Linear Technology LT3755 User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
1
LT3755, LT3755-1
DESCRIPTION
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 out-
put voltage is as high as 60V. The demonstration cir-
cuit 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. Effi-
ciency at 12VIN is as high as 93% for 50W output.
The switching frequency is set at 400kHz to accom-
modate the automotive switching frequency range.
Small ceramic input and output capacitors are used
to save space and cost and a high voltage 60V exter-
nal 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, reduc-
ing 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 topol-
ogy 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 applica-
tions section.
Soft-start is provided with a simple and small capaci-
tor 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 data-
sheet 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 conjunc-
tion 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 perform-
ance. See the datasheet section ‘Layout Considera-
tions’.
Design files for this circuit board are available. Call
the LTC factory.
LT is a trademark of Linear Technology Corporation

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
2
Table 1. Typical Performance Summary for DC1268A-A/B
PARAMETER CONDITION VALUE (TYPICAL)
Input Voltage PVIN Range Operating 8V-40V
Switching Frequency R8 = 28.7k 400kHz
I
LED
1A
Maximum Output Voltage - Open LED Voltage 56V
Efficiency PVIN=12V V
LED
=50V I
LED
=1A 93%
Under Voltage Lockout 8V
Peak Switch Current Limit RS1 = 0.015 Ω 6.6A
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.
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.
3. With power off, connect the PVIN power supply
to the PVIN and GND terminals within the ranges
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.
7. Observe the LED string running at the pro-
grammed 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.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
3
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 docu-
ment. 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 dim-
ming occurs when there is at least a 200ns delay

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
4
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 relation-
ship 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 re-
moved and the VIN terminal can be connected to a
separate supply from the PVIN terminal. The VIN ter-
minal 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 termi-
nal/pin is tied to IntVcc of the controller if PWM dim-
ming 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 al-
ready 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 dim-
ming MOSFET is used properly, the LEDs will not turn
on during shutdown.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
5
R18
OPT
R17
22k
R9
22.1k
Q2
Si7850DP
LED+
1A
C5
4.7uF
LED-
BOOST
LED-
BUCK / BUCK-BOOST MODES
RS2
0.10
R7
140k
PVIN BUCK M ODE
INTVCC
Q1
Si2308DS
C4
1uF
50V
OPENLED
R14
OPT
L1
22uH
PWMD
D1
PDS560
R13
OPT
R10
100k
R5
OPT R6
1M
RS1
0.015
R4
90.9k
C7
0.1uF
CTRL
R2
0 ohm
R8
28.7k
R15
1M U1
LT3755EUD
6
2
7
154
1
16
5
3
8
14
13
11
10
12
9
17
SD/UV
PWM
INTVCC
VCSS
VREF
CTRL
RT
OPENLED
VIN
ISP
ISN
PWMOUT
GATE
FB
SENSE
GND
R1
499k
SD
R3
0 Ohm
Cout
3x 2.2uF
100V
400kHz
C6
2200pF
C1, C10
2x 4.7uF
50V
C9 OPT
Q3
OPT
C8
OPT
10V
PVIN 8V to 40V
PWMD
HC9-220-R
PWM
Figure 2. DC1268A-A featuring LT3755 and Open LED flag.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
6
80%
82%
84%
86%
88%
90%
92%
94%
96%
98%
100%
5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
EFFICIENCY (%)
Figure 3. DC1268A-A/B Efficiency with 50V LEDs at 1A
0.0
0.2
0.4
0.6
0.8
1.0
1.2
5 10 15 20 25 30 35 40
INPUT VOLTAGE (V)
LED CURRENT (A)
Figure 4. DC1268A-A/B LED Current Drops at Low VIN (CTRL) and Turns Off at Lower VIN (UVLO)

