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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
DESCRIPTION
HIGH VOLTAGE LED CONTROLLER
LT3755, LT3755-1
Demonstration circuit 1268A-A, 1268A-B is a high
voltage and high current LED driver controller. The
VIN pin input voltage is as high as 40V. The LED output voltage is as high as 60V. The demonstration circuit is assembled as a boost topology from 8-40V
input and up to 55V LED output at 1A with an open
LED status pin (-A) or a sync input pin (-B).
DC1268A features PWM dimming, analog dimming,
shutdown, and open LED overvoltage protection. Efficiency at 12VIN is as high as 93% for 50W output.
The switching frequency is set at 400kHz to accommodate the automotive switching frequency range.
Small ceramic input and output capacitors are used
to save space and cost and a high voltage 60V external power switch and a 60V catch diode are used for
up to 55W (55V at 1A) output as assembled. The
open LED overvoltage protection will limit the output
to approximately 56V if the LED string is opened. For
low input voltage operation, the CTRL pin voltage is
reduced as the input voltage drops below 9V, reducing LED brightness and restraining the peak switch
currents in order to limit inductor and switch size.
UVLO is employed to turn the LEDs off when VIN
drops below 8V.
DC1268A PWM dimming is simple. The boost topology LED string returns to the LED- terminal and the
PWM dimming MOSFET turns the LED string on and
off given an input to the PWM dimming terminal. For
the highest pwm dimming ratio, it is recommended to
use 100Hz as a pwm dimming frequency. Information
regarding PWM dimming ratios and performance can
be found in the LT3755/-1 datasheet in the applications section.
Soft-start is provided with a simple and small capacitor to maintain control of the switch current during
turn on for high power designs.
Modifications can be made to the DC1268A in order
to convert the board from a boost topology LED
driver to any other topology. Buck-boost mode, buck
mode, and SEPIC schematics are provided in Figures
6-8. Please consult the factory or LT3755/-1 datasheet for details regarding how to customize the
DC1268A or how to design different topologies for
custom specifications.
The LT3755, LT3755-1 datasheet gives a complete
description of the part, operation and applications
information. The datasheet must be read in conjunction with this Quick Start Guide for demonstration
circuit 1268A-A, 1268A-B. The LT3755, LT3755-1 is
assembled in a small 16-lead plastic QFN package
with a thermally enhanced ground pad. Proper board
layout is essential for maximum thermal performance. See the datasheet section ‘Layout Considerations’.
Design files for this circuit board are available. Call
the LTC factory.
LT is a trademark of Linear Technology Corporation
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
Table 1.
PARAMETER CONDITION VALUE (TYPICAL)
Input Voltage PVIN Range Operating 8V-40V
Switching Frequency R8 = 28.7k 400kHz
I
1A
LED
Maximum Output Voltage - Open LED Voltage 56V
Efficiency PVIN=12V V
Under Voltage Lockout 8V
Peak Switch Current Limit RS1 = 0.015Ω 6.6A
Typical Performance Summary for DC1268A-A/B
LED
=50V I
=1A 93%
LED
3.
With power off, connect the PVIN power supply
to the PVIN and GND terminals within the ranges
QUICK START PROCEDURE
Demonstration circuit 1268A-A, 1268A-B is easy to
set up to evaluate the performance of the LT3755,
LT3755-1. Follow the procedure below:
NOTE:
Make sure that the PVIN DC input voltage
does not exceed 40V.
specified on the PCB.
4.
Connect the PWM terminal. If PWM is not used,
connect PWM to a 5V source or to IntVcc on the
PCB. PWM must be pulled high to work. For –B,
the SYNC input can be left floating.
5.
Turn the PVIN power supply on.
6.
Release the shutdown/GND connection.
1.
Connect a string of LEDs with forward voltage
less than 55V, but greater than the PVIN voltage,
to the LED+ and LED- (Boost) terminals on the
PCB as shown in Figure 1.
2.
Connect the shutdown terminal to GND.
7.
Observe the LED string running at the programmed LED current.
8.
For PWM dimming, connect a PWM 100Hz or
higher signal to the PWM terminal.
9.
Observe the reduction of brightness in the LED
string when PWM dimming.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
Figure 1. Test procedure setup drawing for DC1268A-A and DC1268A-B
TWO ICS, THREE TOPOLOGIES
DC1268A features two build versions. DC1268A-A
uses the LT3755EUD with an open LED status and
DC1268A-B uses the LT3755EUD-1 with sync. Each
board is clearly marked indicating which IC is used.
