Power down function

Posted on Apr 22, 2012

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 missequencing can be eliminated Bringing the power down control high (+5V) applies power to the controlled circuit by turning on all MOSFETs. Specifically, raising the power down brings the output of U1C low, causing capacitor C1 to discharge VOL exponentially with time constant R1C1. As the voltage on C1 falls, two events occur.

Power down function
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In the circuit, MOSFETs Q1 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 isapplied is important: The controlled circuits may be dama

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