With more and more built-in functions, more and more intelligent electronic devices are more attractive and consume more power, so rechargeable batteries have become an economical choice. In recent years, with the emergence of innovative applications, emerging technologies and new battery chemistries, the demand for chargers has continued to evolve. For example, new applications in the field of wearable devices (such as smart bank cards, smart clothing, and medical patches) are leading solutions to become smaller and cheaper, and they also promote the development of smaller batteries with higher power density.
Typical questions or concerns from designers include: "How can I maximize the battery life?" "How can I extend the shelf life of the product?" "Is it possible to over-discharge the battery?" "If the battery is missing or damaged, it will happen What?" "How do I make my product work with a weaker adapter?", and "Can I use the same charger for different designs and different batteries?" In this article, I will discuss the linear charger How different functions can help solve these problems.
Power path
The power path function can add a switch inside the charger to make a separate output power the system and charge the battery. This architecture will lead to other features. Let us first look at Figure 1, which shows a simple linear charger with no power path. The system input and battery electrode are connected to the same charger output node. This non-power path architecture is popular for its simple and compact solution; examples include bq24040 and bq25100 battery chargers. Figure 2 shows the evaluation module (EVM) of the bq25100.
However, this architecture has some limitations. The charger output, battery electrode and system input are all connected at the same point. If the battery is deeply discharged or defective, the system may not be able to start even if an external power source is connected. Before the system starts, the battery needs to be charged to a certain voltage level. Therefore, if the battery of the product is deeply discharged, the end user may think that the system is damaged when the adapter is inserted, and nothing happens because the system cannot be started. If the product’s battery is indeed defective, the system may never start. For products that use removable batteries, it may be necessary to use different packages to remove and replace defective batteries. However, if the battery is embedded in the device, the defective battery makes the entire product unusable.
Another problem with this architecture is that the charger can only detect the total current flowing into the battery and the system. If the system is running, how does the charger determine whether the battery current has reached the termination level?
The solution to all problems is very simple-just add another switch between the system input and the battery electrode. Figure 3 shows the power path linear charger architecture. In addition to the Q1 field-effect transistor (FET) that draws current from an external power supply, another switch (Q2) needs to be added to separate the battery from the system if necessary. The system always has the priority of input power, when the end user plugs in the adapter, the system can be turned on immediately. If the adapter has extra power left while supporting the system load, the battery can be charged.
This method also allows the charger to independently monitor the charging current of the battery (as opposed to the total current of the adapter) to allow proper termination and check for any fault conditions.
The bq24072 device family includes a stand-alone power path linear charger. bq25120A is a new device with higher integration. In addition to the power path linear charger, it also integrates a DC-DC step-down converter, LDO, button controller and I2C interface with customer programmability.
Transportation mode
The transportation mode is usually the lowest quiescent current state of the device. In order to maximize the storage life, the manufacturer has enabled this state before the product leaves the factory, hoping that the battery will not run out when the end user obtains the product. By turning off Q2 in the power path charger, the transport mode circuit essentially disconnects the battery to prevent the battery from leaking into the system. When the end user turns on the product for the first time, Q2 turns on again and the battery is connected to the system.
Dynamic Power Path Management (DPPM)
DPPM is another function of the power path device. It can monitor the input voltage and current of the device, and automatically prioritize the system when the adapter cannot support the system load. The input source current is shared between the system load and battery charging. If the system load increases, this function can reduce the charging current. When the system voltage drops to a certain threshold, the battery can stop charging and discharge the battery to supplement the system current requirements. The implementation of this feature can effectively prevent system crashes.
Input voltage dynamic power management (VIN-DPM)
Another feature that is often confused with DPPM is VIN-DPM. This mechanism sounds very similar, but the focus is completely different. The input power supply or adapter has a rated power. In some cases, the power of the input power supply is not enough to meet the requirements of the equipment. In the case of different USB standards, designers nowadays recognize this more generally. The device being charged may need to adapt to various types (even unknown) adapters. If the input source is overloaded and causes the input voltage to fall below the undervoltage lockout (UVLO) threshold, the device will shut down and stop charging. The power load disappears and the adapter is restored. Its voltage rises above UVLO and starts charging again, but the adapter will immediately overload again and crash. This undesirable situation is called "hiccup mode". See Figure 4.
The VIN-DPM function can successfully solve this problem because it can continuously monitor the charger's input voltage. If the input voltage is lower than a certain threshold, VIN-DPM will adjust the charger to reduce the input current load, thereby preventing the adapter from crashing.
Now you can find that VIN-DPM and DPPM are actually two completely different functions. VIN-DPM can monitor the output of the adapter (or the input of the charger) and keep it at a certain level. DPPM can monitor the charger output (or system rail) and keep it at the lowest predetermined level. These two functions can co-exist and play a role in order to operate smoothly under different operating conditions. Not all chargers have these two functions. You can also implement VIN-DPM on non-power path chargers.
This article uses the linear charger topology as an example to illustrate some basic characteristics related to the power path function, but of course switching chargers can also have this function. For more information on battery charger options, please refer to the battery charger solutions page.
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