“This application note describes how the MLX10803 LED driver controls high-brightness LEDs. This document also describes circuits that can also be applied to other applications, as long as it is within the similar specifications of MLX10803 (such as MLX10801).

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This application note describes how the MLX10803 LED driver controls high-brightness LEDs. This document also describes circuits that can also be applied to other applications, as long as it is within the similar specifications of MLX10803 (such as MLX10801).

EMR / EMC

Each type of active current regulation will produce ripples on the regulated output, which will generate electromagnetic radiation (EMR) and electromagnetic coupling (EMC) with surrounding Electronic circuits. MLX10803 and the applications described in this document are designed to minimize EMR. When designing circuit boards and physical applications, extreme care must be taken. Melexis does not make any statement on whether any of these circuits meets EMI/EMC and EMR compliance with international regulations. It is recommended to conduct a conformance test, and the user conducts such a test before selling in a specific country and market.

Electromagnetic interference

Very low current is used to set the peak current detection level in MLX10803. In some cases, it is recommended to connect a capacitor in parallel with the resistors connected to IREF1 and ground and IREF2 and ground. Another option is to use short lines only for these inputs. MLX10803 is a very flexible circuit that consumes very little energy to implement the functions described in this application note. A person with general electrical engineering skills should be able to decide when or if decoupling capacitors are needed, and these capacitors will not be added in the circuit examples described in this application note.

Application area

PWM control on IC power supply

The most direct way to create a PWM modulation of the LED current is to apply the PWM signal directly to the power supply of the MLX10803. The MLX10803 circuit has a very fast power-up time. If the PWM signal has some of the given characteristics below, there is no problem using the following circuit.

To use the above diagram, the PWM signal must be less than 0.3 V for low level (off) and greater than 5V for high level (on).

It is recommended to use a 12 V (or slightly higher) PWM signal. For most common N-FETs, this will cause the connected FET to be driven to the lowest drain-source resistance (Rdson). The DRVGATE signal is clamped to 12 V, so any higher voltage on the PWM signal will only cause the connected PWM generator to produce higher energy consumption.

Between 5 V and 12 V, the high-level voltage connected to the PWM signal will be the same as the gate voltage in the DRVGATE output. Note that some N-FETs have lower gate voltages to achieve a fully-on state, but most transistors can also handle 12 V gates.

PWM control on Vref pin

If the PWM voltage level is at an appropriate level, applying a PWM signal on the Vref pin is also very simple. This method also has some other interesting effects.

When the low level (off), the voltage Vref pin must (the voltage on Iref1 or Iref2), the voltage Vref pin should not be higher than 4.5 V (high) (on) level. Please refer to the MLX10803 IC specification.

High frequency PWM

Theoretically, there is a way to allow unlimited high switching frequencies with unlimited resolution. Well, of course, the speed of the LED sets a practical limit. Please refer to the LED data sheet of your choice.

Basic function schematic

As can be seen from the following schematic diagram, the switching is done directly in the current path of the LED. During the LED extinguishment period, measures must be taken to deal with the energy stored in the coil. The simplest method is to add a diode to create a current path for the coil when the transistor T2 is off. Note that a series resistor must be used to limit the current. Otherwise, the current may be high. Please note that this is a very energy-efficient shutdown cycle method. In the worst case, you will only waste energy on D1, D2 and series resistors. However, this energy-saving method has a disadvantage, it makes your PWM adjustment a bit non-linear, but you can set the non-linearity more or less freely through series resistors.

HF PWM curve form of MLX10803

The following is the LED adjustment curve form used in the example. Choosing a larger ripple can better show the non-linearity that may occur. If you want to minimize this effect, choose the smallest possible ripple and power consumption during the off (dark) time, which should be as close as possible to the power consumption of the LED.

It is also possible to compensate for this type of non-linearity in the overall adjustment of the system. The nonlinearity of LEDs may be a bigger problem. For example, LEDs emit light with different intensities at different temperatures.

PWM curve form below the switching frequency

Note that the gray dashed line represents the discharge effect. The gray dashed line is directly related to the power consumption of the AC current path during shutdown. The smaller the loss, the slower the decline of the discharge line, and the faster the rise of the charging line.