Create an LED Circuit in Minutes
National Semiconductor and Avnet Make Design Easier
By Jeff Perry, National Semiconductor, Senior Manager, WEBENCH Group
Spurred by the increasing cost of energy and concerns about climate change,
governments and industry are pushing for higher efficiency lighting. LEDs
provide an excellent solution due to their high efficiency and long lifetime. At the
same time, LED technology is undergoing a period of rapid change and
innovation, with ever brighter and more efficient parts being introduced, and this
makes it difficult for the average engineer to keep up with the latest products.
Fortunately, online tools for LED selection and implementation make choosing an
LED and LED driver easier. But even with these tools, the user should have an
understanding of the parameters affecting LED selection.
LED Selection
The first step is choosing a color for the LED. Colored LEDs are characterized
by their dominant wavelength and are available in wavelengths from UV to
infrared. White LEDs are specified by their color temperature with warm white
LEDs, often used for room lighting, in the 2800K to 3500K range. This compares
to an ordinary Tungsten filament light bulb at about 3000K. Also available are
cool white LEDs in the 6300K to 7500K area and white LEDs in the middle range
of 3600K to 6200K.
How Bright Should It Be?
Luminous flux, in units of lumens, is the measurement normally used for the
brightness of LEDs. This is the amount of light emitted in the spectrum to which
the human eye is sensitive. Table 1 shows typical luminous flux values for some
light sources:
Type
40W Tungsten bulb
100W Tungsten bulb
25W Compact fluorescent
55W Halogen auto
headlight
35W HID auto headlight
150W Projector bulb
180W Low-pressure
sodium street lamp
Lumens
500
1500
1500
1500
3250
5000
27,000
Table 1: Typical Luminous Flux Values for Light Sources
Individual high-brightness LEDs typically have luminous flux values of less than
100 lumens, although this is climbing rapidly. LEDs are typically combined into
arrays to achieve higher brightness values. Multiple LEDs can be arrayed in
parallel strings using one current control driver, but this can lead to differences in
brightness in the strings due to the variance in the LED forward voltage and thus
current in each string. Therefore, it is preferable to use LEDs in a series for
consistent brightness and color. However, the series voltage gets higher with
more LEDs, which affects which driver topology can be used – buck vs. boost.
Also keep in mind that LEDs are directional in nature. This is specified by the
viewing angle, which is the point at which the brightness falls off to 50% intensity.
In directional applications, the actual brightness perceived will be higher than a
point source of light with a spherical emission pattern, or conversely, if a
spherical emission pattern is desired, the array must be designed accordingly.
Optics can be used to diffuse or focus the light.
A measure of lighting elements efficiency is luminous efficacy, which has units of
lumens/watt. With the explosion of LED R&D, parts with values of 75lumens/watt
are readily available and LEDs with 115 lumens/watt have been recently
introduced. This compares with tungsten filament bulbs at about 17 lumens/watt,
CFLs at 60 lumens/watt, and low pressure sodium street lamps at 100 to 200
lumens/watt. Figure 1 has a table of LEDs with a graph of the luminous flux vs.
power showing a range of efficiencies for various LEDs.
Figure 1: LED Selection Showing Graph of LED Luminous Flux vs. Power
Voltage and Current
The forward voltage of an LED is a characteristic of the process used to
manufacture the LED with yellow/orange/red LEDs in the 2-3V range and
blue/green/white LEDs in the 3-4V range. The current through the LED controls
the brightness and affects the color. Therefore, LEDs are run in a constant
current mode. High-brightness LEDs typically come in currents of .35A, 0.7A,
1.0A, 1.4A, and up. Also to be considered is the footprint and height of the LED.
Provision must also be made for heat sinking, which becomes vital in highcurrent applications. Cost, is of course, another critical parameter.
LED Selection Made Easy
LED selection has been made much easier with the introduction of online
selection tools from National Semiconductor and Avnet. The ability to do a sideby-side comparison of multiple solutions along with graphical analysis provides
the user with unprecedented power to rapidly get to the best solution.
Temperature Control for LEDs
Why do we need temperature control and monitoring for LEDs if they are
supposed to be so efficient? Don’t LEDs run cooler than incandescent light
sources? It turns out that while LEDs are much more efficient than tungsten
filament bulbs, they still generate a lot of heat. Incandescent lights generate heat
which largely leaves the system as infrared radiation. On the other hand, LEDs
generate heat in the diode semiconductor structure in addition to photons. This
heat is not part of the radiated spectrum and it must exit the system through
conduction and convection.
