Author: Dan Simion, Senior Member of Technical Staff, Maxim Integrated Products

Nowadays, every one of us owns handheld electronic appliances. Think about the cell phones, laptops, netbooks, tablets, e-readers, MP3 players, cameras and camcorders, GPS devices, and…the list grows daily. Did you notice that all these devices have something in common? The battery inside, and it requires repetitive and careful charging. Now, try to collect all the so-called “chargers” that came with them. You will certainly end up with a heavy load, and what a waste! What if there was a method to charge any portable appliance by plugging it into a standard port? What if that port was also present on many, if not all, modern fixed appliances like desktop PCs, flat-panel TVs, DVD players, game consoles, routers, switches and hubs?

This article reviews the recent history of battery charging and then focuses on the new USB Implementers Forum (USB-IF) Battery Charging Specification, Revision 1.2. That specification describes the configurations and terminology employed in USB-compliant battery charging. It offers application suggestions for USB-compliant battery charging on both ends of the chain: host and device. The article explains how new USB host adapter emulators and USB device adapter detectors are standardizing battery charging and, ultimately, reducing the need for device-specific charging adapters.

Fundamentals of Battery Charging

The most ubiquitous portable device today is the cellular phone. Since it appeared on the market as an analog appliance, the cell phone has driven the evolution of rechargeable batteries. In fact, before cell phones the only known rechargeable batteries were lead-acid, mostly used in vehicles, and alkaline, intended mainly for special applications. With the advent of the mobile phone, three new types of rechargeable batteries emerged: NiMH, Li-ion (Li+), and Li-polymer. These new chemistries require special procedures and care during operation, and especially during charging.

• Discharging a battery below a manufacturer’s specified level is not recommended. This safe level is sometimes called the “dead battery” or “wake-up” threshold. When this particular level is reached, the appliance generally shuts down to prevent further discharge. Batteries showing voltage below this threshold need to be charged in a preconditioning or wake-up mode.
• A battery within nominal condition can be charged in constant-current mode, sometimes referred as nominal-, normal-, or fast-charging current. To maintain a battery’s life within specification, the fast-charge current needs to be regulated precisely.
• The final charging stage happens under a constant voltage, or top-off condition. In this charging state the voltage at the battery’s terminals remains constant for a wide range of charging currents. Unlike older lead-acid and alkaline batteries, this constant state cannot be maintained for a long time or indefinitely. After a prescribed time in voltage-mode charging, a modern battery must be disconnected from the charger. Consequently, the precision requirements for constant-voltage-mode charging are even more severe than for constant-current charging.

These basic charging parameters lead to two important conclusions: the charging controller is specific to each type/model of battery; the controller must be physically very close to the battery in order to achieve the precision required.

With little need or interest in understanding these charging basics, consumers ask the same questions all the time. “What, then, is the bulky black thing that plugs into the mains outlet on the wall? Is that a charger?” Well, yes and no. Yes, it is called a charger. No, it is not the charger, but rather a very simple AC-DC converter that provides basic power to the actual charger controller inside the phone, close to the battery in most cases. In fact, calling that black unit a charger is like calling a car key its engine—after all, this key is what makes the car move, or does it?

The analogy with the car key is not as farfetched as it may seem; the AC-DC converter is as particular to the phone as the key is to the car. Even if the mechanical connectors match, there is no guarantee that an AC-DC converter from one device manufacturer will charge a similar device from another manufacturer.

Basic Charging Configurations

Figure 1 shows a popular mobile phone that is ready for battery charging. Charging the battery of a mobile device involves three components: the actual device hosting the battery and the charger controller; the charging/data transfer cable; and the AC adapter. Note that while in a car or other transportation vehicle like a boat or airplane, the charging adapter might be DC instead of AC.

Figure 1. A typical cellular phone ready to be charged.

This configuration is valid for almost all battery-operated handheld devices, the large majority of them being consumer, but there are industrial applications which behave almost identically. Examples include point-of-sale (POS) and inventory-management terminals.

Here the AC-DC adapter is mainly a power supply with output rated 5VDC nominal and with a current capability between 500mA and 2.2A. Battery charging cannot, however, happen by just connecting a plain, generic power supply to the mobile device. Device manufacturers want to ensure that the adapter is safe and able to perform according to the device’s charging specifications. Therefore, some type of very simple handshaking is required between the device and the adapter before charging can occur. There is, admittedly, another reason that manufacturers want this handshaking to occur: to protect business by preventing consumers from using any AC-DC adapter with any host device.

