DEMO MANUAL DC1772A
LT3761EMSE
High Voltage LED Controller
with Internal PWM Dimming
Description
DC1772A is a high voltage LED controller with internal
PWM dimming. It generates its own PWMOUT waveform
for accurate PWM dimming with up to 25:1 brightness
ratio. It accepts an input voltage from 8V to 60V (8V UVLO
and 9.1V rising turn-on), and drives up to 60V of LEDs
at 1A (when PVIN is less than V
LED
). DC1772A features
both PWM and analog dimming of the LED string. It has
an
OPENLED
flag that indicates when the LED string has
been removed.
DC1772A features high efficiency at 350kHz switching
frequency. At high LED string voltages up to 60V and
1A of LED current, the single switch controller has 94%
efficiency. The open LED overvoltage protection uses
the IC’s constant voltage regulation loop to regulate the
output to approximately 69.8V if the LED string is opened
although it may reach 74V peak during transient from
running LEDs to open. The maximum LED string voltage
can be raised from 60V to 65V (or higher) with a simple
change of feedback resistors.
For low input voltage operation, the CTRL pin voltage is
reduced as the input voltage drops below 10V, reducing
LED brightness and restraining the peak switch currents
in order to limit inductor and switch size. UVLO turns the
LEDs off when PVIN drops below 8V.
DC1772A PWM dimming is simplified when compared
with other LED drivers. The LT
®
3761 generates its own
PWMOUT dimming waveform at a frequency determined
by the capacitance on the PWM pin (C8 gives 300Hz for
DC1772A). The PWMOUT duty cycle is determined by the
voltage on the DIM terminal. Between 0V and 7.7V VDIM
gives between 4% and 96% PWM duty cycle. Information
regarding PWM dimming ratios and performance can be
found in the LT3761 data sheet in the Applications Infor-
mation section. Analog dimming is also simple to use with
a single voltage source on the CTRL terminal.
Modifications can be made to DC1772A in order to convert
the board to higher or lower power 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 LT3761 data sheet for details. It can be
modified to provide LED+ to GND short-circuit protection
as well.
The LT3761 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 DC1772A. The LT3761EMSE is
assembled in a 16-lead plastic MSOP MSE package with
a thermally enhanced ground pad. Proper board layout
is essential for maximum thermal performance. See the
data sheet section Layout Considerations.
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.
dc1772af
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DEMO MANUAL DC1772A
performance summary
PARAMETER
Input Voltage PVIN Range
Switching Frequency
I
LED
Low PVIN I
LED
(CTRL Foldback)
V
LED
Range
Open LED Voltage
Typical Efficiency
PVIN Under Voltage Lockout (Falling Turn-Off)
PVIN Under Voltage Lockout (Rising Turn-On)
INTVCC
Peak Switch Current Limit
PWMOUT Dimming Duty Cycle
Specifications are at T
A
= 25°C
CONDITION
Operating
R6 = 29.4k
RS2 = 0.25Ω 10V < PVIN < V
LED
(60V)
RS2 = 0.25Ω PVIN = 8.5V
RS2 = 0.25Ω PVIN = 9V
R3 = 1M R4 = 18.2k
R3 = 1M R4 = 18.2k
PVIN = 14V V
LED
= 60V I
LED
= 1A PWM = INTVCC
R1 = 499k and R2 = 90.9k
R1 = 499k and R2 = 90.9k
Operating
RS1 = 0.008Ω
VDIM = 7.7V
VDIM = 4V
VDIM = 1.5V
VDIM = 0.4V
C8 = 0.047µF 0V < VDIM < 7.7V
VALUE (TYPICAL)
8V to V
LED
(Up to 60V)
350kHz
1A
930mA
975mA
PVIN < V
LED
< 60V
69.8V
94%
8V
9.1V
7.85V
12.5A
96%
50%
10%
4.3%
300Hz
Internal PWM Dimming Frequency
Quick start proceDure
Demonstration circuit 1772A is easy to set up to evaluate
the performance of the LT3761EMSE. Follow the proce-
dure below:
1. Connect a string of LEDs that will run with forward volt-
age less than 60V, but greater than PVIN, to the LED+
and LED– terminals on the PCB as shown in Figure 1.
2. Connect the EN/UVLO terminal to GND.
3. With power off, connect the input power supply to the
PVIN and GND terminals. Make sure that the PVIN DC
input voltage will not exceed 60V (or V
LED
).
4. Connect the DIM terminal to a voltage between 0V and
7.7V to set the internal PWMOUT dimming duty cycle.
If this terminal is left floating the converter will run with
approximately 12% PWMOUT dimming duty cycle. Pull
the PWM terminal high to INTVCC to set the converter
at 100% duty cycle.
5. Turn the input power supply on and make sure the
voltage is between 8V and 60V (or V
LED
).
6. Release the EN/UVLO-to-GND connection.
7. Observe the LED string running at the programmed
LED current and brightness related to the programmed
PWMOUT duty cycle.
8. To change the brightness with PWM dimming, simply
vary the VDIM voltage between 0V and 7.7V with the
PWM terminal floating.
9. To change the brightness with analog dimming, simply
attach a voltage source on the CTRL terminal and reduce
the voltage below 1.2V.
10. Observe the reduction of brightness in the LED string
when PWM or analog dimming.
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dc1772af
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