Presenting resistor pullups or pulldowns on the configuration channel (CC) lines establishes a USB-C connection. A USB-C source-only port requires a pullup resistor on the CC line, known as Rp. The value of Rp will change depending on how much current you would like to advertise. The most common current levels supported by a USB-C source-only port are default USB power (500mA for USB2 and 900mA for USB3), 1.5A and 3A. The USB Type-C specification’s Termination Parameters section lists the corresponding Rp resistor values for each of these current values.

In general, when designing a system for a 5V USB-C source-only port, a CC controller IC will ensure the presentation of the correct value of Rp on the CC line automatically.

In addition to presenting Rp on the CC line, a 5V USB-C source-only port will also need to be able to protect against noncompliant sink devices that draw more current than negotiated by the Rp. For example, when presenting a 3A Rp, the connected sink device must ensure that its current draw does not exceed 3A. Although the sink is responsible for ensuring that it does not exceed the negotiated current level, the source is actually responsible for implementing overcurrent protection in the event that a noncompliant sink device draws more current than negotiated.

There are several ways to implement this current limit. For example, you could design a discrete power path with some form of current measurement, or use a load switch with an integrated current limit. The simplest way is to use a CC controller with an integrated power path, which would ensure automatic implementation of the current limit based on the value of Rp presented.

Implementing a 5V USB-C source-only port requires two key blocks: a CC controller and a 5V load switch. See Figure 44.