ATX Version 3 Multi Rail Desktop Platform Power Supply

Design Guide

ID 336521
Date 11/01/2023
Document Table of Contents


+5VSB is a standby supply output that is active whenever the AC power is present. This output provides a power source for circuits that must remain operational when the five main DC output rails are in a disabled state. Example uses include soft power control, Wake on LAN, wake-on-modem, intrusion detection, Alternative Low Power Modes (ALPM) or suspend state activities.

The power supply must be able to provide the required power during a “wake up” event. If an external USB device generates the event, there may be peak currents as high as 3.5 A, lasting no more than 500ms.

Over current protection is required on the +5VSB output regardless of the output current rating. This ensures the power supply will not be damaged if external circuits draw more current than the supply can provide.

With new modes of operation for computers like Alternative Low Power Modes (ALPM) the continuous current rating of the 5VSB rail is recommended to be at least 3 A. Some scenarios like USB Power Charging in ALPM could require more current on the 5VSB rail.

Figure 4-5: +5VSB Power On Timing versus VAC