1
dc1470af
DEMO MANUAL DC1470A
DESCRIPTION
LT3743 High Current
Synchronous Step-Down LED
Driver with Three-State Control
Demonstration circuit 1470A is a high current synchronous
step-down LED driver with three-state control featuring
the
. The demo board is optimized for 20A
output from a 12V input. Being an LED driver, the output
current is being regulated until the output voltage reaches
a programmed voltage limit. This voltage limit on this
demo board is set to around 6V by R5 and R6. The 6V is
chosen because of the 6.3V voltage rating of the output
capacitors. The ideal load to be used with this demo board
is a single LED, such as PT120 from Luminus Devices.
Smaller LEDs may not be able to handle the high current,
even for a short period of time. At 20A, the demo circuit can
operate continuously. However, it is necessary to mount
the LED load on a proper heat sink and possibly with a
fan to avoid exceeding its maximum temperate rating.
Note:
The DC470A will drive a LED with its cathode
grounded. Grounding cathode of the LED allows customers
to use positive input supply.
The input voltage range of the LT3743 itself is 6V to 36V.
However, the demo board utilizes 30V MOSFETs to demon-
strate best efficiency so the maximum recommended input
voltage is 24V for the demo board. When input is above
20V during PWM dimming, a slightly larger inductance
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.
PERFORMANCE SUMMARY
may be needed. The typical efficiency of the demo board
is 94% from a 12V input to 5V, 20A load. The lower the
input voltage, the higher the efficiency tends to be with
a given load. At output power level of 100W, a couple of
percent of efficiency improvement is a huge advantage in
minimizing temperature rise. If an efficiency measurement
is needed in an application, the output voltage must be
measured at the output capacitors instead of at the LED
load. This prevents cable loss from being counted as a
loss of the board. If efficiency measurement is required
during PWM dimming, the average current and voltage
values should be measured with proper equipment. All
LT3743 circuits turn off unnecessary circuits during PWM
off period to minimize power losses. As a result, efficiency
in many PWM dimming applications is almost the same
as efficiency of the constant current applications.
The DC1470A uses a split output capacitor configuration
to achieve <2μs current rise time from 0A to 20A. To see
the real rise time, connecting wires between the LED and
the board should be minimized to no more than 2 inches
Specifications are at T
A
= 25°C
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
PV
IN
*
Input Supply Range, P
VIN
= V
IN
L1 is Optimized for 12V
12
26.4
V
V
OUT
*
Output Voltage
With One PT120 LED, 6V is the Over
Voltage Limit
4.5
6
V
I
OUT
Output Current
CTRL_SEL > 1V
18.8
20
21.2
A
f
SW
Switching Frequency
400
430
460
kHz
I
RISE
Current Rise Time
Following a PWM Rising Edge
2
μs
EFF
Efficiency at DC
V
IN
= 12V, V
OUT
= 5V, I
OUT
= 20A
94
%
*PV
IN
and V
IN
of the demo circuit are limited to 24V typical by the selection of MOSFETs. The LT3743 input range is 6V to 36V. V
OUT
maximum of the
demo circuit is limited to 6V due to the selection of the output capacitors.
Design files for this circuit board are available at
