ATX Version 3.0 Multi Rail Desktop Platform Power Supply

Design Guide

ID 336521
Date 02/01/2023

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Document Table of Contents

Timing, Housekeeping and Control – REQUIRED

Figure 4-3. Power on Timing


Table 4-9. Power Supply Timing




Legacy Timings1


Recommended for ALPM


AC power on time





Power-on time

< 500 ms

< 200 ms

<150 ms


Rise time


0.2 – 20 ms



PWR_​OK delay

100 ms2 – 500 ms

100 ms2 – 250 ms

100 ms2 – 150 ms


PWR_​OK rise time


< 10 ms



AC loss to PWR_​OK hold-up time3


> 16 ms



PWR_​OK inactive to DC loss delay


> 1 ms


  1. Value in the Legacy column list timings for power supplies designed before the year 2020. In 2020, the T1 and T3 timings have moved from the Legacy timing to the new Required column for all new power supply designs.
  2. T3 minimum time faster than 100 ms is not recommended for previous generation motherboards and systems. All design tolerances must be considered before allowing T3 faster than 100 ms.
    1. A T3 time less than 100 ms may be designed based on system requirements and a need to provide faster PSU and system turn on capability. However, PSU and system designers are highly recommended to verify and ensure no PSU and system compatibility problems exist, especially for previous generation motherboards and systems.
  3. T5 to be defined for both max/min load condition.
  4. PSUs are recommended to label or indicate the timing value for system designer and integrator reference for T1 and T3. This allows system designers to optimize “turn on” time within the system.