This article focuses on microcontroller-based LED drivers . It examines the various topologies that can be used with a microcontroller as the core of the system. It also discusses in detail the trade-offs of various topologies, focusing on their main features and limitations: communication, voltage and current capacity, dimming technology, and switching speed.
What is a high-brightness LED and what does it need to drive?
High Brightness Light Emitting Diode (HI-LED) is a semiconductor device that allows current to flow in only one direction. It is formed by a combination of two semiconductor materials to form a PN structure. High-brightness LEDs differ from standard LEDs in their output power. The output power of conventional LEDs is generally limited to 50 mW, while high-brightness LEDs can be up to 1-5 watts.
Figure 1 shows a typical relationship between the internal voltage and current of a HI-LED. There is almost no forward current (IF) flowing through the HI-LED before the forward voltage (VF) exceeds the internal gate voltage. If the VF rises further, the curve will suddenly and rapidly rise with a linear slope, forming a curve that resembles a knee.
Figure 1: LED voltage vs. current curve
The output brightness of the LED is proportional to the forward current, so if the IF is not properly controlled, the output brightness may be unacceptable. In addition, if the manufacturer exceeds the maximum IF limit specified, it may also seriously shorten the life of the LED.
High-brightness LEDs should be controlled by electronic drivers whose primary function is to form a constant current source. Using the techniques described later in this article, these circuits can provide luminosity control and, in some cases, compensate for temperature changes.
To ensure color consistency in the system, HI-LED manufacturers recommend brightness adjustment of the LED with a constant nominal current pulse output.
Simple topology and its trade-offs
The challenge in designing high-brightness LED drivers is to construct a well-controlled, programmable, stable current source with high efficiency.
Using series resistors (linear method)
The easiest way to adjust the current is to add a series resistor as shown in Figure 2A. The advantages are low cost, simple implementation, and no noise due to switching. Unfortunately, this topology has two major drawbacks: first, the large loss on the resistor leads to a reduction in system efficiency; second, it does not change the luminosity. Moreover, this solution requires a regulated source to achieve a constant current. For example, if we assume VDD is 5 volts and the VF of the LED is 3.0 volts, then if you need to produce a constant current of 350 mA, you will need: R = V / I, then R = (5V-3.0V ) /350mA = 5.7Ω.
It can be seen that with these values, R will consume R x I2, or 0.7 watts (almost equivalent to the power of the LED), so the overall efficiency is inevitably below 50%.
This method assumes a constant VDD and a constant VF. In fact, the VF changes with temperature, causing the current to change. Using a higher VDD minimizes the overall current fluctuation caused by VF, but it introduces a large loss on the resistor, further reducing efficiency.
When we construct a constant current through the LED, we need to find a way to set different luminosity. We know that these LEDs always need to be driven with their nominal current, so we can use a programmable duty cycle to turn the current on and off to achieve luminosity control. This requires a switch, as shown in Figure 2B.
Figure 2: LED Driver Topology
Linear current source
With a transistor and / or an operational amplifier, the current can be set very accurately to 350 mA. Unfortunately, the overall efficiency and R power loss issues remain.
Low-side switch (switch mode method)
Figure 2C shows this concept. As shown in Figure 3, we can regulate the current flowing through the LED by allowing the current on the inductor L to rise when the switch is turned on and down when the switch is off. As with any inductive load, we need to provide a path for the current when the switch is open. This can be done with the freewheeling diode in Figure 2D, where we replace the switch with an N-channel MOSFET and add a resistor R to measure the current flowing through the LED.
When the current drops to a low current threshold (such as 300mA), the switch will turn on, and when the current rises to a high current threshold (such as 400mA), the switch will be turned off.
In this case, the switch is placed at the low end (the method is named), and the implementation is very simple. Turning on the FET requires only adding 5V to its gate, which can be provided directly from an output of the microcontroller. Moreover, this topology no longer requires a constant VDD voltage to maintain regulation current even when the input voltage is fluctuating.
The current sense resistor R must be located in the "high end" portion of the circuit. If you connect it to the source of the MOSFET, you can only measure the current on the LED when the switch is turned on. It cannot be used to adjust another threshold, see Figure 3.
Figure 3: LED and Switch Current
This topology looks like the front end of a boost converter, which has the advantage of using N-channel, low-cost FETs, but requires voltage differential measurements across R to get the current flowing through the LEDs.
Note that the switch actually provides two functions: first, it produces an adjustable current on the inductor; second, it allows for illuminance adjustment.
High-end switch
This circuit is identical to the previous one except for the load and transistor swap locations. The switch shown in Figure 2E is located at the "high end". We also changed the FET from N to P. N-channel FETs require VGS >5V to be fully turned on: In this topology, the source voltage of the N-channel will constantly change, and often above 3 volts, so at least 8 volts is required on the gate. This requires a door drive circuit similar to a charge pump, making the entire circuit a bit more complicated. If you use a P-channel FET and you can provide a -5V VGS directly from the microcontroller's output, it's much simpler. This topology is similar to the front end of a buck converter.
