Design experience sharing of the hottest portable

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Design experience sharing of portable instruments

1 low power design

portable instruments generally adopt battery power supply. Of course, users do not want to charge or replace the battery frequently, so the length of standby time is often an important factor for users to consider. This requires designers to take various methods to reduce power consumption

1.1 choose low-power components

with the development of integrated circuit technology, the power supply voltage of integrated circuits has shown a downward trend. CMOS technology is widely used in operational amplifiers, a/d converters and various digital devices. The working current of micro power IC has been reduced to several μ A ~ dozens μ A. A voltage comparator max918 with a reference voltage source, whose working current is only 0.8 μ A. This significantly reduces power consumption. Based on his own design experience, the author puts forward the following suggestions:

because low power supply voltage helps to reduce power consumption, in recent years, 3.3V low-voltage CMOS devices have been widely used in design, and 2.5V power supply chips have also appeared in newer portable instruments. In the future, the power supply voltage of the chip will even continue to drop to 0.9V

mcu (microcontroller) and MPU (microprocessor) are often the components that consume the most power in the system. Try to choose RISC chips, because the low-power records of chips are mostly created by RISC chips

single power supply can improve the efficiency of power supply. Try to adopt single power supply chips in the design, especially operational amplifiers

LCD (liquid crystal) display can be used as the display element. It is not necessary to close some circuits according to different working states, especially for high current devices. In the early days, the power IC was mainly used to control the shutdown function, but now it has gradually developed to operational amplifiers, comparators, a/d converters and other devices. In the off state, the IC does not work, and the power consumption is between a few microamps and a few microamps. When the circuit inevitably uses high current devices, such as infrared transmitters, wireless communication transmitters, etc., it should be designed to make the circuit unit with high current work only in a short time when it is required to work, and keep it in a power-off state for the rest of the time. The working response time of the circuit should be considered when designing this kind of circuit

reduce the clock frequency of the system. The power consumption of digital chips is related to the clock frequency. After weighing the running speed, using a lower clock frequency can reduce the current consumption. Taking PIC16C71 low-power MCU as an example, when the power supply voltage is 5V and the clock frequency is 4 MHz, the power consumption is about 10 MW; Under the same supply voltage, when the clock frequency is reduced to 32 kHz, the power consumption is about 0.15 MW. Power consumption is significantly reduced

The design of power supply system is an important aspect of low power design. When a system is powered by batteries, designers must consider factors such as maximum current consumption, operating voltage range, size and weight constraints, operating temperature range, and operating frequency. The working voltage of various types of batteries is different from each other. The lithium battery is 3.0V, while the nickel cadmium battery can provide a current of up to 30A. When choosing batteries, designers must consider all the characteristics of each type of battery. High efficiency and small size chips should be considered for power chips

function and power consumption should be evaluated at the design stage. Generally speaking, more functions must mean larger hardware scale and greater power consumption, and some optional functions should be reduced as much as possible

1.3 optimize software design and make full use of sleep mode

there are MCU in most portable instruments. The best way for MCU to save internal power consumption is to enter sleep state. In the sleep state, the oscillator of MCU is turned off, which can make it consume very little current, the typical value is a few microamps. The MCU can be awakened from sleep by using the monitoring timer or external interrupt. For example, the dynamic electrocardiograph, because the human heartbeat is very slow relative to the MCU clock, you can use the timer to interrupt, regularly wake up the MCU, and enter sleep again after processing, which can greatly reduce power consumption

2 anti interference design

the human body is in a space full of electromagnetic fields, just like an antenna receiver. The human body induces voltages of various frequencies, which is likely to interfere with portable instruments. Moreover, portable instruments may work in various environments, especially when some industrial instruments face industrial sites with harsh electromagnetic environment, and then the external interference is even greater

there are three basic elements of interference: interference source, propagation path and interference coupling device. 1. When clamping the jaw, the interference source is the component, equipment or signal that produces interference, such as lightning, motor, high-frequency clock, etc. The propagation path refers to the path and medium from the interference source to the interference coupling device. Interference coupler refers to the object being interfered, and each IC and sensor may be interfered. For portable instruments, external interference sources are not selectable because of the uncertainty of their location, so we can only make an issue from reducing internal interference and eliminating the propagation path of interference

several anti-interference principles in general circuit design should still be followed. Such as reasonable distribution of components, block layout of strong and weak signals, digital and analog signals; Try to avoid 90 ° broken lines. If the router supports circular arc lines, try to use circular arc lines; Digital and analog are separated, and finally this part of consumer demand is rigid in power supply; Isolate the digital area and analog area with ground wire; When wiring, try to reduce the area of the loop, and the power line and ground wire should be as thick as possible. It is best to adopt multi-layer board design, one layer of power supply and one layer of ground, so as to reduce the coupling of noise; For power low-frequency filtering, each IC power input terminal on the circuit board is connected in parallel with a 0.01 μ F~0.1 μ F high frequency filter capacitor; For idle pins of chips, try not to hang in the air; It is better to use power monitoring and watchdog circuit for single chip microcomputer system; High frequency devices shall be placed on the edge of the circuit board as far as possible; Reduce the clock frequency as much as possible, etc