If LEDs are allowed to get hot, a number of issues arise. The brightness of LEDs
goes down markedly with temperature. Also, the color of LEDs changes with
temperature, which can lead to problems in applications that require consistent
color integrity such as RGB-generated white light. Electrical characteristics, such
as the forward voltage of the LEDs, will drift with temperature which may have to
be taken into account when designing the driver circuitry. This change can also
be an issue if the LEDs share current in parallel configurations. Constant
exposure to high junction temperatures accelerates the degradation of LEDs and
reduces their life and reliability. Thus, it is essential to design the system so it
runs within the temperature specification of the LEDs. This is normally
accomplished using heat sinks, such as large copper areas on the printed circuit
board, attached fin heat sinks, and/or thermally-enhanced/metal PCBs to mount
the LEDs. Forced airflow can also be used.
However, if an unusual event occurs, such as extraordinary weather-related heat
or failure of a heat sink, a failsafe mechanism can be implemented. Most buck
topology LED drivers have thermal shutdown, so if the drivers exceed a specified
temperature, typically 125C to 150C, they will turn off along with the LED.
Boost topology drivers will protect themselves, but not the load when they shut
down so they require a crowbar circuit or other protection for LEDs. In any case,
the LED temperature may be higher than the driver, and the LEDs thus require
their own temperature sensor and monitoring circuit for failsafe protection. This
temperature sensor circuit can either reduce/turn off the current to the LEDs, turn
on a cooling fan, and/or provide an alert mechanism to the user or maintenance
personnel.
Types of Temperature Sensors for LEDs
In general, temperature sensor accuracy needs to provide enough margin to be
able to both detect an over temperature problem and at the same time not trigger
a false alarm under normal operating temperatures. For example, if a system is
normally operating at up to 80C, and it is desired to detect a fault condition at no
more than 100C, a temperature sensor system with +/- 2C accuracy set to trip
at 98C should be fine, but one with +/-10C accuracy set to trip at 90C would
be marginal.
Discrete temperature sensors appropriate for LEDs include thermistors which
change resistance as the temperature changes. Thermistors are inexpensive
and have high sensitivity, but are non linear, thus requiring initial calibration.
Thermistors are available in the desired temperature range of 50C to 150C.
Another choice is thermocouples. The voltage of these sensors changes as the
temperature changes and they also generate a current so they don’t necessarily
require a power source. They are less sensitive than thermistors, but are good
enough for LED use. They come in a wide range of temperatures, well beyond
what is required for LEDs and are used widely in other applications. The cost is
high relative to thermistors. Both of these sensors require some analog circuitry
either to interface with a microcontroller which can then take action to correct the
temperature problem or to interface directly to the LED driver via a shutdown or
dimming pin.
Silicon temperature sensors, which come in temperature ranges of -50C to
150C, may also be used for LED applications. These inexpensive sensors
provide a wide variety of options ranging from analog voltage output which is
proportional to temperature, temperature triggered on/off output with hysteresis,
fan control, and digital interfaces such as I2C and SPI/ Microwire to interface with
a microcontroller.
Temperature Sensor Application Example
A simple circuit for interfacing an LM26LV temperature sensor directly to an
LM3404HV buck LED driver is shown in figure 1. The temperature sensor should
be placed as close as possible to the LEDs. In this circuit, the OVERTEMP pin
of the LM26LV is normally high when the temperature is below the specified
value, but the pin goes low when the temperature is high, thus shutting off the
LM3404HV via the dim pin. When the temperature goes back 5C below the
specified value (the sensor has a hysteresis feature), then the OVERTEMP pin
goes high and the LM3404HV will turn back on.
Figure 2: LM26LV Silicon Temperature Sensor with LM3404 LED Driver
A simple temperature sensor can be used to monitor and correct for over
temperature situations in LED applications. More sophisticated systems can be
implemented which proportionally reduce current to the LEDs without shutting
them down as the LED temperature rises above a threshold. Or a fan can be
turned on (and RPM increased) after the LED temperature exceeds the
specification. None of these systems are a substitute for good thermal design for
the LEDs, but they can be used as a failsafe shutdown to enhance LED life and
reliability when the normal thermal controls fail.