The schematic for each is attached to this document. The open LED indicator terminal has a pull-
up resistor to the internal 7V regulator IntVcc. The –
B version with the sync terminal has an RC filter
between the PWM input pin and the SYNC input
pin. This filter provides a small delay between the
PWM signal and the SYNC input when PWM is
used in order to optimize high PWM dimming ratio
waveforms. If high ratios are not needed, this RC
filter may not be needed. If the sync input is used,
this filter may be removed. Please note that when
PWM and SYNC are both used, optimal PWM dimming occurs when there is at least a 200ns delay
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
from the PWM rising edge and the SYNC rising
edge – with the PWM edge occurring first. For more
questions regarding this matter, please consult the
factory.
Although DC1268A-A/B is assembled as a boost
topology LED driver, it can be reconfigured for
buck-boost mode, buck mode and SEPIC LED
driver topologies. A few schematics featuring these
other topologies are attached. Please keep in mind
that the chosen topology should reflect the relationship between the entire input voltage range and the
LED voltage range. Other circuit requirements such
as short circuit between terminals or ground can be
affected by the choice of LED driver topology.
TERMINAL OPTIONS
DC1268A-A/B has both PVIN and VIN terminals. The
schematic shows that the PVIN and VIN terminals are
shorted together with a resistor. As assembled, only
the PVIN terminal needs 8V to 40V power in order for
the board to operate. However, resistor R2 can be removed and the VIN terminal can be connected to a
separate supply from the PVIN terminal. The VIN terminal is tied directly to the VIN pin.
The VOUT terminal is available for the customer to
monitor the state of the output capacitor directly. The
VOUT terminal can also be used as an input for buck
mode topology.
The PWM terminal must be pulled high in order for the
board to operate. It is preferred that the PWM terminal/pin is tied to IntVcc of the controller if PWM dimming is not used, but for board evaluation it can be
tied to an external 5V source as well.
The CTRL terminal is used for analog dimming. The
CTRL pin is tied directly to the terminal and it is already being used to reduce the LED current when VIN
drops too low. The CTRL pin voltage can easily be
overridden with an external voltage supply and the
resistor divider from VIN can be removed.
The shutdown/undervoltage terminal is used to shut
down the demonstration circuit. Tie the terminal to
GND and the converter will stop switching. The
PWMOUT pin will go low and the PWM dimming
MOSFET will be turned off. As long as the PWM dimming MOSFET is used properly, the LEDs will not turn
on during shutdown.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
PVIN 8V to 40V
R15
1M
CTRL
R7
140k
OPENLED
SD
PW M
C8
OPT
R1
499k
R4
90.9k
R18
OPT
INTVCC
C5
4.7uF
10V
R14
OPT
D1
PDS560
Q2
Si7850DP
RS1
0.015
0 Ohm
C1, C10
2x 4.7uF
R5
OPT
R10
100k
50V
C4
1uF
50V
6
1
16
2
3
7
C7
0.1uF
SD/UV
VREF
LT3755EUD
CTRL
PWM
OPENLED
INTVCC
VIN
R2
0 ohm
8
U1
VCSS
154
GATE
GND
R17
22k
C6
2200pF
Q3
OPT
C9 OPT
L1
22uH
HC9-220-R
10
SENSE
FB
ISP
ISN
PWMOUT
RT
5
17
9
12
14
13
11
R8
28.7k
400kHz
Figure 2. DC1268A-A featuring LT3755 and Open LED flag.
R13
OPT
Cout
3x 2.2uF
100V
R3
PWMD
LEDBUCK / BUCK-BOOST MODES
PVIN BUCK MODE
RS2
0.10
R6
1M
R9
22.1k
Si2308DS
LED+
1A
LEDBOOST
PWMD
Q1
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
100%
98%
96%
94%
92%
90%
88%
86%
EFFICIENCY (%)
84%
82%
80%
5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
Figure 3. DC1268A-A/B Efficiency with 50V LEDs at 1A
1.2
1.0
0.8
0.6
0.4
LED CURRENT (A)
0.2
0.0
5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
Figure 4. DC1268A-A/B LED Current Drops at Low VIN (CTRL) and Turns Off at Lower VIN (UVLO)
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
PVIN 8V to 40V
SD
R15
1M
CTRL
R7
140k
SYNC
C8
100pF
R1
499k
R4
90.9k
PW M
R18
10k
INTVCC
C5
4.7uF
10V
R14
OPT
PDS560
Q2
Si7850DP
RS1
0.015
C1, C10
2x 4.7uF
R5
OPT
R10
OPT
50V
C4
1uF
50V
6
1
16
2
3
7
C7
0.1uF
8
SD/UV
VREF
VIN
U1
LT3755EUD-1
CTRL
PWM
SYNC
INTVCC
R2
0 ohm
VCSS
154
R17
22k
C6
2200pF
L1
HC9-220-R
10
GATE
PWMOUT
GND
17
Q3
OPT
C9 OPT
22uH
SENSE
FB
ISP
ISN
RT
5
R8
28.7k
400kHz
9
12
14
13
11
Figure 5. DC1268A-B featuring LT3755-1 and SYNC input.
