Intel ATX Power Supply Design Guide
Version 0.9
3.4.2 PS_ON#
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 five main DC output rails: +12VDC, +5VDC, +3.3VDC, -5VDC, and -12VDC. When PS_ON# is pulled to TTL high or open circuited, the DC output rails should not deliver current and should 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.
The power supply shall provide an internal pull-up to TTL high. The power supply shall also provide debounce circuitry on PS_ON# to prevent it from oscillating on/off at startup due to activation by a mechanical switch. The DC output enable circuitry must be SELV- compliant.
Table 10: PS_ON# Signal Characteristics
| Min. | Max. |
VIL, Input Low Voltage | 0.0 V | 0.8 V |
IIL, Input Low Current, Vin = 0.4 V | | -1.6 mA |
VIH, Input High Voltage, Iin = -200 µA | 2.0 V | |
VIH open circuit, Iin = 0 | | 5.25 V |
| | |
3.4.3 +5VSB
+5VSB is a “standby” supply output that is active whenever the AC power is present. It 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, or suspend state activities. +5VSB is required for the implementation of PS_ON#.
The +5VSB output should be capable of delivering a minimum of 720 mA at +5 V ±5% to external circuits. Because trends indicate a growing demand for standby power, it is recommended that a family of designs be created to supply 720 mA, 1.0 A, or 1.5 A to meet various customer requirements. Overcurrent 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.
3.4.4 Power-on Time
The power-on time is defined as the time from when PS_ON# is pulled low to when the +5VDC and +3.3VDC outputs are within the regulation ranges specified in Section 3.2.1. The power-on time shall be less than 500 ms.
+5VSB shall have a power-on time of 2 seconds maximum after application of valid AC voltages.