Its main advantage is that it can measure current directly at both ends of R, so no differential measurement method is needed.
Brightness adjustment technique
There are many technologies that can adjust the brightness of LEDs, many of which are patented. Here are a few of them. In all methods, the average luminosity is obtained by completely illuminating (at its nominal current) and then turning off the LED at a very fast rate (avoiding flicker) and is proportional to the percentage of LED lighting time.
Pulse width modulation
This technique uses a fixed frequency with a period of T, as shown in Figure 4. The adjustment of the brightness is achieved by changing the pulse width. Figure 4 shows three different luminosity levels with duty cycles of 6%, 50%, and 94%, respectively.
Figure 4: Pulse Width Modulation
Frequency modulation
This technique was announced by Artistic Licence, which uses the concept of fixed-width control pulses, as shown in Figure 5. Pulse A is always the same width and luminosity is controlled by the repetition interval of pulse A.
Figure 5: Frequency Modulation
Angle modulation
This is a new technology invented by Artistic Licence based on a string of binary pulse trains containing luminous intensities. Each bit in the pulse train is stretched in proportion to its bit value. If the duration of the lowest bit b0 is 1, then the duration of the b1 bit is 2, and accordingly, the durations of the b2 to b7 bits are 4, 8, 16, 32, 64, and 128, respectively, as shown in FIG. .
Figure 6: Angle modulation
letter of agreement
DMX512
DMX512 is a standard published by USITT (American Academy of Theatre Technology). Originally used to control lighting dimmers, the protocol has now been extended to control luminaire movements, slide projectors and many other lighting fixtures. The DMX512 runs on the EIA-485 standard. The data is transmitted on the basis of 8-bit asynchronous serial communication, one start bit, two stop bits, and no parity. It has 256 brightness adjustment levels.
DALI (Digital Addressing Lighting Interface)
DALI is a standard developed for the communication of electronic ballasts and is included as an appendix in the ECG standard IEC 929. DALI is designed for standard components and simple wiring, ie low cost applications. The field of application may be to adjust the lighting and preset values ​​of different lighting environments, adjust the lighting settings according to the direction of sunshine and energy saving factors.
The basis of DALI is the master-slave principle: the user operates the system through the controller (host), and the controller sends messages containing addresses and commands to all ballasts (slave). The address determines whether the ballast should follow the instructions. Each ballast is digitally addressed, so it is not sensitive to electromagnetic noise (better than analog 1-10 volt dimmer switching systems).
ZIGBEE
Zigbee is a communication protocol that combines Home RF lite and IEEE 802.15.4 specifications. Zigbee operates in the 2.4 GHz and 868/915 MHz ISM bands. Lighting applications have become one of its main markets due to its low power consumption at a lower cost. The network capabilities provided by Zigbee are also very useful in lighting systems, and it also has the advantage of wireless control.
Limitations of using microcontrollers
Voltage and current
If VDD is the common source for the LED and the microcontroller, then this voltage can only drive one LED. The simple topology we have discussed does not allow the LED voltage to be higher than VDD, see Figure 2 and Figure 7. If LEDs are used in series, all LEDs have the same current, which is an advantage, but VDD must be higher and the microcontroller requires a separate power supply.
Figure 7: Microcontroller-based LED driver
Physical interface supporting communication
The microcontroller only provides simple synchronous (SPI) or asynchronous (SCI) communication. In order to implement DALI, DMX, LIN communication protocols, etc., it also requires additional hardware and software.
Constant current regulation and switching speed
The key parameter in this application is the switching speed. The slower the switching speed, the larger the inductor and the higher the cost. Most microcontrollers can perform A/D conversion in approximately 15 microseconds. Coupled with some instructions for comparing readings and internal thresholds, we can now say a complete switching cycle of 30 to 40 microseconds plus an uncertainty of 15 microseconds. This error defines the minimum inductance value shown in Figure 8. Another option is to arbitrarily set the duration of the turn-on and turn-off, and then re-adjust these values ​​according to the actual situation, try and reach the two current thresholds. This indirect approach allows for smaller, lower cost inductors with less accuracy.
Figure 8: Basic design ideas
Dimming and modulation speed
There is no need to modulate the transistor at 100% luminosity. At the other extreme, for the lowest luminosity level (such as 1%), the transistor needs to be turned on for 1% of the time. Assuming that the brightness adjustment must be done at 100 Hz or higher to avoid flicker, the PWM frequency must be 10 kHz or higher. However, the naked eye can distinguish subtle changes in the low illuminance interval, so the 100 level is not enough. If 4000 levels (12-bit resolution) are required, the PWM frequency must be above 400 kHz, which is almost impossible for a simple microcontroller.
in conclusion
Now, we have seen how easy it is to design a high-brightness LED driver based on a microcontroller. The three main limitations are processing speed and inductor size and dimming resolution, industry-standard communication capabilities, and drive capability for multiple outputs and/or LED strings. (Text / Freescale Semiconductor Pedro Pachuca)
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