DC876A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 876
DUAL 1.3A WHITE LED STEP-UP CONVERTERS WITH WIDE DIMMING RANGE
LT3486
DESCRIPTION
Demonstration circuit 876 is a dual 1.3A white LED
step-up converter with wide dimming range featuring
the LT3486. The board is optimized to drive a string
of six, eight, or ten white LEDs at 100mA from each
of its two outputs with a PVIN input voltage range of
3V to 25V. The maximum LED output voltage must be
less than 36V and the over-voltage protection at the
output will limit the maximum output voltage to 36V
in the case of an open circuit load. The high input
voltage range, high-efficiency low-side internal 1.3A
npn power switches, ground-referenced low-voltage
current sense resistors, pwm pins for high dimming
ratio, control pins for brightness adjust, overvoltage
protection, shutdown control pin, externally programmable switching frequency, low shutdown current and internal soft-start capacitor make the LT3486
an extremely powerful and versatile LED driving IC for
portable applications and displays.
DC876 has overvoltage protection at 36V if 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, but 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 in shutdown. The CTRL terminals
can be used to decrease the LED current by connecting the CTRL jumpers to ‘terminal’ instead of ‘VIN’.
The PWM terminals can be used to dim the LEDs with
a PWM signal greater than 3.5V when the PWM
jumper is connected to ‘terminal’ instead of ‘VIN’.
When the ‘VIN’ setting is used at either jumper, the
function is disabled and the part will run at full current or without PWM dimming. If VIN is below 3.5V,
the gate of the MOSFET may not be activated and the
LEDs may not turn on. For proper operation with VIN
voltages below 3.5V, either external PWM signals
must be used to turn the MOSFETs on or another,
lower voltage, MOSFET must be used in place of the
FDN5630. Some gate voltage curves are provided below.
A simple modification can be made to the DC876A
board in order to change the constant current output
of either channel. The sense resistor can be changed
according to the equation in the datasheet and that
will change the constant LED current.
The silkscreen on the demonstration circuit and the
table in the schematic give a guideline of the input
voltage range for a 100mA LED output at a given LED
forward voltage in which the LT3486 remains below
its maximum current limit.
The LT3486 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 876. The
LT3486 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’. An additional 16-lead (5mm x
3mm) plastic DFN DHC package is available, but not
on this demonstration circuit.
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 876
DUAL 1.3A WHITE LED STEP-UP CONVERTERS WITH WIDE DIMMING RANGE
QUICK START PROCEDURE
Demonstration circuit 876 is easy to set up to evaluate the performance of the LT3486. Follow the procedure below:
sure that the power input voltage (PVIN)
does not exceed 25V and that the input voltage (VIN)
does not exceed 10V.
2.
Connect a 2.5V to 10V input power supply to the
VIN and GND terminals on the PCB and a 3V to
25V input power supply to the PVIN and GND terminals on the PCB.
3.
Set the CTRL and PWM jumpers in the VIN position or if VIN is below 3.5V, set the PWM jumper to
the terminal position and use a PWM or DC signal
with a voltage higher than 3.5V.
4.
Observe the bright LEDs and the constant LED current.
NOTE: Make
1.
Connect an array of LEDs with forward voltage less
than 35V, but greater than the input voltage, to the
LED+ and LED- terminals on the PCB as shown.
Figure 1. FDN5630 typical gate charge characteristics
Figure 2. FDN5630 typical transfer characteristics
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 876
DUAL 1.3A WHITE LED STEP-UP CONVERTERS WITH WIDE DIMMING RANGE
Figure 3. Proper Measurement Equipment Setup
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 876
DUAL 1.3A WHITE LED STEP-UP CONVERTERS WITH WIDE DIMMING RANGE
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