DEMO MANUAL DC2176A
LT3909
2-String × 60mA, 2MHz Step-Up LED
Driver with ±2% Current Matching
Description
Demonstration circuit 2176A features the
LT
®
3909,
a
2-string × 60mA, 2MHz step-up LED driver with ±2%
current matching.
The LT3909 precisely regulates two LED strings for equal
current, supports LED dimming that reduces brightness
down to a 1/40,000 ratio, protects against faults and also
reports faults. The LT3909 consists of a step-up converter
with a fixed 2MHz switching frequency and current sources
for the LED strings.
The operating voltage range for DC2176A is from 7V to
36V. The maximum current in each LED string is 40mA
and is programmed by 24.9k I
SET
resistor R1. The I
SET
resistor can be changed to 20k or 16.7k to program the
string current to 50mA or 60mA respectively. 16.7k is a
0.1% resistor—a 1% 16.9k resistor will result in slightly
less than 60mA string current. See the data sheet for ap-
plication examples with 50mA and 60mA string currents.
The LT3909 utilizes voltage feedback during PWM dim-
ming and when there are faults on both LED strings. Volt-
age feedback resistors R2 and R3 on DC2176A optimize
operation for LEDs that are 35V maximum. Efficiency is
86% at 12V input, 35V for the LED string forward voltages
and 40mA per string.
LED current can be reduced by an external voltage applied
to the CTRL terminal. Resistor R8 pulls up CTRL to the
INTV
CC
voltage of the LT3909 so the LED current is at the
40mA maximum when there is no external CTRL voltage
applied. There are placeholders to add voltage dividers
either from V
IN
or INTV
CC
to the CTRL pin. A voltage
divider from V
IN
to the CTRL pin can be used to reduce
LED current at low input voltages. A resistor divider from
INTV
CC
to CTRL allows an NTC to reduce LED current at
high temperatures. There are placeholders for an RC filter
between the CTRL terminal and the CTRL pin. The RC filter
limits the ramp time of the CTRL pin and prevent false
faults when CTRL ramps too quickly. The
FAULT
terminal
is pulled up by 100k R6 to V
IN
, but there is a placeholder
to pull up
FAULT
to INTV
CC
instead.
Resistor divider R4 and R5 from the input to the EN/UVLO
pin programs the circuit to turn off when V
IN
decreases
below 5V with 0.9V of hysteresis. The circuit can also be
turned off by connecting the EN/UVLO terminal to ground.
An external voltage must be applied to the PWM terminal
in order for DC2176A to operate. The external voltage can
be a DC level or an appropriate PWM dimming signal. A
common frequency for PWM dimming is 100Hz. There is
a placeholder for a resistor to pull up PWM to INTV
CC
so
an external voltage on PWM for operation is unnecessary.
There is a placeholder that allows an external diode to
be placed in parallel with the internal LT3909 rectifier for
higher current capability. DC2176A uses an inductor that
is optimized for high efficiency, and the input supply for
power to the inductor can be separated from the LT3909
V
IN
pin to allow operation from two input sources. Ceramic
capacitors are used at the input, output and for INTV
CC
. A
50V tantalum polymer capacitor protects the input from
ringing that may occur when the input is hot-plugged using
long leads. There are placeholders occupied by 0805 size
0Ω jumpers on the back of the PCB for two LED strings
and also placeholders for SOD323 diodes to clamp the
LED1 and LED2 voltages from ringing below ground.
2-string operation, close current matching, an extremely
wide dimming range and fault protection make the LT3909
ideal for functionally advanced back lighting and illumi-
nation circuits. DC2176A features the LT3909EDD in a
thermally enhanced 12-lead 3mm × 3mm DFN package.
The LT3909 data sheet must be read in conjunction with
this demo manual to properly use or modify DC2176A.
Design files for this circuit board are available at
http://www.linear.com/demo/DC2176A
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.
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DEMO MANUAL DC2176A
performance summary
PARAMETERS
Minimum Input Voltage, V
IN
Maximum Input Voltage, V
IN
Maximum LED Voltage, V
LED1
, V
LED2
LED Current, I
LED1
, I
LED2
Input EN/UVLO Turn-On Voltage, V
IN
Input EN/UVLO Turn-Off Voltage, V
IN
Efficiency
Switching Frequency
CONDITIONS
V
LED1
= V
LED2
= 35V, I
LED1
= I
LED2
≤ 40mA
V
LED1
= V
LED2
= 35V, I
LED1
= I
LED2
≤ 40mA
R2 = 976k 1%, R3 = 31.6k 1%
R1 = 24.9k, V
CTRL
= INTV
CC
R4 = 402k, R5 = 130k, V
IN
Rising
R4 = 402k, R5 = 130k, V
IN
Falling
V
IN
= 12V, V
LED1
= V
LED2
= 35V, I
LED1
= I
LED2
= 40mA
36
35
40
5.9
5
86
2
Specifications are at T
A
= 25°C
MIN
TYP
MAX
7
UNITS
V
V
V
mA
V
V
%
MHz
Quick start proceDure
To evaluate the performance of the LT3909 using DC2176A,
refer to Figure 1 for the proper measurement equipment
setup and then follow the procedure below:
NOTE:
An external voltage must be applied to the PWM
terminal in order for DC2176A to operate. See Step 4 for
details.
