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TFX12V Power Supply Design Guide
Thin Form Factor with 12 V Connector
Version 2.0
22
2.3.1 PWR_OK
PWR_OK is a “power good” signal. This signal should be asserted high by the power supply to indicate that
the +12 VDC, +5 VDC, and +3.3 VDC outputs are above the under voltage thresholds listed in Table 2 in
Section 2.2.1 and that sufficient mains energy is stored by the converter to guarantee continuous power
operation within specification for at least the duration specified in Section 2.2.12, “Voltage Hold-up Time.”
Conversely, PWR_OK should be de-asserted to a low state when any of the +12 VDC, +5 VDC, or +3.3 VDC
output voltages falls below its under voltage threshold, or when mains power has been removed for a time
sufficiently long such that power supply operation cannot be guaranteed beyond the power-down warning
time. The electrical and timing characteristics of the PWR_OK signal are given in Table 13 and in Figure 7.
Table 13. PWR_OK Signal Characteristics
Signal Type +5 V TTL compatible
Logic level low < 0.4 V while sinking 4 mA
Logic level high Between 2.4 V and 5 V output while sourcing 200 µA
High-state output impedance
1 k
Ω from output to common
PWR_OK delay 100 ms < T
3
< 500 ms
PWR_OK rise time
T
4
≤ 10 ms
AC loss to PWR_OK hold-up time T
5
≥ 16 ms
Power-down warning T
6
≥ 1 ms
2.3.2 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 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 should not
deliver current and should be held at zero potential with respect to ground. PS_ON# has no effect on the +5
VSB output, which is always enabled whenever the AC power is present. Table 14 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
10ms to 100ms during the decay of the power rails.