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Tying 74xx05 and 74xx12 together via resistors?

Mark Underwood
Prodigy 30 points

Hello.  I'm trying to figure out why my Bad Thing(tm) meter is going off, and I can't quite put my finger on it.

We have a situation with a proprietary open-collector low frequency bus.  A number of devices may have their logic inputs connected to ("reading") this bus with 10K (or greater) pullups to the device's supply rails.  Some of the devices operate at 12V and some at 5V.   Basic assumption that both rail voltages are being generated by 78xx type LDO regulators.  Some of the devices are in-house design, but some are third-party legacy gear that we will have to accommodate.

So far, best I can figure is that since the LDO outputs are connected through two 10K (min) series resistors, the actual bus voltage will be halfway between the supply voltages (~ 7.5V assuming a 7812 and a 7805), and there will be a small current (~ 0.35mA) flowing from the 12V through the pullups into the 5V circuit load, reducing the load slightly on the 5V regulator and increasing the load on the 12V regulator.  I want to think that the resistors between the outputs will allow the two regulators to operate independently, but something tells me I am missing something.

I've read plenty about how it is a bad thing to directly tie the two different LDOs outputs together, but is it also bad to allow this path through the pullups?

Thank you!

- Mark Underwood

  • 0
    TI__Expert 8905 points

    Mark,

    Do I understand your question correctly in that you are trying to set a common bus voltage of 7.5V, or that you are trying to OR the supplies together?

    Could you also give me the full part numbers to ensure that I am looking at the right parts?

    Thank you, 

    David

  • 0 in reply to David Hopkins
    Prodigy 30 points

    Well, no.  I'm not trying to set the bus voltage... I just see that as one possible side effect of sharing the bus between varied pull-up voltages -- the voltage divider effect of the two pull-up resistors between the two different rail voltages.

    I'll try to share more without writing a thesis...

    We have a distributed system, with nodes interconnected by a common communications bus.  Detector nodes will drive one of several bus wire low (active) or let the wire float (inactive), while Listener nodes will monitor the bus and take action if it is seen to be driven low.  Somewhat analagous to I2C, electrically but simpler.  The Detector node outputs are open collector, and the Listeners have high impedance inputs to their logic inputs.... really just a FET with a pullup on the gate.

    Because the network is assembled ad-hoc and can be reconfigured a bunch of different ways, there is not really an option to have, say, a single dedicated "master" node with one set of pullups to a selected voltage.  So each Listener node has its own power supply and its own set of pullups on its inputs.  All nodes share a ground reference through the bus.

    The legacy third party devices operate at 12V, so the Listeners all have pullups to 12V on their logic inputs.

    This presents a problem for newer devices which are designed around 5V or 3.3V logic.  Level-shifting the inputs from the 12V bus is easy enough, but adding a separate 12V rail just for the bus pull-ups adds cost and complexity that the designers would prefer to avoid.

    So we end up with the situation of two adjacent Listener nodes, Node X and Node Y.  Node X is a legacy device with pull-ups to 12V and Node Y is a newer 5V device, with its pull-ups to its 5V rail.  This creates a situation where there's a current path from the 12V rail through the two pull up resistors (one on each Node) to the 5V rail.  This is what I was trying to describe in the OP.

    Assume for discussion a LM78L05 and LM78L12 with 10K pullups between the output voltage and the bus wire, though in practice it could be any of the 78xx history of regulators.

    I know that directly connecting the outputs of the two regulators would be a very bad thing, but it seems that with the pullup resistors in between, this would allow the two regulators to... regulate... independently and just cause a small trickle current through the pull-ups, slightly reducing the load on the 78L05 and increasing the load on the 78L12.  

    The question is, is there something else about this arrangement that makes it a Bad Idea with respect to the voltage regulators themselves, given the constraints that we must support the legacy devices, there's no provision in the bus spec for a reference "high" voltage, and (to complicate things even further) even the legacy 12V was not actually specified, so there are also some 9V devices in the mix, too.

  • 0 in reply to Mark Underwood
    TI__Expert 8905 points

    Mark,


    Thank you for the information. The 12V LDO should be stable, however these LDO's are not meant to sink current and arenot able to maintain regulation while sinking a current (a small current should be fine, but any appreciable current could cause errors with the LDO). As such, so long as the 5V LDO is still sourcing more current than is coming from the 12V LDO, it will work as expected. As you have the 5V also running the listening nodes, it will most likely still be stable, although 100kohm pull-ups would lessen the chance of instability as the 5V would only have to sink 35 uA, which would most likely be consumed by the standby current on the listeners.


    Regards,

    David Hopkins

  • 0 in reply to David Hopkins
    Prodigy 30 points

    David,

    Thank you. Your answer makes sense.  Since we are in the process of revising the spec, it would not be difficult to require a higher pull-up value for the lower-voltage devices.

    - Mark