microATX Motherboard Interface Specification

Version 1.2

Page 20

Ground Ground

+12 V +12 V

1 2

3 4

Figure 9: 2x2 Connector Configuration

3.2 Signal Control Definitions

3.2.1 PS-ON

PS-ON is an active low TTL signal that turns on all of the main power rails including 3.3V, 5V, -5V, 12V,

and -12V power rails. When this signal is held high by the PC board or left open-circuited, outputs of the

power rails should not deliver current. These outputs should be held at a zero potential with respect to

ground. Power should be delivered to the rails only if the PS-ON signal is held at ground potential. This

signal should be held at +5VDC by a pull-up resistor internal to the power supply.

3.2.2 5VSB

5VSB is a standby voltage that can be used to power circuits that require power input during the powered-

down state of the power rails. The 5VSB pin should deliver 5V ± 5% at a minimum of 10mA for PC board

circuits to operate. This power can be used to operate circuits such as soft-power control. It is highly

recommended that the 5VSB line be capable of delivering a minimum of 2.0A. This current is needed for

features such as Wake on LAN technology*.

5VSB must be short-circuit protected to prevent damage to the power supply in the event a motherboard

demands more 5VSB current than the power supply is rated for.

3.2.3 Power Good Signal, PW-OK

A “Power Good” signal, PW-OK, will be asserted (i.e., high) by the power supply to indicate that the outputs

are above the undervoltage thresholds of the power supply and that sufficient mains energy is stored by the

converter to guarantee continuous power operation within specification for at least the duration specified as

“Hold Up Time.” Conversely, when one of these output voltages falls below the undervoltage threshold, or

when mains power has been removed for a time sufficiently long to no longer guarantee power supply

operation beyond the hold-up time, PW-OK will be held low. The recommended electrical and timing

characteristics of the PW-OK signal are provided in the Power Supply Design Guidelines.

Motherboards should be designed so that the signal timings recommended in the Power Supply Design

Guideline are used. If timings other than these are implemented or required, this information should be

clearly specified.