Different LED Driving Topologies for Portable Applications
By Muhamad Moussa, Engineer, Future Lighting Solutions
There are different methods for driving LEDs in
portable applications. Driving directly from an AA
battery, adding current limiting resistors or using
constant current drivers are all possible options.
Factors to consider when selecting the most
efficient method are:
- Cost
- Power efficiency
- LED output optimization
- Rated life
LEDs must operate at a constant forward current
and within the voltage range indicated in the data
sheets of the LUXEON® based LEDs.
Direct Drive from AA Battery
The battery is connected directly to the LED. Two
cases will be introduced depending on the number
of cells.
- Case one: 1-cell and 2-cell: In these cases,
the batteries either do not have sufficient output
voltage to operate the LED, or they provide
output voltage close to the minimum voltage
limits required to operate a LED.
- Case two: 3-cell and 4-cell: This configuration
will provide 4.5V and 6V respectively,
which is higher than the forward voltage range
specified in the data sheets. As a result, the
LED will be electrically over stressed, thereby
increasing heat production, shortening the
life span and decreasing the light output. On
the other hand, when the batteries are empty,
they will provide voltage between 2.4V – 3.2V,
which is at the lower limits of the forward voltage
needed to turn the LED on.
Using Current Limiting Resistors
Only the 3-cell and 4-cell cases will be considered
since the 1-cell and 2-cell fail to provide the
needed power to operate the LED.
- 3-cell: A 3-cell series configuration will
provide 4.5V to the LED. In order to step that
voltage down to the operational voltage range
recommended, and limit the forward current
of the LED, a 1.5W, 1.5Ω resistor is required.
Using Ohm’s Law, the resistor will dissipate,
as heat, approximately 1.5W (33%) of the
power from the source.
- 4-cell: In order to step down the voltage, a
2.5W, 2.5Ω resistor is needed. Using Ohm’s
Law, the resistor will dissipate 2.5W (41%) of
power as extra heat.
Constant Current Drivers
These drivers provide constant current and voltage
at all times with minimal variations, regardless
of the state of the power source until the
source output voltage falls below the turn on voltage
of the driver.
This performance is desired because it allows the
customer to predict the light output of the LED
since the current is constant over time. It also
provides thermal stability since there are no unexpected
sources of heat such as extra voltage
drop across the LED which leads to longer LED
life span.
There are three different types of constant current
drivers, Buck, Boost and Buck-Boost depending on
the source voltage—i.e. input voltage. If the LED
voltage is less than the input voltage at all times,
then the driver needed is a Buck regulator, if the
LED voltage is greater than the input voltage at all
times, then a Boost regulator is needed. Finally, if
the input voltage is varying within a certain range
and the LED voltage is within that range, then a
Buck-Boost regulator is needed.
Figure 2: Load Current vs. Efficiency for SP6652
Conclusion
Advantages of Using a Constant Current Driver Are:
- Highest efficiency percentages and maintains
the thermal stability
- Longer battery life
- Maximum light output of the LED
Any portable application that requires these 3 performance
criteria should adopt the constant current
driver.
For more information on constant current drivers,
please see www.futurelightingsolutions.com/products/Power.asp or call 1-888-LUXEON2 (589-3662).
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