Portable device battery management strategy

A1: From an engineering perspective, I suggest you add a chip that can use a single I/O communication device to implement battery voltage measurement, such as Maxim's 1-wire battery management device DS2762 or ADC chip DS2450.

As a technical discussion, if this digital I/O is the only resource, the system has a timer available, and it is planned to spend some effort on the software, it is still possible to roughly measure the battery voltage. The specific approach is related to the logic level of the I/O output that can be supplied. Both the output and input states are used. This method requires the output 0 level and the power supply voltage as the voltage standard value, and uses the input discrimination threshold Vt for voltage detection.

The measurement first sets the I/O to the output logic 0 (Vz), and the voltage on one capacitor C is lowered by a resistor R1 to an initial value lower than the logic high threshold when the I/O is taken as an input. This capacitor is passed through R2 with the battery to be measured for voltage E. After the balance, the voltage VC on the capacitor, that is, the voltage difference between Vz and E, is divided by the voltage divider of the R1 and R2 resistor strings. In addition to connecting this capacitor through R1, this I/O needs to be pulled up to the supply voltage through a resistor RV. Then change the I/O from the output to the input and start timing. Due to the pull-up of RV, the voltage across the capacitor will gradually increase. Stop timing when the voltage seen by the I/O input rises above a high logic level. The duration T counted by the timer during this process is related to Vz, E and Vt (but not linear). As a logic device, Vz and Vt are not very stable, so this method can only get a very rough voltage value.

Q2: How to choose between self-recovery fuse and disposable fuse? Compared with disposable fuse, self-recovery fuse has poor reliability, long response time and low rated current; why is the current power supply design more self-recovery? Fuse? Is it recommended for industry standards or national standards?

A2: First of all, not all disposable fuses are fast, for example, the sand tube fuse is very slow. If I choose a fuse for the power supply, as long as the power is not very large, I will choose from the insurance. The main reason is that it will not cause a return or repair due to accidental damage; if the self-recovery insurance is burnt out, the customer is probably too embarrassed to find me. The second is that the self-resetting fuse has a long continuous operation life, and there is basically no aging that affects practical applications. For medium and small power supplies, basically, the circuit components have sufficient margin and internal current limiting mechanism, and there is no problem waiting for the self-recovery safety circuit. I am afraid that it will not work with higher power. If you do not consider it and short-circuit the surge, you may burn a lot of parts. In addition, from the mechanism point of view, if the current is too large, the process of contact separation is likely to burn out the self-recovery insurance.

I am not sure if the reliability you are talking about also includes problems such as inaccurate currents, incomplete current interruptions, or just high failure rates. I estimate that the first two items are limited by their working mechanism and should not be able to catch up with the fuse. The problem of high failure rate should not be solved. If it is related to heat, at least it can be improved by selecting a larger operating current. I have not seen the standard for self-recovery insurance, but some standards stipulate the action when overcurrent occurs or not. If you need to have a self-recovery function, it is definitely cheaper to choose from the complex insurance than the self-recovery circuit.

Q3: Battery protection circuit, how to test the battery protection board? What equipment is needed?

A3: The test needs to be designed in two directions according to the quality assurance of the production process and the design characteristics; one focuses on the productivity efficiency and key parameters, and the other one is practical and comprehensive. Generally, more attention should be paid to production test equipment; if your company plans to mass produce, it is best to make a number of special equipments to meet the general standard equipment (DC power supply, digital panel meter) to complete the production test efficiently. A simple scan of the voltage and current and a short-time hold and display of the trigger voltage and current can be performed by the self-made device (here, two voltage thresholds, one or two current thresholds and withstand voltage capability are considered).

Q4: About the early warning of multiple strings of batteries. I am doing a lithium battery protection circuit. I would like to ask if there is any IC that protects the multi-string lithium battery and alarms when the capacity is low. Because the lithium battery protection IC is mostly designed for lithium battery protection, it is not considered. To the problem of early warning when the user uses it, I want to find an IC that can raise an alarm before the single voltage protection.