Alternative Standardized USB Battery Charging

An AC-DC adapter is not the only way to charge a handheld device. Generally you can plug a mobile appliance into a USB receptacle on a host computer, either desktop or laptop, or even into a USB hub (provided that the hub is self-powered and there is a USB host present and active somewhere in the network hierarchy).

Assuming both the handheld device and host/hub combination are powered on and running, the mobile device “enumerates,” the applicable USB driver loads on the host, and the resident software initiates the charging process (among other processes, such as file and application synchronization).

The above scenario suffers from an inherent drawback: it needs two processors simultaneously running USB stack applications. One processor resides in the host, the other in the mobile device. The moment when one of those processors stops, either by going into standby to save power or even turning completely off, the whole battery-charging process is disrupted. Resuming from standby does not necessarily resume USB stacks. The device might actually need to be unplugged and then plugged in again to restart the whole process. This situation is certainly not practical or convenient for users, but workarounds exist and more than one company of handheld devices is actually doing that.

The only way to ensure compatibility between charging platforms is to follow the USB Implementers Forum (USB-IF®) Battery Charging Specification, Revision 1.2.¹ The USB-IF specification also allows compatibility between computer/hub USB ports and a so-called USB charger: AC-DC or DC-DC adapters featuring one or more USB receptacles, or USB captive cables. These later devices do not generally feature data-transfer capabilities and are intended for battery charging only.

Essential USB-IF Terminology Before summarizing the main characteristics of the host ports, let us briefly review a few definitions.

• Attach versus connect. A downstream device is considered to be “attached” to an upstream port when there is a physical cable between the two. A downstream device is considered to be “connected” to an upstream port when it is “attached” to the upstream port, and when the downstream device has pulled either D+ or D- high through a 1.5KΩ resistor in order to enter low-speed, full-speed, or hi-speed signaling.
• Downstream Port. This is either a Standard Downstream Port or a Charging Downstream Port.
• Charging Port. This is either a Dedicated Charging Port or a Charging Downstream Port.

* A Charging Downstream Port (CDP) has to provide a minimum of 1500mA, no matter what state it is in.

Table 1 shows the main characteristics of USB ports involved in device battery charging.

Hosting Scenarios

Earlier in this article we briefly explained what happens if a mobile device is attached to a host. It is now time to elaborate on the principal characteristics of host ports. With USB-IF BCS Rev.1.2 there are two possible scenarios when the handheld device will be attached to a host port: first, when both the mobile device and the host are powered on, with USB stacks running; and second, when the host is not data capable but implements a charging port, either in accordance with USB-IF BCS Rev.1.2 or a special proprietary specification.

In the first case, whenever the device and the host are attached, there is only one course of action. The device connects and the link configures according to USB 2.0, leading to a maximum of 500mA available for battery charging (SDP).A CDP will allow more hosting current (see Table 1), but to date very few implementations use it. There is no way to avoid this setup and still be USB-IF compatible.

Historically, workarounds were found when more power was required, the most obvious way being to parallel two SDP ports by using a “Y” USB peripheral cable. This patchy solution was ultimately abandoned because it was not a true USB-compliant implementation and paralleling power supplies was not an orthodox approach to the problem.

To summarize the first scenario, if a “powered-up and running” mobile device is connected to a USB2.0 data-capable device, then according to USB-IF BCS Rev.1.2 the device can identify the USB port as either SDP or CDP.

Implementing a Charging Port

The second scenario defined above is more interesting because it covers two classes of devices: USB hosts and USB chargers.

A usual USB port shuts down when the host enters standby. This is, of course, normal and on purpose. If one wants to save power, all power-consuming processes must be terminated, including battery charging of any attached handheld device. But, what if we want to continue battery charging without leaving the standby state? There is a solution for this that requires only a permanent 5VDC power supply and a USB host emulator circuit, like the MAX14566.

If a mobile device has Charging Port Detection capability, it will start a charging session when attached to a USB port and the voltage on VBUS will be greater than its internal session valid threshold. If charging conditions are met, the device will then start the charger detection algorithm. Note that this algorithm does not use USB 2.0 stacks, but is entirely specific to USB-IF BCS Rev.1.2.

A mobile device that does not support charger detection will assume that it is attached to a SDP and the Dead Battery Provision, as defined in Section 2 of USB-IF BCS Rev.1.2.However, the device is allowed to check if it is attached to an SDP or CDP and, consequently, to draw currents according to Section 3.4 of the same specification. Table 2 briefly illustrates the Charging Port Detection process.

