Analysis of key design points of high-performance and low-power solar scale SoC

The threat of used batteries to the human living environment has become a social consensus. Reducing battery usage by reducing power consumption is becoming the choice of many electronic product companies. With the advancement of solar cell technology and the development of low-power semiconductor design technology, the application of solar cells to replace traditional batteries has gradually emerged. This article will introduce the design of a solar electronic scale based on the low-power CSU11 series scale SoC chip.

Key points for designing low power solar scale

For weighing instruments powered by traditional batteries, they are usually limited by size and cost. If solar panels are used, their area must be limited, which limits the power supply of solar panels. In addition, the power generation of solar panels is low (indoor μA level), and there is a problem of unstable current with the difference in light intensity. Therefore, the weighing instrument solution powered by solar panels needs to solve the following problems: add energy storage circuits to store the power generated by solar panels (at 200 lumens, it can generally only provide tens of μA current); reduce power consumption, and the shutdown and operating currents of the whole system must be at nA and μA levels respectively.

The main power consumption of electronic weighing instruments are sensors, ADC, and MCU. Therefore, the common solution needs to adopt the following methods to achieve overall low power consumption: use intermittent power supply or pulse power supply for sensors to reduce sensor power consumption; use high-resistance strain sensors; increase ADC output rate, reduce each sampling time and thus reduce sampling power consumption; increase ADC effective bits to ensure the accuracy of weighing instruments at high speeds; increase SoC operating speed to reduce power consumption per MHz; reduce LCD driver circuit power consumption; increase integration and reduce peripheral devices.

The high-precision 24-bit ADC chip CS1242 launched by Xinhai Technology fills the gap in the field of domestic mid-to-high-end electronic weighing instrument chips. While meeting the high-precision requirements, the company has launched the CSU11 series weighing instrument SoC chips with excellent low-power characteristics in response to the current development trend of low-power applications. The biggest advantage is that it simultaneously meets the low-power requirements of automatic weighing body scales and solar-powered body scales, as well as the high-precision requirements of pocket scales. This series of products takes into account the above-mentioned low-power design factors and achieves excellent low-power characteristics. The average standby power consumption of the automatic weighing body scale designed with the CSU11 series SoC can be as low as 3μA, the weighing current of the solar-powered body scale is less than 20μA, and the division of the pocket scale can reach one in thirty thousand.

CSU11 series SoC integrates 8-bit RISC MCU, 4K*16 OTP (can be used as user data PROM), 256 RAM, 4*18 LCD, and 16 I/O ports (except these 16 I/O ports, all "Seg" ports can be reused as output, making the total number of IO ports up to 26), dual-channel high-speed, high-precision ADC, with a maximum data output rate of 16kHz (precision of 12 bits, gain of 32), and a maximum precision of 18 bits (gain of 128, rate of 32Hz). In order to meet the requirements of low power consumption and high precision on one chip, CSU11 series SoC also provides a "power consumption-precision-speed" balance option, that is, users can achieve low power consumption by configuring a lower ADC operating current, but in this case the linearity is poor and can only be used for scales below 5000 points, or they can configure a larger operating current to achieve 30,000 divisions of accuracy and linearity, which provides convenience for solution engineers to develop various types of solutions. Only a slight change in configuration is needed to meet different market needs.

The emergence of solar body scales is mainly to avoid the harm of ordinary batteries to the environment and achieve the purpose of environmental protection. However, the solar body scales currently on the market require expensive sensors and PS08 main chips. The high cost of manufacturing has greatly limited the market share of solar body scales. The CSU11 series provides features specially designed for solar body scales. The design uses an AD output rate of 1KHz and the fourth AD sampling value. If weighing once per second, the dynamic power consumption required is 4 times * 2.5mA * (1/1000 second) = 10μA, and the power consumption required for LCD display is 5μA (driving circuit working current) + 5μA (glass consumption) = 20μA.

