Electrical
Design Guide 23
Figure 3-4: PS_ON# Signal Characteristics
3.3.4 +5VSB – Required
+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 Sleep Modes
(ASM) 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 Sleep Modes (ASM) the
continuous current rating of the 5VSB rail is recommended to be at least 3 A. Some
scenarios like USB Power Charging in ASM could require more current on the 5VSB rail.
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