LED transient occurs either at start up or because the LED string opens, the peak output voltage may overshoot to 100V but FB will regulate the settled output voltage to 95V.

The demo circuit uses ceramic input and output capacitors. An aluminum electrolytic capacitor can be easily added to the input if it is necessary for stability during conducted emissions testing. The 120V switching MOSFET allows 110V on the input. FB programming inhibits switching at high output voltages so the 100V rating of the rectifier is not exceeded.

The demo circuit is designed to be easily reconfigured to buck mode, buck-boost mode and SEPIC topologies. There are example schematics in the data sheet. Consult the factory for assistance.

Maximum input and output voltages of 110V, spread spectrum switching, fault protection and full monitoring make the LT3795 attractive for high voltage and high power LED circuits, battery chargers and voltage regulators that require an accurate current limit. DC1827A uses the LT3795EFE which is packaged in a thermally enhanced 28-lead TSSOP. The LT3795 data sheet must be read in conjunction with this demo manual to properly use or modify DC1827A.