Vcc delay
It was with humble spirit and a good dose of Mea Culpa that a semiconductor company, from whom some very large-scale digital large-scale integration (LSI) chips were purchased, had a problem (later corrected, thank goodness) in that their chips would malfunction when powering up if their +5V rail voltage rose too slowly as the system was being turned on.
The vendor’s recommendation was to apply a 0 V (off) to +5 V (on) rail voltage with a steeper rise time (< 45 ms) than our power supply could deliver. We decided that we needed a switching arrangement that would operate as follows in Figure 1.
Figure 1 Providing a steep +5-V rail voltage rise time.
One problem with making something like this was that the input voltage could indeed rise very slowly through ½ volt to 1 volt to 2 volts, and so forth, which were voltage levels that were well below specification limits for any voltage monitoring IC we could find.
The resulting operations were erratic and unpredictable at arbitrarily low input voltages. This did not help the LSI situation even one little bit. (Yes, I am aware of the pun.)
Remedy was achieved using the following circuit in Figure 2.
Figure 2 Rail voltage switch, four loads.
The result obtained was as follows:
Figure 3 Rail voltage delay and rise time speedup.
This worked predictably down to arbitrarily low power supply voltages because there would be no response whatsoever, as long as the TLV431 didn’t see some voltage high enough to get itself conducting.
When the power supply voltage did get high enough to turn on the TLV431 at the time we’re calling “t1”, the power MOSFETs would turn on, and there would be a downward but very short-duration transient voltage drop from the power supply, which would be recovered from very quickly. The rail voltage thus presented to the LSI chips had a sufficiently quick rise time of its own to make those chips happy.
The end result made a bunch of human beings happy, too.
John Dunn is an electronics consultant and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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