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Intel ATX Power Supply Design Guide
Version 0.9
Page 17
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.
V
IL
, Input Low Voltage 0.0 V 0.8 V
I
IL
, Input Low Current, Vin = 0.4 V -1.6 mA
V
IH
, Input High Voltage, Iin = -200 µA 2.0 V
V
IH
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.