Design of high brightness LED control based on low cost microprocessor

Important characteristics of high brightness LEDs

As in the case of low-intensity LEDs, the luminous intensity of high-brightness LEDs is proportional to the current through which it passes. This current is usually called the forward current (IF), which ranges from 100mA to 1000mA in HBLEDs. At the same time, whenever the HBLED is polarized, a voltage drop occurs, called the forward voltage (VF). In HBLEDs, the luminosity and chromaticity are proportional to the IF, so it is critical to accurately control the current through the HBLED.

HBLEDs with the same part number and technical specifications do not necessarily have exactly the same VF value. When the current IF through two HBLEDs is the same, their backward voltage VF may be different. Therefore, controlling the LED intensity by means of a constant voltage may result in different densities between HBLEDs, and to ensure that all HBLEDs have the same brightness, a current control must be provided.

Not only is the luminous intensity related to the current through the HBLED, the chromaticity is also related to the HBLED current. In order to maintain the HBLED color, the HBLED must be driven with a constant current. This solution will use PWM (Pulse Width Modulation) to provide a lower average current in the HBLED (light intensity) while maintaining the same instantaneous current (LED color).

As the HBLED current increases, the power dissipation will also increase. A 350mA HBLED with a 3V dropout will consume approximately 1 Watt of power, and without proper thermal management, this dissipation may cause the HBLED to overheat and degrade in long-term performance. Another important aspect of thermal design is that the HBLED luminous intensity is inversely proportional to the LED junction temperature, and as the temperature increases, the color of the emitter will move into higher wavelengths.

Challenges of driving high-brightness LEDs

In low-intensity LEDs, it is very common to use resistors to limit the IF current. In HBLEDs, the power rating of the resistors must be higher, which results in inefficient systems. Therefore, in HBLED systems, switch-mode power supplies (SMPS) are used to improve efficiency and reduce power consumption. Since SMPSs require energy storage components (inductors and capacitors), they are usually more expensive; at the same time, SMPSs may also cause noise or EMI issues.

A group of HBLEDs can be driven simultaneously in parallel or in series. Parallel driving allows each HBLED to have a different light intensity, but if a control loop is required, each HBLED will require a dedicated control, which is too expensive for a large number of HBLEDs.

When HBLEDs are connected in parallel, only one drive and control loop is needed for each string, and the current through all HBLEDs in the series is the same, providing them with a relatively constant brightness. Depending on the number of LEDs in the series, the voltage required for the line may be lower or higher than the input voltage.

Using a microcontroller-based solution

There are a large number of solutions on the market for driving HBLED constant current, some of which are based on dedicated intelligent analog drivers, while others use digital signal processors (DSPs) or microcontrollers with independent analog drivers.

MCU-based solutions are not the best way to perform HBLED constant current control, especially when the system uses a switch-mode power supply built with discrete components, it will become unstable and it is impossible to pass EMC certification. Based on the S08MP16 eight-bit microcontroller, the MCU is responsible for measuring the current feed from the LED string and processing it using a PID control algorithm to control the operation of the independent buck-boost switch-mode power supply to ensure the best current flow through the HBLED string.

The microcontroller is also responsible for monitoring user input, battery voltage and temperature sensors, diagnosing the real-time LED power supply status, and some special communication functions such as LIN functions can also be implemented in the same microcontroller.

A switch-mode power supply is used to provide power to the HBLEDs. It is a discrete buck-boost topology that can operate from 1 to 18 LED strings (continuous range of 0V-5V) and runs at 500mA output current at a frequency of 350kHz. The application block diagram is shown in Figure 1.

Figure 1 Switching power supply applied to HBLED power supply

Challenges of Switch-Mode Power Supply Design

For a large number of HBLEDs, a buck-boost power supply is required to sense the output voltage ( VOUT ) above or below the battery voltage ( VBAT ).

There are many buck-boost topologies available, such as the CUK circuit or the SEPIC converter, each with different requirements and advantages in terms of the number of components required, positive and negative voltage references, and efficiency.

The switch-mode power supply chosen in this design combines a buck converter and a boost converter, which use a common inductor and capacitor. Changing the operation mode from buck to boost or from boost to buck depends on the state of transistors Q1 and Q2, as shown in Figure 2.

Figure 2 Buck-Boost Conversion Example

This topology reduces cost and eliminates the need for additional inductors and capacitors. In addition, depending on the mode in which the switch mode power supply operates, its transfer function is reduced to a common buck or boost converter, simplifying the design from a control perspective.

To control EMC using independent switch-mode power supply topologies, buffer filters need to be set and added to the switching transistors Q1 and Q2; and the software control strategy needs to be set to center-aligned PWM and on/off delay between the two channels.

Choosing the right microcontroller for constant current HBLED control

Switching mode power supply (SMPS) requires accurate switching frequency and duty cycle, PWM signal jitter will be reflected in the output voltage, and then reflected in the HBLED intensity. At the same time, in order to save the cost of inductor and capacitor size, the switching frequency must be increased to tens of thousands of hertz. The resolution and channel availability of analog-to-digital converters are also important for monitoring and controlling HBLED current and voltage at any time.

To achieve HBLED constant current control, S08MP16 measures the HBLED string current reflected in the current sense resistor, which is connected in series with the HBLED string. S08MP16 embedded 12-bit analog-to-digital converter, can use small resistor value, power consumption is very small. In addition, by using ADC and resistor divider, it can measure SMPS output voltage and diagnose open load in overcurrent and overvoltage conditions.

Figure 3 Embedded PID control block diagram

To control the switching power supply frequency and duty cycle, the FlexTimer (FTM) module can be used; using a timer operating frequency of up to 40MHz in the automotive version, high-frequency and high-resolution pulse width modulation (PWM) can be generated, and more HBLEDs can be operated on each string without unstable HBLED intensity when operating in small quantities. In addition, the Programmable Delay Module (PDB) is used in this application to synchronize the ADC reading with the PWM switching frequency, ensuring that the ADC reading is only displayed when the current stabilizes in the ON state.