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dc1827afa

DEMO MANUAL DC1827A

DESCRIPTION

LT3795

110V LED Controller with Spread

Spectrum Frequency Modulation

Demonstration Circuit 1827A features a 110V LED con-

troller 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 volt-

age 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

switchingfrequencyis250kHz.Theefficiencyis92%when

the input is 12V and the LED voltage is at 87V which is

the maximum LED voltage.

Spreadspectrumswitchingisavailabletosimplifyconduct-

ed emissions compliance. There is a 6.8nF capacitor from

the RAMP pin to ground to set the rate at which frequency

modulationoccurs,butresistorR20shortingtheRAMPpin

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

programmedmaximum.CTRL1andCTRL2arepulledupto

are open-collector status flag outputs that are pulled up

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

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common frequency for PWM dimming is 100Hz. The

high-side PMOS FET that is used for pwm dimming also

disconnects the output to protect against shorts.

Thesoft-startpin(SS)isconfiguredsothecircuitwillhiccup

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.

Thedemocircuitusesceramicinputandoutputcapacitors.

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 regula-

tors 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.

Design files for this circuit board are available at

http://www.linear.com/demo/DC1827A