1. Connect two strings of LEDs with a forward voltage of
35V or less and corresponding meters.
2. Connect the EN/UVLO terminal to GND with a clip-on
lead.
3. With the power off, connect the input power supply and
corresponding meters.
4. Apply an external voltage to the PWM terminal. The
external voltage can be a 2V to 5V DC source or a 100Hz
or higher PWM dimming signal. Install resistor R10 to
pull up PWM to INTV
CC
if an external voltage on PWM
is not desired.
5. Turn the V
IN
power supply on and verify that the input
voltage is between 7V and 36V. The V
IN
voltage is al-
lowed to exceed the LED voltage by a few volts, but
a V
IN
that is much higher than the LED voltages may
activate overtemperature protection or dissipate too
much power in the LT3909.
6. Release the EN/UVLO to GND connection.
7. Once the LED strings are running at the programmed
LED current; adjust the input voltage within the operat-
ing range and observe the LED current regulation or
reduction in brightness with LED dimming.
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DEMO MANUAL DC2176A
Quick start proceDure
Figure 1. Proper Measurement Equipment Setup for DC2176A
100
95
90
85
EFFICIENCY (%)
80
75
70
65
60
55
50
5
10
15
20
V
IN
(V)
25
V
LED1
= V
LED2
= 35V
I
LED1
= I
LED2
= 40mA
30
35
40
DC2176a F02
Figure 2. Efficiency vs Input Voltage V
IN
with V
LED1
= V
LED2
= 35V and I
LED1
= I
LED2
= 40mA
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DEMO MANUAL DC2176A
Quick start proceDure
90
80
TURN-OFF
TURN-ON
70
I
LED1
+ I
LED2
(mA)
60
50
40
30
20
10
0
0
5
10
15
20
V
IN
(V)
25
V
LED1
= V
LED2
= 35V
30
35
40
DC2176a F03
Figure 3. Total Output Current I
LED1
+ I
LED2
vs Input Voltage V
IN
with V
LED1
= V
LED2
= 35V. UVLO Occurs
at V
IN
= 5V Input Falling with 0.9V of Hysteresis
V
OUT
20V/DIV
V
IN
= 12V
V
LED1
= V
LED2
= 35V
I
LED1
= I
LED2
= 40mA
I
LED1
20mA/DIV
I
LED2
20mA/DIV
V
PWM
2V/DIV
500ns/DIV
DC2176a F04
Figure 4. LED Current When Brightness is Reduced to a 1/20,000 Ratio by PWM Dimming at 100Hz
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DEMO MANUAL DC2176A
parts List
ITEM
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
QTY
1
1
1
1
1
1
1
1
1
1
0
1
0
1
0
1
1
28
REFERENCE
C1
C2
C3
L1
R1
R2
R3
R4
R5
U1
C4 (Opt)
C5
D1 (Opt)
R6
R7, R9, R10, R11 (Opt)
R8
R12
PART DESCRIPTION
Cap, 1µF X7R 50V 10% 0805
Cap, 2.2µF X7R 50V 20% 1206
Cap, 1µF X5R 6.3V 20% 0603
Inductor, 6.8µH 4020
Res, Chip 24.9k 0.06W 1% 0603
Res, Chip 976k 0.01W 1% 0603
Res, Chip 31.6k 0.10W 1% 0603
Res, Chip 402k 0.06W 1% 0603
Res, Chip 130k 0.06W 1% 0603
IC, LED Driver DFN(12) (DD) 3mm × 3mm
Cap, 10nF X7R 100V 10% 0603
Cap, Tant Polymer 10µF 50V 20% 7343
Schottky Diode, 1A SOD-323
Res, Chip 100k 0.06W 5% 0603
Res, 0603
Res, Chip 100k 0.06W 1% 0603
Res, Chip 1k 0.06W 5% 0603
Vishay CRCW0603100KFKEA
Vishay CRCW06031K00JEA
Vishay CRCW08050000ZOEA
MANUFACTURER/PART NUMBER
Murata GRM21BR71H105KA12L
TDK C3216X7R1H225M
TDK C1608X5R0J105M
Wurth Electronik 74437324068
Vishay CRCW060324K9FKEA
Vishay CRCW0603976KFKEA
Vishay CRCW060331K6FKEA
Vishay CRCW0603402KFKEA
Vishay CRCW0603130KFKEA
Linear Technology Corporation LT3909EDD#PBF
AVX 06031C103KAT2A
AVX TCJD106M050R0120
Diodes Inc SBR1A40S3
Vishay CRCW0603100KJEA
Required Circuit Components
Additional Demo Board Circuit Components
R13, R14, R15, R16, R17, R18, R19, Res/Jumper, Chip 0 0.125W 0805
R20, R21, R22, R23, R24, R25, R26,
R27, R28, R29, R30, R31, R32, R33,
R34, R35, R36, R37, R38, R39, R40
E1, E2, E3, E4, E5, E6 ,E7, E8, E9,
E10, E11, E12
MH1 to MH4
Turret, Testpoint
Stand-Off, Nylon 1/4" Tall (Snap On)
Hardware: For Demo Board Only
1
2
12
4
Mill Max 2501-2-00-80-00-00-07-0
Keystone, 8831(Snap On)
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