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
7
C1, C10
2x 4.7uF
50V
R6
1M
R18
10k
PW M
Q2
Si7850DP
R9
22.1k
L1
22uH
C5
4.7uF
1A
Q3
OPT
U1
LT3755EUD-1
6
2
7
154
1
16
5
3
8
14
13
11
10
12
9
17
SD/UV
PWM
INTVCC
VCSS
VREF
CTRL
RT
SYNC
VIN
ISP
ISN
PWMOUT
GATE
FB
SENSE
GND
LED-
BOOST
LED-
BUCK / BUCK-BOOST M ODES
C4
1uF
50V
HC9-220-R
R13
OPT
D1
PDS560
SD
C7
0.1uF
RS1
0.015
R2
0 ohm
PWMD
Q1
Si2308DS
C6
2200pF
LED+
CTRL
Cout
3x 2.2uF
100V
PWMD
R10
OPT
R17
22k
R1
499k
R3
0 Ohm
INTVCC
RS2
0.10
SYNC
400kHz
R8
28.7k
C9 OPT
R4
90.9k
PVIN BUCK M ODE
PVIN 8V to 40V
R15
1M
C8
100pF
R14
OPT
R5
OPT
R7
140k
10V
Figure 5. DC1268A-B featuring LT3755-1 and SYNC input.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
8
Q1
Si2308DS
R2
0 ohm
R13 510
R4
93.1k
R9
22.1k
R6
1M
R8
28.7k
RS2
0.125
Lfilt1
22uH C2, C3
2x 2.2uF
100V
CDRH127
R1
499k C4
1uF
50V
OPENLED
R14
510
R10
100k
Cfilt1
22uF
PW M
10V
C6
4700pF
L1
22uH
14V
U1
LT3755EUD
6
2
7
154
1
16
5
3
8
14
13
11
10
12
9
17
SD/UV
PWM
INTVCC
VCSS
VREF
CTRL
RT
OPENLED
VIN
ISP
ISN
PWMOUT
GATE
FB
SENSE
GND
RS1
0.025
Q3
Si2319DS
SD
R15
549k
C7
0.01uF
R17
4.7k
D1
PDS560
800mA
R3
0 Ohm
C1, C10
2x 2.2uF
50V
PVIN 8V to 36V
C5
4.7uF
Q2
Si7850DP
400kHz
R7
75.0k
LED+
LED-
BUCK-BOOST M ODE
Figure 6. DC1268A-A Customized Buck-Boost Mode LED Driver Schematic

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
9
Qovp
FMMT558
C5
4.7uF
10V to 36V
Rx5
510
SD
Q2
Si7850DP
RS1
0.033
OPENLED
PVIN BUCK MODE
8.5V
D1 30BQ060
PVIN
R13
0
1.5A
Rx1
1M
Rx4
365
R17
15k
U1
LT3755EUD
6
2
7
154
1
16
5
3
8
14
13
11
10
12
9
17
SD/UV
PWM
INTVCC
VCSS
VREF
CTRL
RT
OPENLED
VIN
ISP
ISN
PWMOUT
GATE
FB
SENSE
GND
C7
0.01uF
R9
22.1k
LED+
C6
2200pF
R8
28.7k
R10
100k
400kHz
R4
267k
C9
22uF
16V
C2, C3
2x 2.2uF
50V
Rx2
100k
C4
1uF
50V RS2
0.068
R5
0
Rx3
249k
10V
L1
10uH
PWM
Q3
Si1471DH
Qx1
2222
R3
0
CDRH8D43
Figure 7. DC1268A-A Customized Buck Mode LED Driver Schematic

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1268A-A, 1268A-B
HIGH VOLTAGE LED CONTROLLER
10
C4
1uF
50V
Cout
6x 47uF
Q2
Si7454DP
CDRH127/LD
PVIN 9V to 36V
Ccoup
270uF 50V
C1, C10
2x 47uF
R8
30.9k
RS1
0.0165
50V
C5
4.7uF
CDRH8D43
C7
0.01uF
RS2
0.10
50V
35V
LED-
L1 22uH
CTRL
U1
LT3755EUD-1
6
2
7
154
1
16
5
3
8
14
13
11
10
12
9
17
SD/UV
PWM
INTVCC
VCSS
VREF
CTRL
RT
SYNC
VIN
ISP
ISN
PWMOUT
GATE
FB
SENSE
GND
BOOST
R18
10k
SD
1A
D1
PDS3100
C6
22nF
R1
499k
R6
732k
350kHz
Q1
Si2318DS
10V
R2
0 ohm
R3
0 Ohm
R9
24.9k
PWM
C8
100pF
L2
33uH
R17
51k
R4
84.5k
LED+
Figure 8. DC1268A-B Customized SEPIC Schematic with PWM Dimming and Aluminum Electrolytic Capacitors