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dc2079af
DEMO MANUAL DC2079A
Description
LT3954
40V
IN
40V
OUT
LED Driver
Demonstration circuit 2079A is a 40V
IN
, 40V
OUT
LED driver.
It generates its own PWMOUT waveform from its internal
PWM generator for accurate PWM dimming with up to
33:1 brightness ratio. It accepts an input voltage from
5V to 30V and drives up to 32V of LEDs at 650mA (when
PV
IN
is less than V
LED
). DC2079A features both PWM and
analog dimming of the LED string. It has an OPENLED flag
that indicates when the LED string has been removed.
DC2079A features high efficiency at 350kHz switching
frequency. At high LED string voltages up to 32V and
650mA of LED current, the single switch controller has
94.6% efficiency (14V input). The LT3954 has an internal
40V, 5.4A switch that simplifies the schematic and layout.
Small ceramic input and output capacitors are used to save
space and cost. The open LED overvoltage protection uses
the IC’s constant voltage regulation loop to regulate the
output to approximately 35V if the LED string is opened
although it may reach 38V peak during transient from
running LEDs to open string.
For low input voltage operation, the CTRL pin voltage is
reduced as the
input voltage drops below 8.5V, reducing
LED brightness and restraining the peak switch currents
in order to limit inductor and switch size. UVLO turns the
LEDs off when V
IN
drops below 5.3V.
DC2079A PWM dimming is simplified when compared
with other LED drivers. The LT3954 generates its own
PWMOUT dimming waveform at a frequency determined
by the capacitance on the PWM pin (C16 gives 300Hz for
DC2079A). The PWMOUT duty cycle is determined by the
voltage on the DIM terminal. Between 0V and 8V VDIM
gives between 3% and 97% PWM duty cycle. Information
regarding PWM dimming ratios and performance can be
found in the LT3954 data sheet in the applications sec-
tion. Analog dimming is also simple to use with a single
voltage source on the CTRL terminal.
Modifications can be made to DC2079A in order to convert
the board to power different LED strings or from an LED
driver to a constant voltage regulator or battery charger. It
can easily be changed from a boost topology to a SEPIC,
buck mode, or buck-boost mode LED driver. Please
consult the factory or the LT3954 data sheet for details.
It can be modified to provide LED+ to GND short-circuit
protection as well.
The LT3954 data sheet gives a complete description of
the part, operation and applications information. The data
sheet must be read in conjunction with this demo manual
for demonstration circuit DC2079A. The LT3954EUHE
is assembled in a 36-lead plastic QFN package with two
thermal pads underneath the IC
. Proper board layout is
essential for maximum thermal performance. See the
Layout Considerations section in the data sheet.
Design files for this circuit board are available at
http://www.linear.com/demo
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
