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Circuit offers simpler power-supply-sequence testing

09 Sep 2015  | Dan Karmann

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The voltage level on Pin 7 of IC1 determines the delay, under firmware control, between turning on or off the first and the second power supply. The microcontroller reads this delay voltage with its 10bit ADC and uses the value to determine the delay according to the following equation: Delay=(VDELAY/VCC)×1024×1 msec, where VDELAY is the delay voltage. This equation yields a delay range from a few microseconds to a bit more than 1 second. As an example, if the delay-voltage value from R3 is the midwiper value of 2.5V, the sequencing delay is approximately 512 msec: (2.5/5V)×1024×1 msec. The delay value is approximate because the microcontroller uses its internal 9.6MHz RC oscillator to generate the timing with a simple firmware delay loop.


Figure 2:


A second press of trigger switch S1 causes the power supplies to turn off in the same sequence and with the same delay with which they turn on. A constant off_sequence can be changed to alter the turn-off sequence with the second press of S1 (figure 2). This constant OFF_SEQUENCE is currently SEQUENCE_same to operate as the original Design Idea did, but if you set the OFF_SEQUENCE to SEQUENCE_REVERSE, the turn-off sequence will be in the opposite order of the turn-on sequence. Alternatively, if you set the constant off_SEQUENCE to SEQUENCE_none, both power supplies will turn off at once. This feature exemplifies the versatility of this follow-on Design Idea with a simple firmware change. Because the circuit uses only about half the code space in the ATtiny13, you could easily add other desired changes. Although this circuit uses an Atmel microcontroller, you can use almost any low-pin-count microcontroller with a built-in ADC. However, other brands may not have the readily available free development tools that exist for Atmel devices.


Reference
Ban Hok, Goh, "Circuit eases power-sequence testing," EDN, July 9, 2009, pg 50.


About the author
Dan Karmann is with DL K Engineering.


This article is a Design Idea selected for re-publication by the editors. It was first published on December 3, 2009 in EDN.com.


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