DC949A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B
4.5A BOOST LED DRIVER WITH PWM DIMMING
LT3478-1, LT3478
DESCRIPTION
Demonstration circuit 949A-A/949A-B is a 4.5A peak
switch current boost LED driver with PWM dimming
featuring the LT3478-1/LT3478. DC949A-A uses the
LT3478-1, a fixed 1A LED driver with the LED current
sense resistor inside the IC. DC949A-B has the
LT3478 adjustable LED current converter requiring an
external sense resistor (R11). Both circuits are optimized to drive a 350mA LED string with a total LED
voltage above the input voltage and below 40V. The
wide input voltage range, internal 4.5A npn power
switch, floating current sense amplifier, LED current
dimming control, inrush protection, programmable
switching frequency, soft-start, and overvoltage protection make the LT3478-1/LT3478 an extremely
powerful and versatile LED driving IC. It can be used
for boost, buck mode, and buck-boost mode LED
drivers.
The LT3478-1/LT3478 has programmable overvoltage
protection in case the LEDs are open or not properly
attached to the terminals on the PCB. The shutdown
function is activated by pulling the shutdown terminal
to ground. In shutdown the boost configuration allows the input voltage to be seen on the output and if
the attached LED array leaks current at the input voltage, the LEDs may not completely turn off although
IC switching will cease. Tying the PWM terminal low
will disable the LEDs in shutdown or during normal
running operation. PWM dimming can be applied to
DC949A as built. CTRL dimming can be used for analog control of LED current if the resistor networks
attached to the CTRL1 and/or CTRL2 pins are removed. Inrush protection limits the VS input current
to 6A during a hotplug or startup condition. Soft-start
prevents high inrush current in most conditions. See
details in the ‘Soft-Start’ section in the LT34781/LT3478 datasheet.
The switching frequency is set at 1MHz and can be
adjusted by changing a single resistor. Please read
the datasheet for details on setting the switching frequency and selecting components such as the inductor and capacitors. The maximum rating on VS is 36V.
However, in a boost converter, the maximum normal
operating voltage of VS is the maximum LED voltage
connected to the output. Using a higher voltage on VS
results in loss of control of LED current.
Modifications can be made to DC949A in order to
convert the board from a boost LED driver to either a
buck mode or buck-boost mode LED driver. The
LT3003 three string ballaster is also available as an
option for any configuration of the DC949A. Please
consult the factory for customization details. Some
schematics are supplied in this quick-start guide.
The LT3478-1, LT3478 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 949A-A/949A-B. The LT3478-1, LT3478 is assembled in a small 16-lead plastic TSSOP FE 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 949A-A/949A-B
4.5A BOOST LED DRIVER WITH PWM DIMMING
Table 1.
Typical Performance Summary for both DC949A-A and DC949A-B
PARAMETER
CONDITION
VALUE (TYPICAL)
Input Voltage VS Range
VLED > 25V, ILED = 350mA
2.8 – 25V
Input Voltage VIN Range
Non-transient
2.8 – 16V
ILED
350mA
Maximum Output Voltage
36V
Open LED Output Voltage
36V to 40V
Switching Frequency
R6 = 31.6k
1MHz
Efficiency
VS=12V VIN=5V VLED=31.5V ILED=.35A
88%
Maximum PWM Dimming Ratio
VS=12V VIN=5V VLED=31.5V ILED=.35A
3000:1
3.
With power off, connect the VS power supply to
the VS terminal and connect the VIN power supply to the VIN terminal within the ranges specified on the PCB.
4.
Connect the PWM and VIN terminals.
5.
Turn the VS and VIN power supplies on.
6.
Release the shutdown/GND connection.
7.
Observe the LED string running at full 350mA
LED current at 100% duty cycle.
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.
QUICK START PROCEDURE
Demonstration circuit 949A-A/949A-B is easy to set
up to evaluate the performance of the LT3478-1,
LT3478. Follow the procedure below:
NOTE: Make sure that the VS DC input voltage does
not exceed 25V and the VIN DC input voltage does
not exceed 16V.
1.
2.
Connect a string of LEDs with forward voltage
less than 36V, but greater than the VS input voltage, to the LED+ and LED- terminals on the PCB
as shown.
Connect the shutdown terminal to GND.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B
4.5A BOOST LED DRIVER WITH PWM DIMMING
Figure 1. Test procedure setup drawing for DC949A-A DC949A-B
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B
4.5A BOOST LED DRIVER WITH PWM DIMMING
for details regarding the use of the sense resistor
and setting the LED current. Both DC949A-A and
DC949A-B are built to provide 350mA LED current
in the boost LED driver topology.