Table 2. Flowchart of Charging Port Detection

Before attempting Charging Port Detection, the portable device needs to make sure that it is attached to a port and that VBUS is valid. Due to the mechanical design of the USB connectors, VBUS and GND are connected first, before the data contacts D+ and D-. After validating VBUS, the device can start the so-called DCD process.

It goes without saying that the flow in Table 2 requires careful timing. For clarity purposes only, the values of time constants have been omitted. Please check the USB specification1 for those values.

Figure 2 outlines the hardware configurations of various USB ports. These configurations are expected during both DCD and Charging Port Detection.

Figure 2. Various USB port configurations expected by USB-IF BCS Rev.1.2.

Apple Adapters: a Special Case

Any Apple device expects to see voltages on the data pins according to the resistor-dividers shown in Figure 3. Three Apple adapters (0.5A, 1.0A and 2.1A, respectively) were manufactured and shipped either with the appliance or sold separately. There are also countless third-party adapters that employ the same scheme to ensure compatibility (e.g., Belkin™, Kensington™, and Macally™ adapters, just to name a few).

Figure 3. The resistor-dividers in an Apple adapter.

Table 3 illustrates the differences among the three known Apple adapters. The 0.5A adapter is very difficult to find and is probably already phased out. Most iPhone® and iPod® appliances use the 1.0A adapter, while the 2.1A is dedicated to the Apple iPad® product.

* Note that the 1.0A and 2.1A adapters have D+ and D- dividers mirrored.

Table 3. Data Pin Configuration of the Various Apple Adapters

A somewhat similar approach is implemented by other companies including Sony and Samsung®.

The USB Host Adapter Emulator

There is yet another unique approach to implementing host ports in accordance with USB-IF BCS Rev. 1.2. When the port is data capable (i.e., attached and not suspended), then normal SDP configuration, according to USB 2.0, is assumed. This configuration is also known as pass-through mode.

There are three steps to enable this adapter emulator.

• Step 1. Allow some kind of adapter emulation for the port when the host is in standby and only a 5VDC permanent power supply is present. The port should emulate mainly DCP and Apple adapters, since there is, practically speaking, no point in emulating SDP. This is also known as the auto mode. The particular case of CDP will be reviewed below.
• Step 2. Enable some kind of mechanism so that the device can switch seamlessly from pass-through-mode charging to auto-mode charging with re-initiation of DCD and Charging Port Detection. This step prevents usual USB troubles, for example not recognizing plugged-in devices during host standby or failure to continue charging after the host resumes.
• Step 3. Provide full signal integrity for USB2.0 HS data traffic while in pass-through mode. A typical block diagram of the USB host adapter emulator is implemented in an integrated solution (Figure 4).

A typical block diagram of the USB host adapter emulator is implemented in an integrated solution (Figure 4).

Figure 4 The USB host adapter emulation with the MAX14566 family of devices.

This USB host adapter emulator operates in two modes: pass-through and auto. Switching between these two modes is done with the help of a control bit, CB, which is tied up directly to the standby state of the host. If the host is in standby, then CB=0 and the host adapter emulator is in auto mode; if the host is running the USB stack, then CB=1 and the host adapter emulator is in pass-through mode.

The pass-through operation is really simple. The USB transceiver inputs (TDM/TDP) are connected directly to the USB connector data lines (DM/DP) by a low-RON switch and 90Ω characteristic impedance lines. The USB port is now fully USB2.0 SDP compliant. While in auto mode, two courses of action are possible: emulate a DCP or an Apple charger. Immediately after CB=0, the emulator assumes an Apple configuration. If the device attached is USB-IF BCS Rev. 1.2 compliant, then IDM_SINK will pull down DM and the control logic will short DP and DM, thus assuming a DCP configuration. Of course, the Apple configuration is maintained if an Apple device is attached instead.

Anytime CB changes state from 0 to 1 or from 1 to 0, a special CEN signal is issued. This signal toggles the VBUS off for some time (typically 1 second). Theoretically, this is enough to reset the device and re-initiate enumeration (if in pass-through mode) or restart DCD and Charging Port Detection (if in auto mode). CEN could be either active low or active high, and usually acts on the port’s associated current-limit switch by overriding the basic command issued by the system controller. For details and an implementation example, please see the MAX14566 data sheet.

The MAX14566B is a special addition to the device family. It exchanges the CEN output for another control bit, CB1; the previous control bit is renamed to CB0 and keeps the same function. CB1 is significant only in auto mode and serves to force DCP, regardless of the device’s attempts at Charging Port Detection. On the surface this approach might seem peculiar, but it is actually very useful for devices implementing DCD. The auto mode then starts forcing DCP and, after a time determined by the system controller, switches to normal auto mode. Depending on processor speeds and other constraints, this forced short mode might be required to last for a couple of seconds or longer. There is some redundancy built into the MAX14566 device family. One particular combination of CB1, CB0 could be used to allow a different configuration, since only three others are significant so far.

