Design skills and analysis of low-power circuits for portable electronic products


Design skills and analysis of low-power circuits for portable electronic products

 With the rapid development of electronic products, a variety of 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 laptops. And so on, and their functions are getting stronger, more abundant, and more compact, so the development trend of portable electronic products will be smaller, lighter and more powerful.

So how to ensure the battery discharge time of a product with rich and powerful functions without increasing or even reducing the product volume? The volume of the product does not increase or decrease, which means that the volume and battery capacity of the built-in battery are not increased or even reduced. In other words, how to ensure the same when the built-in battery capacity is not increased or decreased. Is the single discharge life of a feature or a more powerful product? Then there is only one answer: a lower power design.

For example, the popular fashion of portable DVDs in the market, with the increasing demand for MP3, MP4, Divx, USB, Card Reader, digital TV modules, etc., how to ensure that products are increasing at the same or even lower battery capacity. Does the discharge time after the function not decrease? This puts higher demands on our designers from a power consumption point of view and will become more challenging. Of course, saving is creation, how to use the least amount of energy to maximize the performance of the whole product, which is the meaning of low-power design.

Let's take a portable DVD product as an example to talk about how to optimize the power circuit design in a specific design to achieve low power consumption. Let's first talk about the concept of power consumption. Power consumption is an indicator of all electrical equipment. It refers to the amount of energy consumed per unit of time, in W.

Now in the portable DVD system, the following four parts are mainly included: TFT liquid crystal display, main board, movement and battery. The other three parts except the main board can be selected to use brand parts with lower power consumption by comparison; The power consumption can only be reduced by the specific design. The motherboard 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 part of the movement. The chip and peripheral circuit module and the movement motor drive module can select the device with lower power consumption by comparing 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. Below I will first 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 into the required operating voltage of each chip of the system. However, the conversion efficiency of the power supply cannot reach 100%. There must be power loss during the conversion process. Then our task is to select the device with higher voltage conversion efficiency to reduce the power loss as much as possible through specific design analysis.

We can use low dropout linear regulators (LDOs), charge pumps and inductor-based DC/DC converters to convert external voltages into the different operating voltages required by the system. A low dropout linear regulator (LDO) 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 differ greatly, and the output current is also large, the LDO itself consumes a lot of power and generates corresponding heat. Therefore, the efficiency of LDO is relatively low when the output voltage is large and the output current is relatively low. LDO is especially suitable for low current, small voltage difference, or high requirements for power supply noise; charge pump uses capacitor to achieve The energy conversion can realize the transformation of back pressure, double voltage and voltage regulation, and the efficiency is about 80%. Due to capacitor capacity and size limitations, the charge pump output current and voltage are limited. The DC/DC converter uses low-impedance switches (such as MOSFETs) and energy storage components such as inductors to convert between buck and boost. The DC/DC converter reduces the power loss during voltage conversion with an efficiency of over 90%. At the same time, the switching frequency is high, reducing the size of external inductors and capacitors.

By comparing and analyzing 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 efficiency varies according to the design of each company. This requires us to design. When choosing these devices, we must pay attention to the efficiency. It is necessary for us to test it ourselves. There is no advantage in this aspect of efficiency on our work platform. As mentioned above, LDO is especially suitable for low current, low voltage difference, or high power supply noise requirements, and it has a more obvious price advantage. If the output current is large, DC/DC is used. Our production costs are also a waste. This also makes us consider designing it when the voltage difference and current are not large. For example, the 1.8V used in our system has higher requirements on ripple and less current. From 3.3V through the step-down to achieve, the pressure difference is not large, here using an LDO to achieve. Another is to try to choose the main chip with lower working voltage. Generally speaking, the power consumption of the chip with low working voltage will be reduced accordingly. Therefore, the main chip that works with low voltage also contributes to the reduction of power consumption.

Low-power design is not just to save power. The more important thing is to reduce the heat generated by the loss of power consumption. As the temperature of the device decreases, the device lifetime is extended (the operating temperature of the semiconductor device is increased by 10). Degree, the life is shortened by half), while the reduction of current also reduces the interference of electromagnetic radiation and thermal noise.

Secondly, for the audio part, it is mainly Audio Amplifier. The existing power amplifiers are Class A, Class B, Class AB and Class D. Among them, Class D amplifiers have the highest efficiency, Class A and Class B. Both the class AB and the linear amplifier output stage need to provide no small bias current. Among them, Class A amplifiers have the lowest efficiency and are generally not used. Class D amplifiers do not require a bias current, so they have higher efficiency and can greatly extend battery life. So Class D amplifiers should be the best choice for portable electronics. However, because of the cost advantages of Class D amplifiers, most of the 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. A typical Class AB amplifier can only achieve 50-70% efficiency, compared to a typical Class D amplifier, which can achieve up to 85% efficiency. Especially for low power output, Class D amplifiers can store more than Class AB amplifiers. energy of. Even with full power or low power output, the efficiency is more prominent. The basic class D amplifier theory (Figure 1) is the input of a given small analog signal as a power amplifier. The internal amplifier of the power amplifier converts the analog to a digital signal (so in a sense it is a digital attack and release), such as pulse width modulation (PWM), but it is still a weak digital signal. The bridge amplifier then amplifies the amplitude of the digital signal. The high amplitude digital signal is then converted back to the analog output. In this case, the amplifier can be fully operated in the switching state. In the switching state, the efficiency of the transistor is very high, because the transistor has a large current when fully turned on, but the voltage is small; The voltage of the transistor is high, but the current flowing through it is small. At the same time, when there is no audio signal, the transistor is fully operated in the off state, so that the power loss can be reduced as much as possible, so that it works in an efficient state. If you use a Class D amplifier here, you can double the efficiency of the Class AB. These types of amplifiers are increasingly being used in portable products. It has been widely adopted in portable DVDs. Figure D class amplifier theoretical diagram

There is also mentioned that the software plays a vital role in improving the efficiency of the whole machine. Because the control of the I/O port is controlled by software to achieve effective control of the power supply, the I/O is set so that it only Power consumption during work, 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 voltages of the screen, and now we have only the background generated by the high voltage board. The high voltage is turned off, but the positive and negative voltage generated by the FAN5331 and the voltage required by the modules of other screens are still working, which causes unnecessary power loss. Now we can directly control the initial energy terminal (EN) of the FAN5331 through the I/O port, so that the FAN5331 can be directly turned off when the screen is closed, so that the modules of the entire screen are in the off state, so that the power consumed is reached. optimize.

The other thing to note is that there are many signals in our system that need to be pulled up or pulled down. Of course, there are many reasons why the signal needs to be pulled up and down, but not all. The pull-down resistor pulls a simple input signal, and the current is tens of microamps or less, but pulling a driven signal will bring the current to milliamperes. Some people think that with the addition of the pull-down, the working state of these signals will be more stable, but this will increase the power consumption of the whole machine. So when designing, you need to figure out which signals must be pulled up. If you can not pull them up, try not to use them.

Based on the above, it is necessary for our designers to have the ability to achieve higher circuit performance with limited power specifications, as well as to meet basic system performance requirements and cost targets. We need to carefully analyze the power of all components in the system to make reasonable choices of the devices and chips involved in the hardware circuit design to provide the best design.

Release Date:2019-08-25 13:15
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