100%
12.5V L ED
95%
19V L ED
90%
31.5V L ED
85%
80%
75%
70%
65%
60%
55%
5V IN
50%
0
5
10
15
20
25
VS INP UT (V)
Figure 2. DC949A-A efficiency
TWO ICS, THREE TOPOLOGIES
DC949A has two build versions. DC949A-A uses the
LT3478-1 1A fixed LED current IC with internal
sense resistor. It can be set at full 1A current or less
than 1A LED current with CTRL1,2 resistor divider
network. DC949A-B uses the LT3478 adjustable
LED current IC with a required external sense resistor to set the LED current. It can be set at LED currents above and below 1A. Please see the datasheet
Although DC949A is a boost LED driver, it can be
reconfigured for buck-boost mode LED driving and
buck mode LED driving. The datasheet details some
of these changes and the figures below show sample schematics with the demonstration circuit reference designators that can be used in the different
modes. There may be a cut or two required on the
PCB to make these changes. Please ask for further
documentation detailing the changes in DC949A for
conversion to buck mode or buck-boost mode LED
driving.
OPTIONAL LT3003 BALLASTER
The LT3003 is available for three-channel LED ballasting. With 1A or less output using the DC949A,
three parallel channels of LED strings can be driven
and PWM dimmed in all three topologies using the
LT3003 with the LT3478-1 or LT3478. For details
regarding the connection of the LT3003 and the
LT3478 please see the LT3003 and LT3478 datasheets. When not being used, the LT3003 circuit
should be left unstuffed. Please note that there is a
small pad near the LT3478 Vc pin that connects internally to the LT3003 /OT2 output for overtemp
protection. It is shown in the schematic of the
LT3478 and it is easy to connect these functions
using the DC949A.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B
4.5A BOOST LED DRIVER WITH PWM DIMMING
VS
2.8V-25V
36VDC MAX
C1
4.7uF
25V
VIN
C6
3.3uF
25V
11
SHDN
13
CTRL2
L1 10uH
3
4
VIN
VS
5
SW
SW
SHDN
CTRL2
U1
OUT
*
LED
LT3478
R3 93.1k
10
12
CTRL1
R4
36.5k
350mA
C21
4.7uF
50V
1
2
LED String
6
7
VREF
15V max
PWM
OVPSET
R10
100k
C3
1uF
LED-
PWM
CTRL1
R9
24k
8
LED+
PDS560
C2
4.7uF
50V
L
*
3
R1
100k
CIN
1uF
25V
R11
OUT
SS
GND
RT
16
17
15
VC
Q1
Si2318DS
14
1
9
R7
10k
C4
0.1uF
R6
31.6k
1%
1 MHz
ASSEMBLY VERSION
2
2.8V-16V
36V transient
D1
* VERSION TABLE
U1
DC949A-A
LT3478EFE-1
DC949A-B
LT3478EFE
R11
DO NOT STUFF
0.1 ohm 1%
Figure 3. Customer friendly schematic for boost (as-built) DC949A. 40V is the maximum that the output will see, but OVP might be as low
as 36V without any adjustments.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B
4.5A BOOST LED DRIVER WITH PWM DIMMING
Q1
Si2302ADS
R21
PWM
510
Q2
Si2315BDS
VS
R7
10k
R20
510
3.8V-6.5V
LED+
1A
LEDC1
10uF
10V
VIN
D1
L1
6.8uH
R1
100k
SHDN
11
13
10
12
3
4
VIN
VS
5
2
SW SW
SHDN
OUT
*
C2
4.7uF
16V
B320
1
L
R11
6
CTRL2
* U1
VREF
LT3478
LED
PWM
CTRL1
OVPSET SS
R9
8
100k
R10
34k
16
GND
VC
17
9
C3
1uF
RT
7
14
15
R6
69.8k
1%
C4
0.1uF
500kHz
* VERSION TABLE
ASSEMBLY VERSION
U1
DC949A-A
LT3478EFE-1
DC949A-B
LT3478EFE
R11
DO NOT STUFF
0.1 ohm 1%
Figure 4. Customer friendly schematic for buck-boost mode LED driver DC949A. This circuit requires changes to the standard demo circuit such as addition of new components and a trace cut on the top-side of the board between the drain of Q1 and the LED- terminal.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B
4.5A BOOST LED DRIVER WITH PWM DIMMING
PVIN
This is the OUT terminal on DC949A.
C2
3.3uF
50V
32V
R11
0.068
LED+
1.5A
LEDR20 365
Q2
Si2319DS
COUT_buck
10uF
VIN
3.3V
C6
4.7uF
10V
R1
100k
L1
R21
510
10uH
VIN L VS
OUT
LED
D1
SW
SHDN
SHDN
PDS560
LT3478EFE
VREF
Q1
2N7002
PWM
CTRL2
R7
CTRL1
10k
R9
PWM
24k
OVPSET
VC
SS
RT
GND
R10
100k
C3
1uF
R6
69.8k
1%
500kHz
C4
0.1uF
Figure 5. Customer friendly schematic for buck mode LED driver DC949A-B. This circuit requires changes to the standard demo circuit
such as addition of new components and a trace cut on the top-side of the board between the drain of Q1 and the LED- terminal. The
input PVIN must be connected to the OUT terminal. The connection between pin L and the inductor must be cut on the topside of the
board. Pin L can be connected to VIN terminal via R22 with a zero ohm resistor. Terminal Vs can be left floating. The COUT_buck capacitor must be added to the board.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B
4.5A BOOST LED DRIVER WITH PWM DIMMING
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