Battery management issues for portable products
With the increase in the complexity of portable devices, consumers need small, lightweight, user-friendly designs with a long battery life. This consumer preference has caused electrical design engineers to face a dilemma. This article introduces one of the portable products. A battery management technology.
For the convenience of users, more and more functions are integrated in the new generation of portable products, for example, the integration of wireless communication and data processing functions in mobile phones and PDAs. The new generation of mobile phones can not only access the Internet, handle mail, but also integrate functions such as taking pictures ; PDA manufacturers have integrated wireless communication, GPS and other functions in the newly launched PDA. The increase in functions will inevitably lead to an increase in system power consumption. In order to meet the requirements of lower power consumption and smaller size, battery management technology has become a crucial factor in the design of portable products. This paper analyzes the common problems in the design of battery-powered products.
Figure 1: The die layout of a single-chip hot-swap device.
1. Choose batteries reasonably
Whether the end user is satisfied with the design of their portable device depends largely on the performance of the battery. The key indicator of the battery is of course the service life of the battery. On the surface, this is just a simple value, but it involves many factors, including: system load (power supply time at full load current, power supply time in standby mode), Power efficiency, system power management, battery type and charging method, etc. The interaction between these characteristics will also affect the end user's feelings. In general, when the user starts to notice the battery, things become more tricky! A good product design does not require frequent battery replacement (such as a TV remote control) or frequent charging of the battery (electric toothbrush), it should make the battery "disappear" from the user's eyes, avoiding the user's attention to the function of the device Just pay attention to the battery.
The mutual restriction between the battery and the system is a problem that is often overlooked in the design, and it is very important to ensure that the capacity of the battery matches the requirements of the system. Commonly used battery types are: alkaline batteries, nickel metal hydride batteries and lithium ion batteries. They are not interchangeable, and there is an optimal choice for most products.
Alkaline batteries
Alkaline batteries are non-rechargeable batteries, but they have extremely low self-discharge rates and costs, and do not require a charger or AC outlet. For applications with low power consumption, alkaline batteries will be a good choice, but they must be used reasonably, and the quiescent current or sleep current must be very low. A common misunderstanding in the design is: only focus on work efficiency, and ignore the current loss in the "off" or "sleep" state, even consuming tens of microamps of battery current will cause frequent battery replacement.
rechargeable battery
When the load is too large for alkaline batteries, rechargeable batteries need to be selected, which has become the standard mode for portable products such as notebook computers, PDAs and cellular phones. Rechargeable batteries "disturb" users as little as possible, which will have a certain promotion effect on product sales, at least not reduce the performance of the product. Currently, there are two commonly used rechargeable batteries for portable products: nickel-metal hydride batteries or lithium-ion batteries.
The cost of nickel-metal hydride batteries is lower than that of lithium-ion batteries. When the product's conditions of use are not safe for the battery, this choice will become very sensitive. For low-cost products that lack a complicated charging design, since nickel-metal hydride batteries are suitable for fully charged and fully discharged applications, products that are used to completely drain electrical energy are more suitable, such as power tools. Another application suitable for nickel-metal hydride batteries is to directly replace alkaline batteries. This is because nickel-metal hydride batteries have the same voltage as alkaline batteries. When the battery is exhausted, the battery is removed from the device and then provided by an external charger battery charging. This application is more common in digital cameras, but requires frequent user intervention.
Many portable products are different from the above situation, for example: PDA, mobile phones need to be regularly charged, but they only occasionally consume power. Lithium-ion batteries are best used for these products. In addition to weight density, this battery also has two important advantages: low self-discharge rate and no limit to short-term charge-discharge. Consumers will not be aware of the "battery management" issue, simplifying product use.
Second, accurate detection of battery power
Electricity metering is useful for products that will not completely exhaust the electric power in each use, especially digital cameras, mobile phones, PDAs and other products. Digital cameras are not frequently used products. Therefore, it is easy for users to forget to charge the battery, and the power is very important when using it. Without a good fuel gauge, there is a danger that the digital camera will run out of power at any time, causing many regrets for users .
Figure 2. Monolithic battery fuel gauge provides higher cost performance
Most portable products do not leave sufficient funds budget for the electricity meter in the design. The designer mistakenly believes that consumers will not pay attention to this feature, but it is not. The increasing storage capacity of handheld systems means that applications and user files will be saved in volatile memory. If the battery is powered down without any precautions, the files generated or purchased by the user will be damaged or lost. Some systems use an auxiliary battery to power the memory when the main battery is disconnected. However, due to the extremely small auxiliary battery capacity, the time to protect the memory is very limited. Therefore, data-centric portable products must be used before the main battery is exhausted. Turn off in time to ensure that the main battery has enough remaining power to protect the contents of the memory. Ideally, the battery of a multi-function mobile phone or wireless PDA should leave 100mAh to 200mAh of remaining power when it is stopped. For example, assuming that an application requires a remaining capacity of 150mAh, Figure 1a) shows the correspondence between the battery discharge voltage and the remaining capacity at different temperatures. From the 20 ° C curve, it can be seen that in order to retain an appropriate remaining capacity, 3.5 should be selected V cut-off voltage. However, the 0 ℃ and 40 ℃ curves have different cut-off voltages under the same residual power. If the battery is cold (0 ° C), the 3.5V cut-off voltage will retain 400mAh of remaining power, and the actual power used for work is less than 600mAh; if the battery is hot (+ 40 ° C), the 3.5V cut-off voltage is used The remaining power is less than 100mAh.
