Posted on 29 June 2019

Design Considerations for Implementing Multiple Sources Charging Solutions

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As battery-powered electronics become power hungry due to the ever-increasing demand for functionality and features, consumers are constantly looking for more convenient and easily available charging solutions. This drives the need to charge handheld devices through multiple sources – AC adapter, universal serial bus (USB) adapters, wireless, solar and any other custom input solutions. This article discusses the challenges and key considerations in providing charging solutions for multiple inputs.

By Miguel Aguirre, Battery Charge Management Systems Engineer Manager, and Sudha Chirra, Battery Charge Management Sector Manager, Texas Instruments


Architecture considerations

As the number of input power sources increases, the design challenges for the charging system become increasingly complex. The choice of architecture depends on design tradeoffs, not for the charging system alone, but also overall product design.

A common misconception is that multiple input sources translate into more than one input connector on the handheld device. This is not always the case. Many applications use single input, multiple source architectures. A common example is the USB connector, which has become a commonly used input source, where the device can be charged via a USB port or AC adapter with a single USB plug. The benefits of using this architecture are low cost and minimal connections to the external world, which minimizes the probability of an external event damaging the system. The drawback of this configuration is that it may create the need of more complex detection schemes to determine the type of source that has been connected (i.e., USB port or AC adapter) or how much power can be extracted without creating problems for the source.

The more common scenario is the multipleinput, multiple-source architecture, where the need for multiple input plugs arises due to the physical design of the system or due to a backward compatibility requirement with previous charging adapters or solutions. The benefit of this configuration is that it is a simpler design, where each source has a unique input. The major drawback is the additional cost from the additional connectors. Moreover, the use of multiple connectors increases the probability of the system being damaged by external events.

Single input, multiple sources

For cases where the system has a single input but supports multiple sources, it is important to identify which source is connected as different sources have different characteristics. As mentioned earlier, many applications nowadays use a micro-USB connector as the only input source on the system and the connector is used for charging from USB ports on the computer or hub and from wall adapters with a micro-USB connector.

If USB compliance is a requirement for the system, it is mandatory to detect if the system is connected to a USB source or a wall adapter. USB standard requires that when a portable device is connected to an USB port, the device will not consume more than 100mA until the device enumerates the USB host. Once enumeration is complete, the portable device can consume higher currents. On the other hand, if the input source connected is a wall adapter, the portable device can use the maximum current available from that source as required by the system. USB detection can be achieved by looking at the D+ and D– lines on the USB connector as shown in Figure 1.

D+/D– detection on bq24050 and/or bq24052 battery chargers

In this example we use the bq24050 to monitor the D+ and D– lines during power up and begin the detection routine using the 100 mA input current limit to ensure the USB specification is not violated. The device completes the detection within 100 ms of the D+ or D– lines being pulled high. After detection is complete, the D+ and D– lines are discon- nected to minimize any possible interactions of the charger detection pins and the normal USB communication.

If the device detects that a USB port is connected to the input, the charger stays in the 100 mA input current mode and waits for the host to enumerate to decide the current capability of the USB port. The host processor may then set a higher input current limit by driving the ISET2 pin. If the device detects that an adapter is connected to the input, the charger changes the input current limit to the programmed current on the ISET pin. Detection of an adapter is based on the D+ and D– lines being shorted together, as defined on the USB battery charging specification.

This kind of detection simplifies the implementation of a single device connected to a single micro-USB input while supporting two kinds of wired input power sources.

Multiple input power sources

A more commonly used architecture to support multiple sources is to simply have a unique input for each of the power sources. Figure 2 shows an example of this configuration where the system needs to support a wall adapter input and a USB port.

Dual-input wall adapter and USB port

In this configuration, the system is able to charge the battery using either the wall adapter, connected to the AC pin, or the USB port connected to USB pin. In this case, the ISET2 pin is used to program the input current limit on the USB input and the ISET1 pin is then used to program the charge current, which also represents the current limit for the wall adapter power source. When both input sources are connected, the battery charger selects the AC source since it will assume that this source is the higher power source and will allow faster charging of the battery. Before selecting the AC source, the device ensures this voltage is higher than the battery voltage by at least 190mV. If this condition is not met, then the device charges from the valid USB source. Implementation of this architecture is simpler as no detection is required since both sources have unique pins on the system.

Another example of a multiple input architecture are systems that support both wired and wireless power sources. Recently, wireless power has gathered a lot of attention and interest in the consumer market, pushing equipment manufacturers to incorporate wireless power into their existing products. In these scenarios, it is important to implement a way to turn off wireless power when an adapter is connected to the wired input. Figure 3 shows a simple implementation for solving this problem. This configuration allows the support of three potential power sources: USB port, dedicated wall adaptor, and wireless power source.

Wired and wireless power support

Another example uses the bq51013, a Qiv1.0 compliant wireless power receiver, and the bq24040 as the battery charger IC for both the wired and wireless power sources. Let’s assume that the portable device is currently being charged using a wireless power source and the user decides to plug in the wired power source. During this condition, the compliant wireless power receiver detects the adapter connected using the AD pin; sends a message back to the wireless power transmitter to stop wireless power transfer, and then drives the AD_EN signal to allow the portable device to be charged with the wired adapter. Q1 blocks in both directions to ensure the wired power is not delivered to the device while wireless power is enabled.

The ISET2 pin that is connected to the host, programs the input current limit of the battery charger. When a wired input is connected, the host floats the ISET2 pin, which programs the input current limit to 100 mA. Once the host enumerates the USB source, it can then decide if it wants to pull 500 mA from the USB port by pulling the ISET2 pin high. If the host detects a wall adapter, pulling ISET2 low programs, the battery charger device pulls the current programmed with the ISET resistor.


More and more battery-powered electronics are now incorporating multiple input connectors to enable powering up the devices through a variety of power sources. As the number of inputs increase, the battery charge management tends to get more complex in the overall system design. This article primarily focuses on USB input, AC and wireless input, the same concepts can be applied to other forms of input power sources – wild charge, solar, or custom solutions.



To learn more about battery chargers from TI, visit:
• Download these datasheets: bq24050, bq51013 and bq24040.
• Download the USB Battery Charging Specification.



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