1.  Field of the Invention
The present invention relates to a power good signal generating circuit for a motherboard. 
2.  Description of Related Art
PS_ON (power supply 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.  When PS_ON is pulled to TTL low, the power supply should turn on full power.  When PS_ON is at a TTL high level, the power supply turns off all of the voltages except the 5-volt standby voltage (5V_SB).  Once the power supply voltages are stable, the power supply will signal that fact to the motherboard with the PWR_GOOD (power good) signal.  When the PWR_GOOD signal is at a TTL high level, the motherboard should start operating. 
According to the ATX (Advanced Technology Extended) standard, the power-on time which is defined as the time from when PS_ON is pulled TTL low to when the motherboard receives the PWR_GOOD signal must be from 100 ms-500 ms.  Otherwise, the motherboard does not start. 
What is needed, therefore, is a power good signal generating circuit which can solve the above the problem.