D1
0 Ohm
R13
OPT
Cout
3x 2.2uF
100V
R3
PWMD
LEDBUCK / BUCK-BOOST M ODES
PVIN BUCK MODE
RS2
0.10
R6
1M
R9
22.1k
Si2308DS
LED+
1A
LEDBOOST
PWMD
Q1
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
PVIN 8V to 36V
Cfilt1
22uF
OPENLED
Lfilt1
22uH
R15
549k
R7
75.0k
R13 510
LEDBUCK-BOOST M ODE
D1
PDS560
Q2
Si7850DP
9
RS1
0.025
12
14
13
11
R3
0 Ohm
2x 2.2uF
SD
R1
499k
R4
93.1k
PW M
R10
100k
C5
4.7uF
10V
C1, C10
2x 2.2uF
50V
C4
1uF
50V
6
1
16
2
3
7
0.01uF
Si2319DS
SD/UV
VREF
LT3755EUD
CTRL
PWM
OPENLED
INTVCC
C7
VIN
Q3
R2
0 ohm
8
U1
VCSS
154
GATE
GND
R17
4.7k
C6
4700pF
L1
22uH
CDRH127
10
SENSE
PWMOUT
RT
17
R14
510
FB
ISP
ISN
5
R8
28.7k
400kHz
Figure 6. DC1268A-A Customized Buck-Boost Mode LED Driver Schematic
C2, C3
100V
14V
R6
1M
R9
22.1k
800mA
LED+
RS2
0.125
Si2308DS
Q1
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
PW M
OPENLED
Rx1
1M
SD
R4
267k
R10
100k
C5
4.7uF
10V
R8
28.7k
400kHz
PVIN BUCK MODE
C4
1uF
50V
6
SD/UV
1
VREF
R5
0
16
CTRL
2
PWM
3
OPENLED
7
INTVCC
5
RT
C7
0.01uF
Figure 7. DC1268A-A Customized Buck Mode LED Driver Schematic
8
VIN
U1
LT3755EUD
VCSS
GND
154
17
R17
15k
C6
2200pF
10V to 36V
C2, C3
2x 2.2uF
50V
PWMOUT
ISP
ISN
GATE
SENSE
FB
11
14
13
10
9
12
R3
0
Rx5
510
R13
0
Qx1
2222
Rx4
365
Si7850DP
Q2
Q3
Si1471DH
RS2
0.068
LED+
1.5A
8.5V
PVIN
L1
10uH
CDRH8D43
RS1
0.033
22uF
D1 30BQ060
C9
16V
Rx2
100k
FMMT558
Rx3
249k
Qovp
R9
22.1k
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
PVIN 9V to 36V
SD
CTRL
PW M
R1
499k
R4
84.5k
R18
10k
C8
100pF
C5
4.7uF
10V
C1, C10
2x 47uF
50V
C4
1uF
50V
6
1
16
2
3
7
0.01uF
SD/UV
VREF
LT3755EUD-1
CTRL
PWM
SYNC
INTVCC
C7
VIN
R2
0 ohm
8
U1
VCSS
154
CDRH127/LD
10
GATE
GND
17
R17
51k
C6
22nF
L1 22uH
SENSE
FB
ISP
ISN
PWMOUT
RT
5
R8
30.9k
350kHz
9
12
14
13
11
Ccoup
270uF 50V
Q2
Si7454DP
RS1
0.0165
D1
PDS3100
L2
33uH
CDRH8D43
R3
0 Ohm
Cout
6x 47uF
50V
R6
732k
R9
24.9k
RS2
0.10
Figure 8. DC1268A-B Customized SEPIC Schematic with PWM Dimming and Aluminum Electrolytic Capacitors
35V
LED+
LED-
BOOST
Si2318DS
Q1
1A
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
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