Simple Power Down Circuit Circuit

  
This circuit, adds a power-down function to analog I/O ports (for example, the AD7769 and AD7774). Moreover, the diodes ordinarily needed to protect the devices against power-supply mis-sequencing can be eliminated (see the figure).In the circuit, MOSFETs Ql and Q2 switch the +5- and +12-V supplies, respectively, in a sequence controlled by two cross-coupled CD4001 CMOS NOR gates (U1C and U1D). The sequence in which power is applied is important: The controlled circuits may be damaged anytime Vcc exceeds VDD + 0.3 V. Consequently, the NOR gates must be powered from a 12-V supply throughout the power-down sequence.Bringing the power down control high (+5 V) applies power to the controlled circuit by turning on all MOSFETs.
Simple Power Down Circuit Circuit - schematic

Specifically, raising the power down brings the output of U1C low, causing capacitor CI to discharge VOL exponentially with time constant0. As the voltage on CI falls, two events occur. First, it puts a negative gate-source voltage on P-channel Ql, turning it on.Second, it causcs output gate U1D to go high. With the output of U1D high, capacitor C2 charges exponentially to VOHabout 12-Vapplying a positive gate-source voltage to turn on Q2. In the power down mode, the Power Down control is brought low and the RC circuits and their delays work in reverse. Consequently, capacitor C2 discharges to the logic input of U1C before CI can charge. Hence, Q2 turns off before Ql.




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