The impact of load current changes is also significant. The curve in Figure 1b) shows the voltage change curves at three different discharge rates: C / 2, C / 5 and C / 10, where C is the charge capacity of the battery. The curve shows that when the 3.5V cut-off voltage is reached, the remaining power changes from C / 10 "100mAh to C / 2" 200mAh. If the cut-off voltage is increased to 3.6V to ensure that there is enough remaining power under C / 10 load, the remaining power at the three discharge rates will vary from 150mAh to 400mAh. Therefore, trying to increase the remaining power by increasing the cut-off voltage will lose a lot of battery energy. In addition, battery aging will also have a certain impact on battery discharge characteristics, as shown in Figure 1c). The aging effect varies from battery to battery, and there are large differences in products from different manufacturers.
Rough electricity metering often results in interruptions in calls, data exchange interruptions, loss of data files, and so on. Users have to start charging when the power indicator is half full. The smart battery monitor can display the estimated running time under the conditions of the above factors, and the user does not need to pay attention to the power consumption mode. Smart battery monitors usually do not query power based on voltage, temperature and current. It measures the charge flowing into and out of the battery, and uses the coulomb counter to track the amount of charge in the battery. By measuring the temperature and the discharge rate, the battery's ability to supply charge is compensated based on a small look-up table that preserves the battery's characteristic parameters. The DS2751 in Figure 2 is one such product. The DS2751 integrates a current-sense resistor with a standby current of 2μA. It communicates with the host through the 1-Wire interface and can provide all necessary measurements and data storage. The final result is calculated with the algorithm provided by the host. When the temperature is "+ 15 ° C, the maximum measurement error when discharged from a fully charged state is expected to be within 3%. The comprehensive measurement error can reach 5% under various temperature, load and aging conditions. If the interval between two full charges exceeds two weeks, the effect of input offset error will become significant. However, most users will fully charge the battery every week.
3. No more current after shutdown
Alkaline batteries are suitable for applications with low power supply currents, and must be used to ensure extremely low quiescent current or sleep current. A common power system design error is to focus only on the efficiency of the product while ignoring the current when it is "off" or "sleeping." Even wasting tens of microamps of current will cause a large amount of power loss, so that products used intermittently also need to frequently replace the battery. It is worth mentioning that this design error is now more common than a few years ago, because the design uses "soft switches" instead of mechanical switches, and mechanical switches can completely disconnect the battery. When entering the soft shutdown mode, the system still maintains an active state, but only enters the low current mode (hopefully). At this time, the system loss will be significantly reduced, because the CMOS logic circuit is static (no clock), and the current loss is basically zero. However, in actual design, it must also be noted that components such as pull-up resistors continue to absorb battery current, and non-working system units are not powered off and consume a certain amount of battery energy. Generally, there is no reason to let the shutdown current of such a system exceed a few microamperes. Even a system powered by one or two AA batteries requires a boost DC-DC converter to provide the power supply voltage of the logic circuit. If you use the MAX1722 boost converter, it will consume less than 2μA of current. The self-discharge current of AA batteries is still low.
4. Avoid using OR logic diodes
In designs that require battery switching, a diode with a 10mV forward voltage drop and no reverse leakage current is a "luxury" for designers. But so far, Schottky diode is still the best choice, its forward voltage drop is between 300mV to 500mV. However, for some battery switching circuits, even the selection of Schottky diodes cannot meet the design requirements. For a high-efficiency voltage converter, the energy saved may be completely wasted by the forward voltage drop of the diode. In order to effectively save battery energy in low-voltage systems, power MOSFET switches should be selected instead of diodes. The MOSFET with SOT package and on-resistance of only tens of milliohms can ignore the on-voltage drop at the current level of portable products.
To determine whether a system must use MOSFETs to switch power supplies, it is best to compare the diode conduction voltage drop, MOSFET conduction voltage drop, and battery voltage, and consider the ratio of the voltage drop to the battery voltage as a loss of efficiency. For example, if a Schottky diode with a forward voltage drop of 350mV is used to switch Li + batteries (nominal value 3.6V), the loss is 9.7%. If it is used to switch two AA batteries (nominal value 2.7V), The loss was 13%. In low-cost designs, these losses may be acceptable. However, when a high-efficiency DC-DC is used, it is necessary to balance the cost of DC-DC with the cost of efficiency improvement brought by upgrading the diode to MOSFET.
In many cases, by rationally using system resources and improving system design, it can also avoid the use of diodes and MOSFET power selection switches. As shown in Figure 3, MOSFET switches are not used in the figure. The system power supply can be connected to a AA battery and USB. Automatically switch between inputs, when the USB power is plugged in, the system will not take power from the AA battery. When plugged into a USB power supply, the MAX8511 LDO boosts the output of the MAX1675 from 3V to 3.3V. It is not necessary to drive the shutdown (/ SHDN) pin of the MAX1675 boost converter to ground to prevent AA batteries
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