LEDs, both low-power indicators as well as high-power Lights, can enable an advanced User Interface as well as a high-end look-and-feel for today’s smart appliances.
Appliances are no longer utilitarian chunks of metal that serve consumers by just keeping their food cold, their laundry and dishes clean, and heat up water for cooking macaroni and cheese. Instead, today’s consumer is often looking to have the most advanced machine available in their price range. Part of realizing this dream is defined by the look and feel of the appliance itself. One of the most noticeable features other than the finish of the machine is the appearance of the User Interface, accent lighting, or even the appearance of the interior itself. The increased usage of indicator LEDs, as well as higher power LEDs used for general illumination, has the appliance designer trying to balance their power budget with board cost. This overview presents the various methods and tradeoffs to drive LEDs for user interface applications.
The standard user interface is comprised of low power LEDs (10mA-20mA) configured in arrays, seven segment displays, or for icon illumination.
The power supply to these LEDs generally comes from one of two sources. The first is taking power from the existing machine power supply from an existing transformer output and ballasting the current through a resistor network. This is often the lowest cost method as very few components are needed. One drawback however is that the energy loss can be significant and thus contributes to the loss of efficiency for the entire machine itself. An alternative method is with the use of a DC/DC converter. The usage of DC/DC converters is often employed when many LEDs must be driven and efficiency is a key element of the design requirement. DC/DC converters also allow the standardization of power supplies so that one main power supply can be used for multiple product models, with the DC/DC adjusting its output based on the number of LEDs to be driven.
Once the power supply configuration has been established, the next and most challenging step is to identify the best method of controlling the LEDs. Traditionally, three methods have been employed: direct drive from the MCU, control through serial communication, and advance serial control by dedicated LED drivers. Each method has its own benefits as well as drawbacks associated with it.
The traditional method of driving LEDs has been to turn the LEDs on and off by having the LED current flow through one of the I/Os of a microcontroller and limiting the current by the use of an external resistor. In cases where a limited number of LEDs are used, this approach is the lowest cost method of control. A few items must be taken into consideration such as having enough I/O available on the MCU, especially “High Current” I/O. The total amount of current through all I/Os must also be taken into account as the MCU has a maximum current rating that must be observed. In cases where the amount of current exceeds either the pin or package max current rating, external switches must be used to control the current. The main drawback with controlling the LEDs with external switches is the additional components needed. This leads to increased PCB insertion costs as well as more opportunities for points of failure on the board.
Both methods are shown below.
Another drawback to both direct drive control methods is that the MCU must constantly monitor and refresh these outputs, and in the case of dimming, pulse and monitor the outputs, thus creating additional software overhead.
When multiple or a large number of LEDs are used, another method of control is often employed where remote control is accomplished by the use of a dedicated device which communicates through one of the MCUs communication ports. This method gives the designer many advantages. The first is often realized in cost savings and reliability by eliminating the multiple transistors and resistors necessary to control the LEDs as well as decreasing board space, with a single device. This type of device is often a type of shift register which takes the instructions from the MCU and holds the outputs in the correct state until otherwise notified, thus decreasing the amount of software overhead necessary to control the LEDs. Another major advantage is that it often eliminates an additional MCU. In many applications, simple user interface boards are located remotely. In these cases, standardized communication methods such as I2C and SPI can be employed to control the shift register through a few wires from the main MCU and often a Darlington array is needed as well. This method is shown below.
The main drawback to both solutions so far has been the lack of an almost identical and constant current through all of the LEDs. Due to variations in resistor values and Vf tolerances of the diodes, the current through different branches of the LED strings may vary greatly. The resulting perceived light output to the human eye is more pronounced with COLORS of LEDs and at lower light levels. A shift in color is also possible when the current is different from one string to another. Using a constant current to supply all the LEDs minimizes these effects. Any perceived difference in intensity from one LED to another, or color change, will greatly reduce the perceived quality of any product. In order to resolve these issues, companies such as STMicroelectronics have invested in the development of dedicated LED drivers, which not only eliminate these previous issues, but also provide additional benefits to the designer.
Dedicated LED Drivers
Dedicated LED drivers provide similar benefits as standard shift register based products. They incorporate key features necessary for driving LEDs that assure consistent light output not only from string to string, but also from array to array when multiple drivers are used.
The first benefit is that the LEDs are supplied with a current that is controlled by the driver itself. Each driver is designed to monitor and adjust the current through each line of LEDs within a predefined tolerance. This assures that each string that is controlled will have a maximum tolerance on the current from one output to another which will in turn provide a similar current to flow through all strings of LEDs. This is especially critical in higher end applications where the LED arrays are close in proximity and any variation in brightness can be readily seen.
Another benefit from an integrated LED driver is that fault detection is often easy and convenient to implement. Features such as LED short or open can be detected and a message relayed to the microcontroller so that either safety critical applications can be monitored or shut down, or in some cases, maintenance can be called for repair or replacement. STMicroelectronics has developed multiple series LED drivers which incorporate these types of protection features.
Additional features have also been added to standard LED drivers to help optimize board space and functionality to meet the needs of today’s LED arrays. One common feature that is almost always used with a user interface is some type of keyboard input. This is often accomplished by using many pins on a microcontroller to monitor key presses. In addition to taking up valuable board space, these keys must also be monitored and filtered by the microcontroller. In products similar to the STLED316 (shown below), the LED driver takes care of this task internally through dual functionality pins which not only drive the LEDs, but also monitor the keyboard inputs.
One of the most useful functions of devices such as the STLED316 is the constant current control that also incorporates dimming. This dimming can be different for different LED strings, which is useful not only for general dimming, but is very valuable when different colors are used. Since the eye interprets different colors of LEDs to be brighter than others, not all colors of LEDs can be driven at the same current. With the capability to have different strings with various constant currents, this allows the LEDs to be driven with the appropriate currents. The other important point to note is that this control is again done through serial communication from the MCU to the driver and no PWMing of the outputs is necessary – thus reducing the software overhead needed to monitor and control the LED currents.
The last important point to make surrounding the use of dedicated LED drivers is that they are designed to meet the demanding EMI needs in industrial and appliance applications. The drivers are designed from the start to provide the highest level of immunity to electrical bursts and other noise sources that can cause displays to flicker, resulting in the appearance of poor quality and frequent service calls.
In summary, the use of dedicated LED drivers are no longer costly, feature rich devices geared toward high end signage applications, but are often the most cost effective system solution for LED-driven user interface boards that not only offer a cost savings, but increased reliability and end-product quality as well.