Recently, USB pass-through mode with CDP emulation was added to the previous operational modes. This capability is present in the MAX14602 family of USB host adapter emulators. In this mode the data lines can be in a high-impedance state or fully connected and awaiting device-resume commands. (Note that this is only if the host USB port is preconfigured to allow remote wake-up).

The current limit in this mode needs to be set externally. Consequently, the mode might not fully comply with USB-IF BCS Rev. 1.2, since there is no way to establish what state the USB communication is in (i.e., LS, FS, HS, chirp, suspend). A cautious approach will assume 900mA, as the vast majority of USB ports today are HS. But in the end, this decision is the responsibility of the system designer.

The USB Device Adapter Detector

The USB-IF BCS Rev. 1.2 covers two aspects of standardized USB charging: host or charger side, and device side. We already covered the host adapter emulation. It is now time to address the USB device side.

Recall that, in the past, portable devices required a specific adapter for offline battery charging. When attached to a host, almost all devices assumed a SDP connection present. Fortunately enough in the recent years there is a strong push to standardize both the mechanical connection to the adapter, with micro-USB becoming widely adopted, and the adapter requirements themselves. Adapter detection is best represented by the newest battery charger detectors (Figure 5).

Figure 5. The MAX14578A in a USB-IF BCS 1.2-compliant mobile device.

Recall that to comply with USB requirements, a mobile device must perform the following tasks: • Detect a valid VBUS and possibly discriminate between a regular USB peripheral connection and USB OTG connection.

• Start the DCD process to ensure that D+/D- are present.
• Identify the USB port to which it is attached and its current status; perform Charging Port Detection.
• Issue a command to the battery charger according to the results of the Charging Port Detection.

This whole process is better illustrated by the states diagram in Figure 6.

Figure 6. A typical states diagram of a mobile device that is compliant with USB-IF BCS Rev. 1.2. This mobile device is implemented with the MAX14578 family of ICs.

From Figure 6, the device needs to know if the adapter is USB compliant or not. If yes, then further tests differentiate between SDP, CDP, and DCP. If no, other tests ascertain Apple or Sony adapters. The MAX14578 can detect eight adapter types (Table 4). A special case is represented by the suspended SDP port. Because of the strict low-current requirements, it becomes equivalent to a disabled charger. The result of DCD/Charging Port Detection is then translated to the state of three GPIOs intended to drive a charger-controller IC (the MAX14578A only). The MAX14578 has two GPIO pins and no CE0.

Table 4. State of GPIO pins on MAX14578 Depend on the Result of DCD/Charging Port Detection.

The significance of the three GPIO pins follows:

• CE2: USB current; signals maximum 500mA when high; gives precedence to the other GPIOs when low.
• CE1: selects DCP current when high; gives precedence to the USB current when low.
• CE0: signals that the host/adapter is in suspend mode when high; used for USB-compliant connections; allows charging when low.

Note that in the special case of dead-battery mode, the charger is just enabled. No decision on the charging current is made by MAX14578A. Usually, charger controllers provide an internal preconditioning, or wake-up current setting.

USB device adapter detectors are designed into circuits differently, depending on the intended end product. While host emulators can afford to be separate circuits, since there is plenty of space and lots of power in a major appliance, the handheld devices are much more constrained for space.

As a result, the USB device adapter detection is most often integrated alongside other functions in what is now known as the USB multiplexed interface circuit, multiplexed USB switch (UMIC/MUIC), or some other variation. At this time the only true and single-function USB device adapter detector is the MAX1578/MAX14578A.

Conclusion

The USB-IF Battery Charging Specification, Revision 1.2 offers a good strategy for building interoperable and compatible modern handheld devices, at least from the battery-charging point of view. The standard also offers tremendous relief to device manufacturers who can deliver their product without an attached charger, since battery charging will occur by plugging it into any USB appliance. Adapter manufacturers can reduce their inventory spread and concentrate more on size, reliability, and green features in their USB products. Industry regulators will probably be happiest of all, because now USB certification can be present on every handheld product and accessory.

Reference

¹ Battery Charging Specification Revision 1.2, December 7, 2010, Copyright © 2010, USB Implementers Forum, Inc. See

http://www.usb.org/developers/devclass_docs

and

http://www.usb.org/developers/compliance/.