Protect your application processors from short circuit at 9V

Smartphones are moving to larger screens, with higher resolution, faster modems and longer battery life. All of these features drain the battery in our phones – and we need them to recharge fast.

As smartphones and other mobile devices go to bigger batteries, more power from the charging adapter is needed to allow fast charging.  There are several proposed methods from different manufacturers, but the common theme is that higher input voltage is required at the power connector. Higher input voltage allows more power into the system for fast charging without increasing the current above the connector’s limited current-carrying ability. The adapter voltage default setting is the usual 5V (USB VBUS) level, but signaling on the D+/D- data lines between the external adapter and the mobile device can negotiate the adapter output higher as needed. Typical values include 5, 9, 12 or 20V output levels depending on the adapter capability.  The charging IC or application processor in the system controls this signaling so that the adapter voltage only goes to the appropriate level.

Typically, when inserting the USB connector, you need an analog switch to transmit power between the application processor (AP) and a charger like the TI bq25890, as shown in Figure 1. Upon first connection, the AP has control of the D+/D-. It will detect whether it is a charger or a USB attachment.

Figure 1: Typical application diagram for an application using USB and charger

TI provides multiple USB switches that cover a wide spectrum in terms of configuration, voltage ranges, on-resistance (RON) and bandwidth.

Application processors cannot tolerate high voltages. Since fast charging generally operates at 9V and the AP cannot tolerate 9V, it becomes crucial that the AP is never exposed to the 9V.

The USB has four pins – VBUS, D-, D+ and GND – shown from right to left in Figure 2. After negotiating the fast charge state, the VBUS will now be at 9V.

Figure 2: USB switches and pin descriptions

If the USB plug is not inserted properly, or if it is removed at an angle when the VBUS = 9V, it might cause the VBUS pin to short with the D- pin of the connector. This exposes the AP connected to the D- pin to VBUS, damaging the AP, as shown in Figure 3.

Figure 3: The consequence of not using TS3USB3000 at 9V when a short happens

The TS3USB3000 USB switch protects the AP at 9V while enabling switching between the AP and the charger/ universal asynchronous receiver/transmitter (UART), as shown in Figure 4. After connecting a charger to the USB port, the AP will detect that a charger is attached and pull the output enable (OE) pin high to disable the switches. The AP will communicate to the charger that it can negotiate for a faster charging mode with VBUS at 9V, also shown in Figure 4. The TS3USB3000 is now in a low-power state, with the switches disabled, and can protect the AP.

Figure 4: TS3USB3000 protects the AP at 9V when a short happens

The TS3USB3000 switch serves two purposes: 1) it switches between the host and the charger, and 2) it protects the AP at 5-9V. For case 1, any USB switch can be used depending on what kind of bandwidth, Ron (on resistance) that the customer wants. But for the case 2, only the TS3USB3000 can protect the AP in case there is a short. The TS3USB3000 serves these two purposes for designs that need to operate at 9V. The device makes switching between the host and AP easy, and if a short between the VBUS and D- happens while inserting or ejecting the USB connector, it protects the AP.  

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