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Grasping the significance of charge detectors

Posted: 21 Nov 2014 ?? ?Print Version ?Bookmark and Share

Keywords:USB? Battery Charging? BC1.2? SDP? DCP?

Think of the SDP as the classic USB port. In addition to USB communication, it provides 100mA of current to peripheral devices when connected; the 100mA is negotiable up to 500mA. While most ports typically do not enforce this current limit, higher currents are not guaranteed. A DCP does not support USB communication, but can supply charge currents beyond 500mA without any negotiation.

A CDP supports both USB communication and high-current charging; it features internal circuitry that is switched on during the charger detection phase. Some electronics manufacturers build proprietary charger identification schemes in addition to the USB port types outlined in the specification. Those various schemes add another layer of charger detection technology that cannot be overlooked.

 USB connector pins and data contact detection

Figure 1: USB connector pins and data contact detection (DCD).

The charger detection process
The charger detection phase outlined in BC1.2 has five basic steps.

1. VBUS detection To ensure proper sequencing for any potential devices connected to a USB port, the VBUS and GND pins on the connector are intentionally made longer than the D+ and D- pins. This ensures that they make contact first (figure 1). Thus, before any detection can occur, the device must first sense that VBUS is present.

2. Data contact detection (DCD) Once the voltage on VBUS is valid, the portable device must ensure that the data pins also make contact before any detection can occur. The end device might incorrectly identify what charger is present if it makes a premature decision before the data pins make contact.To perform DCD, the peripheral device must enable a 7?A to 13?A current source (referenced to +3.3V) on D+ and monitor its voltage. This current range is chosen to maintain proper logic levels across all voltage and resistance tolerances allowed in the specification. If D+ is open, the voltage will be logic high. If closed, D+ will read logic low regardless of the port type. If no data pin contact is sensed after the 1-second timeout period, the end device assumes that an SDP is present.

3. Primary charger detection In this step, the end device differentiates the > 500mA-capable ports with the charging label (CDP and DCP) from the 4. Secondary charger detection After turning off the voltage source and current sink from the previous step, the end device needs to discern a CDP from DCP. To accomplish this, it performs the previous test in reverse. Thus a 0.5V to 0.7V voltage source is enabled on D-, and a 50?A current sink is enabled on D+. If a DCP is present, the 0.5V to 0.7V test voltage will appear on D+. If a CDP is present, the voltage on D+ will be zero.

5. CDP charge current limit Since a CDP supports both data and high-current charging, one last distinction must be made. Because of the large amount of current in the USB cable, the host ground and device ground can only tolerate a ground offset of 375mV.

 BC1.2 charger detection procedure

Figure 2: Summary of BC1.2 charger detection procedure.

While non-BC1.2-compliant chargers differ among manufacturers, many of these proprietary chargers identify themselves to end devices by the voltage level set by a resistive-divider between VBUS and ground. Depending on the level of coverage required by a charger detecting circuit, sensing circuitry can be added to detect the voltage levels on D+ and D- and, thus, to identify different manufacturer-specific chargers.

Charger detection technology
A USB charger detection IC is a single chip that implements many of the features and intricacies associated with BC1.2 charger detection. It is, admittedly, possible to implement the detection scheme discretely. But the number of components, board space, and time spent getting the discrete system to operate successfully would increase dramatically.

Adding a dedicated charger detection chip requires some additional board space, so manufacturers often combine other necessary or desirable features into the same package. Consequently, charger detection ICs are highly integrated with a myriad of additional features such as built-in switches for USB or UART/audio operation, serial control interfaces, over-voltage protection (OVP), USB OTG support, Li+ battery charging capability, or even the ability to perform USB enumeration.

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