ATX Version 3.0 Multi Rail Desktop Platform Power Supply

Design Guide

ID 336521
Date 01/02/2022

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PWR_​OK is a “power good” signal. This signal shall be asserted high by the power supply to indicate that the +12 VDC, +5 VDC, and +3.3 VDC outputs are within the regulation thresholds listed in Table 4-2 and that sufficient mains energy is stored by the converter to guarantee continuous power operation within the specification for at least the duration specified in Section Section 4.2.9. Conversely, PWR_​OK should be de-asserted to a low state when any of the +12 VDC, +5 VDC, or +3.3 VDC output voltages fall below its voltage threshold, or when mains power has been removed for a time sufficiently long enough, such that power supply operation cannot be guaranteed. The electrical and timing characteristics of the PWR_​OK signal are given in Table 4-10.

PSU manufacturers are required to label or tag PSU DG revision or ATX spec revision to show compliance and reflect the timing supported.

Table 4-10: PWR_​OK Signal Characteristics

Signal Type

+5 V TTL compatible

Logic level low

< 0.4 V while sinking 4 mA

Logic level high

Between 2.4 V and 5 V output while sourcing 200 μA

High state output impedance

1 kΩ from output to common

Max Ripple/Noise

400 mV p-p