This paper mainly discusses the microcontroller-based LED driver. It looks at the microcontroller as the core of the system can be used in a variety of different topology. It also discussed in detail the tradeoffs of various topologies, and focus on their main features and limitations: communication, voltage and current capacity, dimming technology, as well as switching speed, etc..
What is the high brightness LED lights, and what it needs to drive?
High-brightness light-emitting diodes (HI-LEDs) is a semiconductor device that only allows current to flow in one direction. It is into the PN structure formed by the combination of the two semiconductor materials. The high-brightness LED and standard LED difference is that their output power. Traditional LED output power is generally limited to less than 50 mW, and high-brightness LEDs up to 1-5 watts.
Exceeds the internal threshold voltage in the forward voltage (VF), HI-LEDs, almost no forward current (IF) to flow through. If VF is further increased, the curve will be linear slope of a sudden rapid increase in the formation of a shape of the knee of the curve.
The output brightness of the LED forward current is proportional to, therefore, if the IF is not properly controlled, the output brightness may not accept the changes. In addition, if more than the maximum specified by the manufacturer IF restrictions could severely shorten the life of the LED.
The high brightness LED flood lights electronic drive control, the main function of these electronic drive to form a constant current source. Using the techniques described later in this article, these circuits can provide a luminous intensity control can also compensate for temperature changes in certain circumstances.
To ensure that systems provide color consistency, HI-LED manufacturers recommend constant nominal current pulse output of the LED brightness adjustment.
Simple topology and its trade-offs
The challenges faced by the design of high-brightness LED driver is to construct a well-controlled, programmable, stable current source, but also high efficiency.
1, using a series resistor (linear method)
The easiest way to adjust the current is to add a series resistor. The advantage is low cost, simple to implement, and not due to switching noise. Unfortunately, this topology has two main shortcomings: First, the substantial loss on the resistor lead to lower efficiency of the system; Second, it can not change the luminosity. Moreover, such programs need to use a voltage regulator to constant current. For example, if we assume that VDD is 5 volts, the VF of the LED is 3.0 volts, so if you need to generate 350 mA constant current, you will need: R = V / I, at this time R = (5V-3.0V ) / 350mA = 5.7Ω.
Can see that these values, R will consume 0.7 watts (almost equivalent to the power of the LED) R × I2, so the overall efficiency is inevitably less than 50%.
This method assumes a constant VDD and constant VF. In fact, VF will change as the temperature changes, making the current changes. Changes to minimize the overall current to a higher VDD can be caused by VF, but a huge loss in the resistor, thus further reducing efficiency.
When we construct a flow through the LED constant current, you need to find a way to set a different light. We know that these LED tube always requires its nominal current to drive, so we can use the programmable duty cycle on-off current, in order to achieve control of the luminosity.
2, using a linear current source
A transistor and / or an op amp can be very precise current is set to 350 mA. Unfortunately, the problem is still the overall efficiency and R power loss.
3, the use of low-end switches (switch-mode method)
Allows the inductor L current when the switch is turned on, rise, drop when the switch is disconnected, we can adjust the current flowing through the LED. With any inductive load when the switch is open, we need to provide a path for current.
When the current is reduced to the low current threshold (300mA), the switch turns on, and when the current threshold rose to a high current (400mA), the switch will disconnect.
This case, switch to low-end (hence the name), the method is very simple. Pass FET is simply its doors to the ends of plus 5V supply, which can be provided directly by an output port of the microcontroller. Moreover, this topology is no longer needed constant VDD voltage, even if the input voltage fluctuations, but also to maintain regulated current.
Current sensing resistor R must be located in the “high end” section of the circuit. If it is connected to the MOSFET source can only be measured on the switch turns on the LED current can not be used to adjust another threshold.
This topology looks like a front-end boost converter, which use N-channel low-cost FET’s advantage, but in R both ends of the voltage differential measurements to obtain the current flowing through the LED.
Please note that the switch actually provides two functions: First, it makes the inductor current can be adjusted; Second, it allows the adjustment of the luminosity.