however, some traditional anti-interference measures cannot be applied to portable instruments. For example, the most effective way to deal with high-frequency radiation interference is to add a shield to the shell. On the one hand, this measure increases the volume and weight, on the other hand, it is not applicable to some instruments that need to contact with the outside world. The alternative method is to spray conductive material on the shell. For another example, if the input signal cannot be fully filtered due to the limitation of space on the board, which means that the analog signal must be filtered by software

most connecting elements are connected with cables, which acts as an unwanted antenna for EMI (electromagnetic interference). Therefore, the connecting elements should be kept as far away from high-frequency signal sources (such as clock signals) as possible in the design. Similarly, circuits that are susceptible to interference, such as reset or interruption, should also be kept away from high-frequency signal sources as far as possible, and capacitive filtering should be increased. When there is not enough space on the board, it is better to fix the connecting element on the shell

3esd protection

strictly speaking, ESD protection is also a part of anti-interference design, but in view of the fact that ESD protection has not attracted enough attention of designers for a long time, and the possibility of portable devices being damaged by electrostatic discharge (ESD) is very large, it is now described separately. The contact and separation of two non-conductive materials will cause the transfer of electrons, thus generating additional charges on each object. When the accumulated static charge discharges to another object with low potential (relatively to the ground), the magnitude and duration of the discharge depend on various factors such as the type of charging material and the surrounding environment. All portable electronic devices, from the most basic to the most complex, are vulnerable to ESD damage

a trusted operator may carry harmful charges even in normal equipment operation. In the past, ESD protection was often added to circuit design as needed. However, due to the increasing complexity of semiconductors, the impact of submicron technology and very small linewidth on transient overvoltage is also becoming more and more obvious. Some of the most sensitive components may be damaged by ESD voltage as low as 20V. The traditional protection methods used in the past, such as spark arresters, zener diodes, RC networks and clamping diodes, are no longer applicable, because they will produce safety illusion and even interfere with the normal operation of the circuit. Moreover, the en and other industrial standards of the European community set strict anti ESD requirements for all electronic products. Obviously, a good system design should consider the threat of ESD to portable devices at the initial stage of circuit design

there are several transient protection devices to choose from, the most commonly used ones are transient voltage suppressor (TVS) diodes. However, you should still be very careful when choosing. If the device is selected improperly, it will not work and will interfere with the normal operation of the circuit. In order to suppress the transient process of portable system, the protection device must have the following characteristics:

extremely fast response time

low clamping voltage and operating voltage

able to handle peak ESD current

can withstand the repeated action of ESD without damage

small size

the reverse leakage current is small

pcb wiring is an important part of anti transient design, especially portable system design. The parasitic inductance in the protection path will produce voltage spikes, which may exceed the damage threshold of the protected IC. This is particularly serious in fast transients such as ESD or EFT (Electrical fast transient burst). The voltage on the inductive load is proportional to the time change rate of the current. According to IEC standard, the transient generated by ESD can reach the peak value within 1 ns. Assuming that the inductance per inch of wiring is 20 NH and the wiring length is 0.25 inch, the voltage spike will be a pulse with a voltage of 50 V and a current of 10 A. All inductive paths must be considered, including the ground circuit, TVs (path between the protected line) and the path from the connector to the TVS device. TVs devices should be as close to the connector as possible to reduce transient coupling to nearby wiring. Radiation will also affect other areas of the circuit board, even if there is no direct path with the connector. Finally, avoid arranging important signal lines at the edge of the circuit board or near the protected wiring

in the design of the housing, there should be a long enough discharge distance between the opening of the housing and the internal circuit to keep the sensitive part of the circuit away from the opening as far as possible. For products with plastic shells, the PCB should also avoid contact with the shell near the opening, because ESD can make the arc connect to the PCB along the plastic surface

4 reduce the volume size

the above are all focused on performance considerations. For portable instruments, volume and weight are also areas of great concern to users. In order to reduce the volume, we need to consider the following aspects

try to use patch elements. At present, more than 90% of electronic products produced abroad use SMD and SMT for assembly, while 100% of portable electronic products use SMD components. Electronic products composed of chip integrated circuits can be mounted on both sides, which is not only small in size, but also has better high-frequency performance

select the IC with integrated functions. It is difficult to further reduce the package size of IC, but you can choose to make several related integrated circuits on the same silicon chip. For example, PIC microcontroller of microchip company integrates MPU, a/d converter, pulse width modulation and other functions, which can form an independent single-chip system. Digital programmable devices, such as CPLD and FPGA, turn on a large number of previous gates

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