With the rapid development of electronic products, various electronic products are becoming more and more abundant, especially portable electronic products, MP3, MP4, mobile phones, portable DVDs, portable digital TVs, portable car navigation systems GPS, portable notebook computers And so on, and their functions are getting more powerful, more abundant, and more compact in size, so the development trend of portable electronic products will definitely be smaller, lighter and more powerful. So how to ensure the battery discharge time of products with rich and powerful functions without increasing or even reducing the size of the product? The volume of the product does not increase or decrease, which means that the volume and capacity of the built-in battery of the product do not increase or decrease. In other words, how to ensure the same when the capacity of the built-in battery does not increase or decrease. Single discharge time of function or more rich and powerful products? Then there is only one answer: a design with lower power consumption. For example, the portable DVDs of popular electronic products in the market, with the increasing functional requirements of MP3, MP4, Divx, USB, Card Reader, digital TV modules, etc., how to ensure that the products are increasing these at the same or even lower battery capacity. Does the discharge time after the function decrease? This puts higher demands on our designers from the perspective of power consumption, and will become more challenging. Of course, saving is creating. How can we use the least energy to maximize the performance of the whole product, which is the meaning of low-power design. Take the portable DVD product as an example, to talk about how to optimize the power consumption circuit design in a specific design to achieve low power consumption. Let me first talk about the concept of power consumption. Power consumption is an indicator that all electrical equipment has. It refers to the amount of energy consumed in a unit of time. The unit is W. At present, the portable DVD system mainly includes the following four parts: TFT liquid crystal display, motherboard, movement and battery. The other three components except the motherboard can be selected to use brand components with lower power consumption by comparison; but the motherboard The power consumption can only be reduced through specific design. The main board has the following four modules: the main chip and its peripheral circuits, the power management part (including the power management part of the TFT), the audio and video part, and the movement motor drive part. Chips and peripheral circuit modules and movement motor drive modules can be selected by comparing the devices with lower power consumption in the low-power design, so the low-power design of the power management part and the audio power amplifier part is the most critical and important, of course the product The system software inside also plays an important role in reducing the power consumption of the whole machine. Next, I will elaborate and analyze the design optimization from the power management part and the audio part. First, for the power management part, this requires a voltage conversion device that converts the external voltage to the operating voltage required by each chip of the system. However, the conversion efficiency of the power supply cannot reach 100%, and there must be power loss during the conversion process. Then our task is to select devices with relatively high voltage conversion efficiency through specific design analysis to minimize power loss. We can use low-dropout linear regulators (LDOs), charge pumps and inductance-based DC / DC converters to convert external voltages into different operating voltages required by the system. Low dropout linear regulators (LDOs) can only convert the input voltage to a lower output voltage. In practical applications, its power consumption is P = (Vin-VOUT) * IOUT. When the input and output voltages are very different, and the output current is also large, the power consumed by the LDO itself is very large and generates corresponding heat. Therefore, the efficiency of the LDO is relatively low when the voltage difference is large and the output current is large. The LDO is particularly suitable for low current, small voltage difference, or high power noise requirements; the charge pump uses capacitors to achieve Energy conversion can realize the conversion of back pressure, voltage multiplication and voltage stabilization, with an efficiency of about 80%. Due to the capacity and size of the capacitor, the output current and voltage of the charge pump are limited. DC / DC converters use low-impedance switches (such as MOSFETs) and energy storage components such as inductors to achieve conversions such as buck and boost. The DC / DC converter reduces the power loss during the voltage conversion, and the efficiency is as high as 90%. At the same time, the switching frequency is very high, reducing the size of the external inductance and capacitance. Through the comparative analysis of the above three voltage conversion devices, the DC / DC converter has higher efficiency and can achieve larger current output, so it is the first choice for power conversion of our portable products. Therefore, DC / DC is widely used in the design of portable electronic products. When choosing DC / DC, the design of each company is also different in efficiency. This requires us to design. When choosing these devices, we must pay attention to the level of efficiency. It is necessary for us to test it ourselves. Are there advantages in terms of efficiency on our work platform? As mentioned above, LDO is particularly suitable for low current, small voltage drop, or high power noise requirements, there