A4: It is safe to say that there is no such chip at present. It is usually "systems" that are currently available for similar functions, not chips. The reason for this situation is related to the "noble" history of lithium-ion battery cells on the one hand, and the complexity of battery residual power estimation on the other hand. The discharge capacity of lithium battery is closely related to the short-term historical performance of discharge conditions including temperature and discharge current. It is necessary to continuously correct the residual power to estimate the residual power. Such a system can be seen in both notebooks and high-end PDAs. The basic method is to use a fuel gauge with parameter correction methods, such as OCV correction method.

Q5: What are the boost chips for two dry batteries? I am now working on a system that uses two dry batteries to power 3V, but other parts need 5v, 12v power, the current can be small, can provide some common What about the chip?

A5: The condition of generating 3V with 2 dry batteries is special; when the new battery starts to use, it needs to be stepped down for a while, and it will be boosted most of the time. Selecting a power supply combination also requires more parameters such as output voltage stability requirements, efficiency (ie, what operating time is expected under load conditions), and size limitations. In terms of cost and efficiency, the MAX711 is an option for 3V output power (from 1.8V). The other two sets of power supply options are large, such as MAX1677, MAX1817, etc. If the output current required by 5V or 12V is small, or if the stability of a certain group of outputs is not high, other combined solutions can be considered.

Q6: Design consultation for ultra-low power LDO circuits powered by alkaline batteries. We are designing a product that requires four alkaline batteries to power a small motor and an LDO step-down for the chip. The battery voltage is about 6V, the IC voltage is 3. 3V, and now I choose a GM6250 LDO, his standby current is 1uA, then what kind of capacitor should we choose to ensure that the small motor will not cause LDO output when it starts? Large fluctuations, as well as ensuring minimal leakage during pure sleep standby. We hope that the leakage of the capacitor will be about 1uA. The small motor will start to work at 1A, and then quickly fall to about 200ma.

A6: Capacitor leakage is not a problem. Almost any general capacitor will not have too much leakage. However, the LDO will be unstable after the capacitor is large. By adding a capacitor can not solve the problem of voltage drop, only a huge capacity may be backed up by 1A of the stall current. Generally, the mountain does not turn water. If the motor is not moving, the voltage of other parts that are sensitive to voltage changes but not large in current will not be affected by the voltage drop during the stalling period, and the voltage of the motor part will drop. time.

Q7: How do two sets of dc to dc converters be applied to positive and negative voltage outputs (same input power supply)?

A7: If two separate chips are selected to implement the two power supplies, the optional range of positive power supplies is much larger and can be considered to be quite conventional. If the space is not very large, if the two power sources cannot be synchronized, the fluctuations of the total amount of the current drawn from the previous stage are related to the difference frequency between the frequencies of the two power sources, and a significant difference can be seen. Shoot" ups and downs. If the beat falls within the range of one hundred to several tens of thousands of weeks, it is difficult to digest it by the front stage power supply and the storage capacitor. At least one of the two power supplies should be synchronizable and need to track the frequency of the other power supply. The two power supplies should also be misaligned so that they do not draw current from the power supply at the same time to reduce the need for source filtering. This involves another requirement: from the point of view of system power-up and security, which power supply needs to be guaranteed (main-slave)?

Indeed, the Fly-back architecture is relatively simple in the transformer mode. But not using the Fly-back architecture is also related to other conditions. Last time I asked a question: Is the 28V stable? It seems that the two power supplies are like subordinate power supplies in a system. It is possible that the 28V is guaranteed. If the 28V is guaranteed, the easiest way is to make a transformer driver, use the transformer to generate an unregulated DC slightly larger than +/-15V, and then use a linear regulator to produce a stable +/-15V. For a 15V output, the 2-3V voltage drop has little effect on efficiency.

To be sure, the positive power supply will not have problems with the normal Buck structure. However, for the negative power supply, the voltage from +28 to -15V varies greatly and the current is not small. The actual design is not difficult. The specific practice of inductance-based Inverter <1A is generally seen. I recommend considering the Fly-back structure on the negative supply side.

