ATX Version 3.0 Multi Rail Desktop Platform Power Supply

Design Guide

ID 336521
Date 01/02/2022

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Document Table of Contents


PS_​ON# is an active-low, TTL-compatible signal that allows a motherboard to remotely control the power supply in conjunction with features such as soft on/off, Wake on LAN*, or wake-on-modem. When PS_​ON# is pulled to TTL low, the power supply should turn on the four main DC output rails: +12 VDC, +5 VDC, +3.3 VDC, and -12 VDC. When PS_​ON# is pulled to TTL high or open-circuited, the DC output rails shall not deliver current and must be held at zero potential with respect to ground. PS_​ON# has no effect on the +5VSB output, which is always enabled whenever the AC power is present. To support systems with ALPM this is required for all power supplies. The power supply may be asked to turn back on before all voltage rails have turned off. The power supply must be able to turn back on via a change in the PS_​ON# signal after 100 ms of the PS_​ON# signal being de-asserted. Table 4-11 lists PS_​ON# signal characteristics.

The power supply shall provide an internal pull-up to TTL high. The power supply shall also provide de-bounce circuitry on PS_​ON# to prevent it from oscillating on/off at startup when activated by a mechanical switch. The DC output enable circuitry must be SELV-compliant.

The power supply shall not latch into a shutdown state when PS_​ON# is driven active by pulses between 10 ms to 100 ms during the decay of the power rails. When PS_​ON# de-asserts (turn on the PSU) with a time that is greater than 100 ms, from when it is first asserted (turns off), the PSU must respond to this request and turn back on all voltages rails no matter where the voltage rails are in ramping down the voltage to an Off state.

Table 4-11: PS_​ON# Signal Characteristics






0.8 V

IIL (VIN = 0.4 V)


-1.6 mA1

VIH (IIN = 200 uA)

2.0 V


VIH open circuit


-5.25 V

Ripple / Noise

400 mV p-p

    1. Negative current indicates that the current is flowing from the power supply to the motherboard.
    2. Due to PS_​ON# toggle on/off frequently, system and PSU component’s reliability should be considered based on the days, months, or years of claimed warranty listed on product specification. Refer to Document #575961 PSU design consideration for S0ix mode.

Figure 4-4: PS_​ON# Signal Characteristics