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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
switching frequency is 250kHz. 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 conduct
-
ed emissions
compliance. There is a 6.8nF 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 V
REF
pin by 100k resistors. SHORTLED and OPENLED
are open-collector status flag outputs that are pulled up
to the INTV
CC
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
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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.
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 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