is a clear price advantage, if you use a larger output current DC / DC, right Our production cost is also a waste. This also makes us to choose it when designing when the pressure difference and current are not large, such as the 1.8V used in our system, which requires relatively high ripple and low current. It is realized by stepping down from 3.3V, and the pressure difference is not large. Here, an LDO is used. Another is to choose the main chip with lower working voltage as much as possible. Generally speaking, the power consumption of the chip with low working voltage will be correspondingly reduced, so choosing the main chip working with low voltage also helps to reduce the power consumption. The low-power design is not just to save power, the more benefit is to reduce the heat generated due to the loss of power consumption, as the temperature of the equipment decreases, the life of the device is correspondingly extended (each semiconductor operating temperature increases by 10 Degree, the life is shortened by half), and at the same time, the reduction of current also reduces the interference of electromagnetic radiation and thermal noise. Secondly, for the audio part, it is mainly audio amplifier (Audio Amplifier). The existing power amplifiers are Class A, Class B, Class AB and Class D, among which Class D amplifiers have the highest efficiency and Class A and Class B And class AB linear amplifier output stage needs to provide a large bias current. The lowest efficiency of Class A amplifiers is generally not used. Class D amplifiers do not require bias current, so they have high efficiency and can greatly extend battery life. Therefore, Class D amplifiers should be the best choice among portable electronic products. However, because of the lack of cost in class D amplifiers, most portable electronic products still use class AB amplifiers. The following highlights the efficiency advantages of Class AB and Class D amplifiers compared to Class D amplifiers. Typical Class AB amplifiers can only reach 50-70% efficiency, while typical Class D amplifiers have an efficiency of up to 85%, especially in terms of low power output. Class D amplifiers can store more than Class AB amplifiers. energy of. Even at full power or low power output, efficiency is more prominent. The basic theory of class D amplifier (as shown in Figure 1) is that a given small analog signal is used as the input of the power amplifier. The internal modulator of the power amplifier converts the analog to a digital signal (so in a sense it is a digital attack and discharge), such as pulse width modulation (PWM), but it is still a weak digital signal. Then, the bridge amplifier amplifies the amplitude of the digital signal. The high-amplitude digital signal is then converted back to an analog output. In this case, the amplifier can work completely in the switching state, and the efficiency of the transistor in the switching state is very high, because when the transistor is fully turned on, the current of the transistor is large, but the voltage is very small; The voltage of the transistor is high, but the current flowing through it is very small. At the same time, when there is no audio signal, the transistor is fully operated in the cut-off state, so that the power loss can be reduced as much as possible to make it work in an efficient state. If the class D amplifier is used here, the efficiency can be doubled compared with the class AB. This type of amplifier is being used more and more in portable products. It has been widely adopted in portable DVDs. Figure 1 Class D amplifier theoretical diagram Also mentioned earlier is that software also plays a vital role in improving the efficiency of the whole machine, because the software controls the I / O port to achieve effective control of the power supply, set the I / O, so that it only It consumes power when working, for example, for the power management part of our portable DVD TFT, when turning off the screen, it is best to turn off all the voltage to the screen. We now have the practice of only turning the background generated by the high-voltage board, etc. The high voltage is turned off, but the positive and negative pressure generated by FAN5331 and the voltage required by other screen modules are still working, which will cause unnecessary power loss. Now we can directly control the start end (EN) of FAN5331 through the I / O port, so that FAN5331 is directly turned off when the screen is turned off, so that the entire screen module is in the off state, so that the power consumption reaches optimize. Other points to note are: In our system, there are many signals that need to be pulled up or pulled down. Of course, there are many reasons why the signals need to be pulled up or down, but not all of them must be pulled. The pull-up and pull-down resistors pull a simple input signal, and the current is below tens of microamps, but when a pulled signal is driven, the current will reach the level of milliamps. Some people think that by adding up and down, the working status of these signals will be more stable, but this makes the power consumption of the whole machine also increase. Therefore, it is necessary to figure out which signals must be pulled up when designing, and try not to use them if you can not pull them up. Based on the above, for our designers, we must have the ability to achieve higher circuit performance under limited power specifications, in addition to meeting basic system performance specifications and cost target requirements. We need to carefully analyze the power of all components in the system to make a reasonable selection of the devices and chips involved in the hardware circuit design and provide the best design solution.
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