QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1315
DUAL STEP-DOWN 1.5A LED DRIVER
1
LT3475-1
DESCRIPTION
Demonstration circuit 1315 features the
LT®3475-1 dual step-down 1.5A LED driver in a
16-lead TSSOP thermally enhanced package.
The demonstration circuit is designed to drive
two 1.5A LED strings (one on each channel)
from a high input voltage. The high switching
frequency scheme permits the use of a small
inductor and ceramic capacitors to save space
and cost. The current mode PWM control
scheme provides fast transient response and
cycle-by-cycle current limiting. DC1315 is
specifically designed for high power LED
applications such as automotive and avionic
lighting, architectural detail lighting and display
backlighting. See the LT3475-1 datasheet for
more detailed information on LED brightness
and dimming control using both the PWM pin
function and the V
ADJ
pin function. The PWM
terminals can be used to dim the LEDs with the
addition of a PWM MOSFET and the resistor-
short connection from LED- to GND removed.
The efficiency of DC1315A with 20V input and 4
white LEDs on each 1.5A channel is 91.5%.
NOTE:
The input voltage is limited to 30V during
startup. During normal (non-startup) operation,
the input voltage can be as high as 36V.
DC1315 allows LED string voltages up to 23V.
However, some combinations of VIN, VLED,
and ILED can result in the LT3475-1 getting hot.
Please consult the datasheet for thermal calcu-
lations and details. The voltages, the LED cur-
rent, and the switching frequency can be ad-
justed for optimal thermal performance.
Design files for this circuit board are avail-
able. Call the LTC factory.
LT is a registered trademark of Linear Technology Corporation
QUICK START PROCEDURE
DC1315 is easily set up for evaluating the
LT3475-1 dual step-down 1.5A LED driver
circuit. Follow the test procedure outlined below
and see Figure 1.
1. Connect a bench supply with power OFF
to the V
IN
and GND terminals.
2. Connect a 1.5A LED string with forward
voltage >13V to the LED1+ and LED1-
terminals.
3. Connect a 1.5A LED string with forward
voltage >13V to the LED2+ and LED2-
terminals.
NOTE:
The boost circuitry on DC1315 is de-
signed to work best with VLED >13V since
there is a 9V zener diode limiting the boost
voltage. If lower voltage LED strings are re-
quired, please lower the voltage of the zener
diode or use another boost method. Please
see the datasheet for details.
4. Turn on the input power supply and set
the voltage above the LED string voltage
and below 30V.
5. Observe the LEDs working properly at
1.5A without looking straight at the LEDs.
The brightness is harmful to the eye.