There is another question: Are you sure to make a transformer? After you have confirmed the master-slave and 28V conditions of the positive and negative power supply, let's discuss what options are available.

The MAX1654 is a good choice for two controlled outputs with a single controller and a pair of switches. The shortcoming is not wrong, and all the energy needs to be injected into the +15V output capacitor first, so that a part of it is transferred to the -15V output. This will require higher input and +15V energy storage and filtering. However, since only one pair of switching tubes is used and the synchronous rectification structure is used, the overall efficiency and cost are still competitive in this power. Another point is the need to adjust the feedback and overcurrent protection part of the sampling circuit to adapt it to +/-15V output (the circuit provided on the Datasheet is <6V).

Q8: When USB is charged, the second half of the USB is connected to the single-cell Li-ion battery and boosted to 5.3V. The booster chip used is SP1308, but in the protection circuit when power is supplied to other devices. The mos tube is too hot. I am in the middle of the USB, let them be left floating. How to connect the middle signal pin when charging and discharging the USB interface?

A8: It seems that you are talking about several problems. The first question is: When using a lithium-ion battery to supply a 5.3V power supply through the booster chip SP1308, the MOS tube in the protection circuit is hot. I believe you must It is unusually hot; if it's just normal because the current is too hot, probably you won't be asked a question. I didn't find the SP1308's Datasheet, and it's not good to judge whether it has a problem with conversion efficiency. Another thing you need to be reminded of is not to let transient high current pulses flow through the battery, which is extremely detrimental to battery life, in addition to causing abnormal heat.

The second question is more complicated and has a lot to do with what you have done. The relevant standards are YD T1591-2006, USB Charger 2.0, USB 2.0, OMTP, USB OTG, etc., which can be found on the website. If you are going to do a USB mobile phone charger, short the two lines (it doesn't seem like the product you are talking about).

Q9: Discharge termination control of NiMH batteries. We now have a project, 12V battery pack, average discharge current 5-8A, peak 10A, EOD: 9V, requires discharge termination control, but the question is, how much static power can be achieved? Because the battery may be shipped after the very For a long time, if the quiescent current is too large, the battery will be over-discharged.

A9: On the whole, the self-discharge rate of NiHM is relatively high, and it is better to use the nominal Ah capacity of 0.001-0.002%. The static power dissipation (current) of the over-discharge protection termination circuit can be referenced to the self-discharge current design (if there is much less practical effect than self-discharge). From your battery, tens of uA of quiescent current will not be a problem. MAXIM has many reference + comparator combinations that can operate at 1-2uA, and slow (basically static) drives NMOS or PMOS without requiring too much current. Carefully design the power consumption of the power supply, this circuit should be able to achieve <10uA.

Q10: The integration of charger and battery protection circuit has been realized! Many companies claim that their charging control IC is an integrated solution with intelligent protection. Why do experts still come up with discussions and say that partial integration, do you need additional protection circuits?

A10: Indeed, many companies, including MAXIM, have this product with protection. The difficulty of actual integration is not in circuit design, but in respect for safety management and the continuation of industry division of labor. It is easy to design and manufacture a chip with cell protection, and it is even possible to achieve single point of failure protection for the protection part and the charge control part on the same silicon. But even if it does, there is not much cost advantage compared with the battery manufacturer's independent protection circuit inside the battery. Only small batteries and a small number of applications that do not allow users to change batteries use batteries that do not have a built-in protection circuit. From the perspective of industry division of labor, the battery pack with built-in protection will be responsible for the safety design of the battery pack, and the responsibility is clearly divided. If the charging and protection are integrated into one chip, the installation relationship between the chip and the battery needs to be close and cannot be easily changed, and basically needs to be placed in the battery pack. This will bring a series of issues such as heat dissipation, versatility, and cost control related to versatility. The actual promotion of the integration of the fuel gauge and the protection circuit is higher than the fusion of the charger and the protection circuit, and the fuel gauge can be used to complete the charging judgment function, which can be considered as a partial fusion.

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