SLVAFB9 February   2022 TPS25947

 

  1. 1Application Brief

1 Application Brief

USB 2.0 and USB 3.1 power sourcing ports have traditionally been protected with current limiting power switches. Such devices can also incorporate short circuit and reverse current/voltage protection. Texas Instruments has a wide range of fixed and adjustable current limit power switches such as TPS25221 which offer this protection.

GUID-B6582F97-084F-4A41-906B-AD72831C3B14-low.gifFigure 1-1 Typical Application use as USB Power Switch

More recently IEC62368-3 Safety aspects for DC power transfer through communication cables and ports has emerged. This standard mandates that under single fault conditions in the PSE with a fixed, single output voltage, the output of the PSE shall not exceed 130% of the nominal rated output voltage.

In practice, this means that if an upstream voltage regulator fails and applies a higher voltage to the USB power switch, the output must not exceed 6.5 V. Most traditional USB power switches are rated for 5 V with an absolute maximum voltage rating in the range of 6 V. If the upstream failure applies 12 V to the USB switch it is likely to fail and it cannot be guaranteed that it will not fail in short thus applying the 12 V to output port.

To meet IEC62368-3, a different kind of power switch device is required. TPS259472 is a 28 mΩ eFuse with an adjustable current limit that can be programmed between 0.5 A and 6 A. Supplied in a tiny 2mm x 2 mm QFN package, it is rated for operation from 2.7 V to 23 V. Additionally, this device has a pin selectable 5.7 V output overvoltage clamp option. In this case, where the single fault condition applies a voltage up to 23 V to the power switch, the output clamp ensures that the output of the PSE continues to meet IEC62368-3 requirements as shown in Figure 1-2.

GUID-20220128-SS0I-MJSK-NBXZ-GS19MRB3KD7W-low.gifFigure 1-2 Output Overvoltage Clamp Response

Figure 1-3 shows a simplified application circuit. A TPS563202 buck converter generates a 5 V rail to be passed by the TPS259472 to the USB port. If a fault with the dc/dc converter circuit allows 12 V to be applied to the input of TPS259472, the overvoltage clamp (OVC) set by resistor to be 5.7 V, ensures the output voltage never exceeds 130% of its nominal value, meeting IEC62368-3.

GUID-20220203-SS0I-FPH9-DGPC-WVC88ZKF3FZX-low.pngFigure 1-3 Simplified USB 2.0 and 3.1 Circuit with OVC to Meet IEC62368-3

The TPS25947x CB Test Certificate IEC62368-1 can be found in the technical documentation section of the TPS259472 product page.

IEC62368-3 is a system level certification standard and cannot be obtained at device level. However, TPS259472 has been tested successfully to meet the requirements for IEC62368-3. The full IEC62368-3 test report can only be shared under NDA. Please use the TI E2E™ design support forum to request this test report.

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