Introduction to general LED drive circuit

The relationship curve between forward voltage drop (VF) and forward current (IF) shows that when the forward voltage exceeds a certain threshold (about 2V), which is usually called the on-voltage, it can be approximately considered that IF is proportional to VF. The table shows the electrical characteristics of the current main ultra-bright LEDs. It can be seen from the table that the highest IF of the current ultra-bright LEDs can reach 1A, while VF is usually 2 to 4V.

Since the light characteristics of LEDs are usually described as a function of current rather than a function of voltage, the relationship curve between luminous flux (φV) and IF, therefore, constant current source driving can better control the brightness. In addition, the forward voltage drop of LEDs has a relatively large range of variation (up to 1V or more), and from the VF-IF curve in the figure above, it can be seen that a small change in VF will cause a large change in IF, thereby causing a large change in brightness. Therefore, the use of a constant voltage source driving cannot guarantee the consistency of LED brightness, and affects the reliability, life and light decay of the LED. Therefore, ultra-bright LEDs are usually driven by a constant current source.

The relationship curve between LED temperature and luminous flux (φV) is shown in the figure below. The luminous flux is inversely proportional to the temperature. The luminous flux at 85°C is half of that at 25°C, and the light output at -40°C is 1.8 times that at 25°C. Temperature changes also have a certain impact on the wavelength of LFD. Therefore, good heat dissipation is the guarantee for LED to maintain constant brightness.

Introduction to general LED drive circuit

Due to the limitation of LED power level, it is usually necessary to drive multiple LEDs at the same time to meet the brightness requirements. Therefore, a special drive circuit is needed to light up the LED. The following briefly introduces the LED conceptual drive circuit.

The resistor current limiting drive circuit is the simplest drive circuit, and the current limiting resistor is calculated according to the following formula.

Vin is the input voltage of the circuit: VF is the forward current of IED; VF is the voltage drop of LED when the forward current is IF; VD is the voltage drop of the anti-reverse diode (optional); y is the number of LEDs in each string; x is the number of parallel LED strings.

The linear mathematical model of LED is: Vo is the turn-on voltage drop of a single LED; Rs is the linear equivalent series resistance of a single LED. Then the calculation of the current limiting resistor in the above formula can be written as: After the resistor is selected, the relationship between IF and VF of the resistor current limiting circuit is: From the above formula, it can be seen that the resistor current limiting circuit is simple, but when the input voltage fluctuates, the current through the LED will also change, so the regulation performance is poor. In addition, since the power loss of the resistor R is xRIF, the efficiency is low.

Introduction to Linear Regulators

The core of the linear regulator is to use the power transistor or MOSFET working in the linear region as a dynamically adjustable resistor to control the load. There are two types of linear regulators: parallel type and series type.

The parallel linear regulator is also called a shunt regulator (in fact, the load can be multiple LEDs in series, the same below). It is connected in parallel with the LED. When the input voltage increases or the LED decreases, the current through the shunt regulator will increase, which will increase the voltage drop on the current limiting resistor to keep the current through the LED constant. Since the shunt regulator needs to be connected in series with a resistor, the efficiency is not high, and it is difficult to achieve constant regulation when the input voltage range is relatively wide. For the series regulator, when the input voltage increases, the dynamic resistance is increased to keep the voltage (current) on the LED constant. Since the power transistor or MOSFET tube has a saturation conduction voltage, the minimum input voltage must be greater than the sum of the saturation voltage and the load voltage for the circuit to work correctly.

The above driving technology is not only limited by the input voltage range, but also has low efficiency. When used for low-power ordinary LED driving, the loss is not obvious because the current is only a few mA. When used as a driver for high-brightness LEDs with a current of several hundred mA or even higher, the loss of the power circuit becomes a more serious problem. The switching power supply is the most efficient in energy conversion at present, which can reach more than 90%. Power converters such as Buek, Boost and Buck-Boost can all be used to drive LEDs, but in order to meet the constant current drive of LEDs, the output current is detected instead of the output voltage for feedback control.

The LED drive circuit using the Buck converter is different from the traditional Buek converter. The switch tube S is moved behind the inductor L, so that the source of S is grounded, which facilitates the drive of S. The LED is connected in series with L, and the freewheeling diode D is connected in anti-parallel with the series circuit. The drive circuit is not only simple but also does not require an output filter capacitor, reducing the cost. However, the Buck converter is a step-down converter and is not suitable for occasions with low input voltage or multiple LEDs in series.

The LED drive circuit using the Boost converter pumps the output voltage to a higher expected value than the input voltage through inductive energy storage, thus achieving the drive of the LED at a low input voltage. The advantage is that the output of such a driver IC can be used in parallel, effectively increasing the power of a single LED.

LED drive circuit using Buck-Boost converter. Similar to the Buek circuit, the source of S in this circuit can be directly grounded, which facilitates the drive of S. Although Boost and Buck-Boost converters have one more capacitor than Buck converters, they can both increase the absolute value of the output voltage. Therefore, they are more commonly used when the input voltage is low and multiple LEDs need to be driven.

Introduction to PWM dimming knowledge

In mobile phones and other consumer electronic products, 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 function 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 respective 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 widths and narrowness of digital pulses to simply change the output current, thereby adjusting 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 system of a mobile phone, a dedicated PWM interface can simply generate a pulse signal with any duty cycle, which is connected to the EN interface of the driver through a resistor. Most manufacturers' drivers support PWM dimming. However, PWM dimming has its disadvantages. Mainly reflected in: PWM dimming can easily cause the white light LED drive circuit to generate audible noise (or microphonic noise) that is audible to the human ear. 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 happens to fall between 200Hz and 20kHz, the inductor and output capacitor around the white light LED driver will generate audible noise. Therefore, it is necessary to avoid using low frequencies below 20kHz when designing.

We all know that a low-frequency switching signal acting on an ordinary wire winding coil will cause the coils in the inductor to vibrate mechanically with each other. The frequency of the mechanical vibration 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 lower resistor. 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 is beyond 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, so that the energy consumption of the white light LED is also reduced. 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.