USB Power Delivery 2.0 vs 3.0

When I first heard about the movie “Batman v Superman: Dawn of Justice,” I was confused because they are both the “good guys.” Since USB has released a limited distribution of a new USB Power Delivery (PD) 3.0 specification (version 1.0a), there is a similar dilemma: Which version of USB PD is right for your application?

The release of USB PD 3.0 does not mean that USB PD 2.0 is invalid or out of date. Many applications continue to use USB 2.0 even though USB 3.1 is available. Similarly, for USB PD, both USB PD 2.0 and USB PD 3.0 are valid options and they are fully interoperable.

Let’s dispel the erroneous thought that the voltage profiles or power rules are different between USB PD 2.0 and USB PD 3.0. The power rules are identical in both PD versions. The most important rules are:

  • Sources offering more than 15W shall advertise 5V and 9V.
  • Sources offering more than 27W shall advertise 5V, 9V and 15V.
  • Sources offering more than 45W shall advertise 5V, 9V, 15V and 20V.

USB PD 3.0 ensures backwards compatibility with USB PD 2.0. All USB PD 3.0 sources and sinks are required to fully interoperate with USB PD 2.0 sources and sinks. This requirement is enforced by requiring all USB PD 3.0 devices to pass USB PD 2.0 compliance tests. In fact, from the USB Implementers Forum (IF) point of view, there are only USB PD products. The USB-IF does not distinguish between USB PD 2.0 products and USB PD 3.0 products when issuing USB logo certification.

Table 1 lists the actual differences between the two versions as listed in the USB PD 3.0 specification.

Table 1: List of new USB PD 3.0 features.

Why use USB PD 2.0 now?

Here are two example applications that in my opinion should not migrate to USB PD 3.0 and incur the burden of supporting new requirements/features.

The first application is the simplest sink-only device that only wants to negotiate power. None of the new features in USB PD 3.0 really provide any tangible system benefit unless the application requires USB authentication, or if the application wants to report sophisticated information about its battery to the source.

Second, a simple source-only device that cannot make use of extra information the sink may report really does not need any of the new USB PD 3.0 features. TI has recently released TPS25740 and TPS25740A for such applications.

The USB-IF has been certifying USB PD 2.0 silicon since August 2015, and the compliance program was finally completed in June 2016 after the USB PD 2.0 specification stabilized at version 1.2 in March 2016. Today the USB PD 3.0 specification is at version 1.0a, and compliance testing has not yet begun, the USB-IF has not even announced a schedule for when compliance testing for USB PD 3.0 silicon will begin.

In conclusion, a system designer should not automatically assume that USB PD 3.0 is required for their application. In the end, whether or not USB PD 3.0 is really necessary in a given application depends upon whether or not the new features are needed in that application.  The maturity of the USB PD 2.0 specification as well as its lower complexity may very well always out-weigh the benefits offered by USB PD 3.0 in many applications. So USB PD 2.0 is here to stay.

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