DC976A - Demo Manual

DEMO CIRCUIT 976
LT3476EUHF
QUICK START
GUIDE
LT3476EUHF
Quadruple High Power LED
Driver in Buck Mode
DESCRIPTION
WARNING!
Do not look directly at operating LED.
This circuit produces light that can damage eyes.
Demonstration circuit 976 is a Quadruple High
Power LED Driver in Buck Mode featuring the
LT3476EUHF. The LT3476 is a quad output
DC/DC converter designed to operate as a current
source for driving high current LEDs. On the demo
circuit, the default current for each output channel
is 350mA but that can be adjusted to up to 1.05A
via resistor dividers at the VADJ pins. The output
current should not exceed the rated current of the
LEDs used. If 1W LEDs, such as the Lumileds
Luxeon LEDs, are used, 350mA current is the
maximum. 1A output current is reserved for 3W
LEDs, such as Luxeon III. The LED current thermal
de-rating should be considered to protect the LEDs.
The maximum LED string voltage the demo circuit
can drive is limited by the duty cycle and the input
voltage of the LT3476. The demo circuit default
switching frequency is 1MHz. At 1MHz, the maximum duty cycle is 84% (MIN). Lower switching
frequency results in higher maximum duty cycle.
The input voltage is limited by the CAP pin overvoltage protection threshold. As a result, each
channel of the demo circuit can drive up to 27V
total LED voltage. Adjustable switching frequency
allows optimization of the efficiency and the external component size. At 1MHz, 96% efficiency can
be achieved at 1A without sacrificing the solution
size.
Each of the four regulators on the demo circuit is
independently operated. The PWM dimming feature of the LT3476 allows True Color PWM dimming with the dimming ratio up to 1000:1. Additional analog dimming is possible.
The LT3476 datasheet gives a complete description of the part, operation and application information. The datasheet must be read in conjunction
with this quick start guide for working on or modifying the demo circuit 976.
Design files for this circuit are available. Call
the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
PERFORMANCE SUMMARY Specifications are at TA = 25°C
SYMBOL
PVIN
VIN
VLED
ILED
ILED
FSW
η
η
POUT
PWMD
PARAMETER
Power Input Supply
Chip Bias Input Supply
Output LED Voltage, per Output
Output LED Current, per Output
Output LED Current, per Output
Switching Frequency
Conversion Efficiency
Conversion Efficiency
Maximum Total Output Power
PWM Dimming Ratio
CONDITIONS
MIN
TYP
2.8
PVIN = 33V, ILED = 350mA
R2, R25, R27, R29 = 66.5K
R2, R25, R27, R29 = 0
R11 = 21K
ILED = 350mA
ILED = 1050mA
ILED = 1050mA
PWM Frequency = 100Hz
1013
0.85
350
1050
1
94
96
MAX
33
16
27
1087
1.15
100
1000:1
UNITS
V
V
V
mA
mA
MHz
%
%
W
1
LT3476EUHF
QUICK START PROCEDURE
Demonstration circuit 976 is easy to set up to
evaluate the performance of the LT3476EUHF. Refer to Figure 1 for proper measurement equipment
setup and follow the procedure below:
1. With power off, connect an input power supply
to VIN and GND. The maximum rating is 16V. To
achieve highest efficiency, a low voltage supply,
such as 3.3V or 5V, is recommended.
2. With power off, connect a second input power
supply to PVIN and GND. The maximum PVIN
is 33V.
3. With power off, connect PWM1, PWM2, PWM3,
PWM4 to VIN. If an output n is not used, the
PWMn can be left unconnected.
4. With power off, connect a LED (in series) string
to an output channel. Connect the anode of the
first LED to the LEDn+. Connect the cathode of
2
the last LED to the LEDn-. All four channels can
operate simultaneously or individually.
5. Turn on PVIN power supply.
6. Turn on VIN power supply.
7. Check for the proper LED current and LED
string voltage.
NOTE. If there is no output, turn off the power supplies. Check all
connections.
8. Once the proper output current/voltage are
established, adjust the parameters within the
operating range and observe the output current
regulation, efficiency and other parameters.
9. To observe PWM dimming, turn off all power
supplies. Disconnect PWMn from VIN. Connect
PWMn to a pulse generator. Then turn on PVIN,
the PWM pulse generator and VIN.
10. For even faster LED current turn-off, remove
R20, R21, R22, R23 and repeat step 9.
LT3476EUHF
Figure 1. Proper Measurement Equipment Setup
3
LT3476EUHF
4
LT3476EUHF
5
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