At present, a large domestic weighing instrument company has successfully developed solar electronic scale products using CSU1101B chips, and has achieved mass production and has successfully exported to the European and American markets. The average operating current of this product is less than or equal to 25μA, and the scale can be started under a light intensity of 55 lumens, which is completely suitable for various indoor environments including bathrooms. Its accuracy reaches 2,000 divisions, and it has an automatic power-on function, and the average shutdown current is less than 3μA.

The threat of used batteries to the human living environment has become a social consensus. Reducing battery usage by reducing power consumption is becoming the choice of many electronic product companies. With the advancement of solar cell technology and the development of low-power semiconductor design technology, the application of solar cells to replace traditional batteries has gradually emerged. This article will introduce the design of a solar electronic scale based on the low-power CSU11 series scale SoC chip.

Key points for designing low power solar scale

For weighing instruments powered by traditional batteries, they are usually limited by size and cost. If solar panels are used, their area must be limited, which limits the power supply of solar panels. In addition, the power generation of solar panels is low (indoor μA level), and there is a problem of unstable current with the difference in light intensity. Therefore, the weighing instrument solution powered by solar panels needs to solve the following problems: add energy storage circuits to store the power generated by solar panels (at 200 lumens, it can generally only provide tens of μA current); reduce power consumption, and the shutdown and operating currents of the whole system must be at nA and μA levels respectively.

The main power consumption of electronic weighing instruments are sensors, ADC, and MCU. Therefore, the common solution needs to adopt the following methods to achieve overall low power consumption: use intermittent power supply or pulse power supply for sensors to reduce sensor power consumption; use high-resistance strain sensors; increase ADC output rate, reduce each sampling time and thus reduce sampling power consumption; increase ADC effective bits to ensure the accuracy of weighing instruments at high speeds; increase SoC operating speed to reduce power consumption per MHz; reduce LCD driver circuit power consumption; increase integration and reduce peripheral devices.

The high-precision 24-bit ADC chip CS1242 launched by Xinhai Technology fills the gap in the field of domestic mid-to-high-end electronic weighing instrument chips. While meeting the high-precision requirements, the company has launched the CSU11 series weighing instrument SoC chips with excellent low-power characteristics in response to the current development trend of low-power applications. The biggest advantage is that it simultaneously meets the low-power requirements of automatic weighing body scales and solar-powered body scales, as well as the high-precision requirements of pocket scales. This series of products takes into account the above-mentioned low-power design factors and achieves excellent low-power characteristics. The average standby power consumption of the automatic weighing body scale designed with the CSU11 series SoC can be as low as 3μA, the weighing current of the solar-powered body scale is less than 20μA, and the division of the pocket scale can reach one in thirty thousand.

CSU11 series SoC integrates 8-bit RISC MCU, 4K*16 OTP (can be used as user data PROM), 256 RAM, 4*18 LCD, and 16 I/O ports (except these 16 I/O ports, all "Seg" ports can be reused as output, making the total number of IO ports up to 26), dual-channel high-speed, high-precision ADC, with a maximum data output rate of 16kHz (precision of 12 bits, gain of 32), and a maximum precision of 18 bits (gain of 128, rate of 32Hz). In order to meet the requirements of low power consumption and high precision on one chip, CSU11 series SoC also provides a "power consumption-precision-speed" balance option, that is, users can achieve low power consumption by configuring a lower ADC operating current, but in this case the linearity is poor and can only be used for scales below 5000 points, or they can configure a larger operating current to achieve 30,000 divisions of accuracy and linearity, which provides convenience for solution engineers to develop various types of solutions. Only a slight change in configuration is needed to meet different market needs.

The emergence of solar body scales is mainly to avoid the harm of ordinary batteries to the environment and achieve the purpose of environmental protection. However, the solar body scales currently on the market require expensive sensors and PS08 main chips. The high cost of manufacturing has greatly limited the market share of solar body scales. The CSU11 series provides features specially designed for solar body scales. The design uses an AD output rate of 1KHz and the fourth AD sampling value. If weighing once per second, the dynamic power consumption required is 4 times * 2.5mA * (1/1000 second) = 10μA, and the power consumption required for LCD display is 5μA (driving circuit working current) + 5μA (glass consumption) = 20μA.

