Introduction to PWM dimming knowledge

In mobile phones and other consumer electronics, white LEDs are increasingly being used as backlight sources for displays. Recently, many product designers hope that the brightness of white LEDs can be changed accordingly in different applications. This means that the driver of white LEDs should be able to support the adjustment of LED brightness. There are currently three main dimming technologies: PWM dimming, analog dimming, and digital dimming. Many drivers on the market can support one or more of these dimming technologies. This article will introduce the characteristics of these three dimming technologies, and product designers can choose the corresponding technology according to specific requirements.

PWM Dimming (Pulse Width Modulation) dimming method - This is a dimming technology that uses simple digital pulses to repeatedly switch the white light LED driver. The user's system only needs to provide different wide and narrow digital pulses to simply change the output current and thus adjust the brightness of the white light LED. The advantages of PWM dimming are that it can provide high-quality white light, simple application, and high efficiency! For example, in the mobile phone system, a dedicated PWM interface can simply generate a pulse signal with any duty cycle, which is connected to the driver's EN interface through a resistor. Most manufacturers' drivers support PWM dimming.

However, PWM dimming has its disadvantages. The main disadvantage is that PWM dimming can easily cause the white light LED driver circuit to produce audible noise (or microphonic noise). How is this noise generated? Usually white light LED drivers are switching power supply devices (buck, boost, charge pump, etc.), and their switching frequencies are around 1MHz, so in typical applications of the driver, no audible noise is generated. However, when the driver performs PWM dimming, if the frequency of the PWM signal falls exactly between 200Hz and 20kHz, the inductance and output capacitor around the white light LED driver will generate audible noise. Therefore, when designing, avoid using low-frequency bands below 20kHz.

We all know that a low-frequency switching signal acting on an ordinary wire winding coil will cause mechanical vibrations between the coils in the inductor. The frequency of the mechanical vibrations falls exactly on the above frequency, and the noise emitted by the inductor can be heard by the human ear. The inductor generates part of the noise, and the other part comes from the output capacitor. Now more and more mobile phone designers use ceramic capacitors as the output capacitors of the driver. Ceramic capacitors have piezoelectric properties, which means that when a low-frequency voltage ripple signal acts on the output capacitor, the capacitor will emit a squeaking buzzing sound. When the PWM signal is low, the white light LED driver stops working, and the output capacitor discharges through the white light LED and the resistor at the lower end. Therefore, when PWM dimming, the output capacitor inevitably generates a large ripple. In short, in order to avoid audible noise during PWM dimming, the white light LED driver should be able to provide a dimming frequency that exceeds the audible range of the human ear!

Compared with PWM dimming, if the resistance value of RS can be changed, the current flowing through the white light LED can also be changed, thereby changing the brightness of the LED. We call this technology analog dimming.

The biggest advantage of analog dimming is that it avoids the noise generated by dimming. When using analog dimming technology, the forward voltage drop of the LED will decrease as the LED current decreases, which reduces the energy consumption of the white light LED. However, unlike PWM dimming technology, the white light LED driver is always in working mode during analog dimming, and the driver's power conversion efficiency drops rapidly as the output current decreases. Therefore, the use of analog dimming technology often increases the energy consumption of the entire system. Another disadvantage of analog dimming technology is the light quality. Since it directly changes the current of the white light LED, the white light quality of the white light LED also changes!

In addition to PWM dimming and analog dimming, some manufacturers' drivers currently support digital dimming. White LED drivers with digital dimming technology will have a corresponding digital interface. The digital interface can be SMB, I2C, or a single-wire digital interface. System designers only need to give the driver a string of digital signals according to the specific communication protocol to change the brightness of the white LED.