At present, a large domestic weighing instrument company has successfully developed solar electronic scale products using CSU1101B chips, and has achieved mass production and has successfully exported to the European and American markets. The average operating current of this product is less than or equal to 25μA, and the scale can be started under a light intensity of 55 lumens, which is completely suitable for various indoor environments including bathrooms. Its accuracy reaches 2,000 divisions, and it has an automatic power-on function, and the average shutdown current is less than 3μA.

Design suggestions to improve system performance

At present, solar electronic scales are positioned in the high-end market and are mainly exported to Europe and the United States. In order to improve the performance of the whole machine and cope with the strict electromagnetic compatibility test and other related standards in Europe and the United States, the following suggestions are given for the design of products based on the CSU11 series SoC. This kind of design practice has practical guiding significance for other SoCs based on Chipsea Technology and general electronic product designs:

1. Improve product ESD resistance

The most important thing for the ESD performance of the product is the product shell design, which must ensure that the static energy is not introduced into the product PCB as much as possible. However, the buttons and battery compartments are sometimes inevitably affected by ESD events in the shell design. Therefore, the key to improving the ESD performance of the circuit is to take good ESD protection measures for the buttons and power supply.

For buttons that may be exposed to ESD events, an RC circuit needs to be added to guide the ESD energy to the ground. The general capacitance value is 0.1μF and the resistance value is 10K. It is best to cover the buttons with ground wires (use solid ground, not grid ground).

Near the power access point on the PCB (the connection welding point between the battery compartment and the PCB), a 105 capacitor (104 capacitor is also acceptable, but 105 capacitor can increase the upper limit of ESD resistance) must be connected immediately after the power line welding point. This capacitor is mainly used to discharge static electricity energy to the ground when the battery compartment cover is opened and ESD is applied. The ground here should be paved into a large area.

In addition to the above two measures, if you want to further improve the ESD performance, the ground on the PCB should be laid as large solid ground as possible, and do not use a grid-laying method. In addition, the connection between the grounds should be good, and scattered ground laying and thin wires between the grounds should be avoided. In addition, the ground connection design of the sensor is also conducive to improving the anti-ESD performance. In principle, the ground wire solder point of the sensor should be as close as possible to the power supply access solder point so that the electrostatic energy can reach the sensor without passing through the board winding, thereby reducing the impact on the chip.

2. Improve AD performance

For high-precision palm scales, especially those with a precision of more than 20,000 points, circuit and PCB design are crucial. Reasonable design can effectively improve the performance of the entire scale. The capacitor on the sensor should be as close to the chip pin as possible, and the wiring should be as equal and parallel as possible. Connect a 105 capacitor between AVDD and AGND, as close to the chip as possible. When developing a scale with a clock function or a scale that requires communication function, it is usually necessary to use an external crystal oscillator. The crystal oscillator should be as close to the chip as possible, with parallel wiring. The bottom of the crystal oscillator cannot be wired and covered with ground wires.

Conclusion

Xinhai Technology's product development idea of ​​integrating high-performance ADC and MCU provides a highly competitive solution for the weighing industry. At present, related ICs are widely used in more than a thousand domestic enterprises. The introduction of low-power solar electronic scale solutions conforms to the current global trend of low-carbon economic development. The CSU11 series chips introduced in this article recently won the 2010 Hot Product Award of "Electronic Engineering Times", which once again demonstrated the innovative product design ideas and high cost-effective features. In fact, in addition to solar weighing applications, the CSU11XX series is also widely used in industrial application fields such as industrial process control, liquid/other chemical analysis, portable products, smart converters, solar human scales, portable medical equipment and smart meters. In order to shorten the time to market for customers' products and reduce R&D costs, Xinhai Technology has provided application support to end customers or solution providers with "IC+system standard solutions" since the beginning of product launch, and provides a complete set of development tools and rich application software packages.