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CD4504B: Input voltage thresholds when level shifting

David Jeckells
Prodigy 10 points
Part Number: CD4504B
Other Parts Discussed in Thread: CD4093B

Hi,

Background:

I am using the CD4504 to level shift the inputs to my circuit, from the voltage level of a selectable power supply, that can either be 15V, 5V or 3V3, to 3V3 (the inputs will always be driven from the same voltage as the selectable supply).  I need to meet both TTL and CMOS input thresholds for my application (obviously 15V TTL isn't a valid condition).  The circuit for the inputs is shown below.  The signals TTL_EN is driven from the same supply as the selectable power supply.

I am also using the CD4504 to level shift my outputs from 3V3 to the selectable supply level, so again 15V, 5V or 3V3.  The circuit for the outputs is shown below.

Speed and power are not critical for my application, but the L->H and H->L switching voltages are critical for noise immunity.  The 3V3 signals are the ones that interface with my raspberry pi.

Each IC has local 100nF decoupling between each of the supplies and there's good bulk decoupling (e.g. 10uF) spread around the board.

The problem:

When I vary the input voltage to test the V_IL and V_IH levels, I notice that there is no hysteresis.  The output switches at around 1.18V and there is no hysteresis (I'm running CMOS to CMOS) and the switching voltage is pretty much the same for all input supply voltages (i.e. Vcc=15V, 5V or 3V3).  For all inputs Vdd is 3V3.  When I shift to TTL mode for the inputs, the threshold voltage for L->H transition and H->L transition both drop to around 280mV, making it practically useless from a noise immunity perspective.

The level shifting for the outputs shows the same issue, except the voltage for switching changes and at Vdd=3V3 it's around 1.08V, at Vdd=5V it's around 1.62V and Vdd=15V it's around 1.47V.  This time, Vcc=3V3 for all outputs.

These results are different to what's in the datasheet, for V_IL(max) and V_IH(min).

Attempts to resolve:

The CD4504 datasheet states minimum recommended operating voltage of 5V, however the graphs indicate operation at 3V3 and other threads (e.g. https://e2e.ti.com/support/logic-group/logic/f/logic-forum/1116554/cd4504b-ep-can-cd4504b-ep-convert-15vin-to-5vout?tisearch=e2e-sitesearch&keymatch=CD4504B#) also support the principle of what I'm doing.  For the time being, I suspect the design is OK.  However, I have adjusted the supply of one input chips to shift from 5V to 5V, just in case the 3V3 operation was the issue, but the problem remains.

I've checked that the parts fitted are correct and the code on the IC matches what I expect to see.  The parts were bought from Mouser, so I'm not expecting any fake IC issues.

I wondered about damaged ICs, possibly from unconnected inputs (there shouldn't be any, but you never know when testing if something accidentally gets disconnected with the test hook-up) and so I tested a previously unused channel, which exhibits the same problem.  I also tested a second board that had not been powered before.  That also shows the same issues and I can confirm that that board has been handled perfectly.  Full ESD precaution has been taken when handling all parts, so I don't think that's a likely explanation either.

Call for help!!

Is the phenomenon that I'm seeing a known artefact of the CD4504?

Could there be something wrong with my implementation, configuration or operation?

Is there a more suitable device that you would recommend for my application?  Ideally it would be pin compatible, because we have had PCBs built, but that's not the end of the world...

Many thanks in advance!!

  • Guru 311760 points

    You should see the typical switching thresholds as shown in figures 9 and 10. How exactly are you testing this?

    Pages 1 and 4 contradict each other regarding the polarity of the SELECT input.

  • in reply to Clemens Ladisch
    Prodigy 10 points

    I've used an external voltage supply and increased the applied voltage by slowly turning the dial.  The return lines are connected so there's no offset voltage.  I have also used my AFG to generate a 1Hz triangular wave, ranging from 0V to 5V and observed the same response.

    Good spot about the contradiction.  I'm not sure it's the root cause of my issues, but it's worth noting.

  • in reply to David Jeckells
    TI__Mastermind 35696 points

    Hey David,

    For clarification, the polarity of the SELECT pin would require VCC to be pulled high for CMOS mode, whereas tying low via VSS would indicate TTL mode. 

    Regards,

    Jack 

  • in reply to Jack Guan
    Prodigy 10 points

    Hi Jack,

    Thanks for commenting.  It's useful to note that the  information on the datasheet is inconsistent, but at the moment, I'm not sure this is very relevant my problem.  As I described, I observe the same behaviour in both TTL and CMOS modes, where the V_IH is basically the same voltage as V_IL, so there is no hysteresis...

    Kind regards,

    David

  • in reply to David Jeckells
    Guru 311760 points

    This device indeed does not have hysteresis. The datasheet guarantees only that the switching threshold is somewhere between the specified VIL and VIH values. (And the typical values are shown in figures 9/10.)

    From what I understand, you measure thresholds that are below the specified VIL?

  • in reply to Clemens Ladisch
    Prodigy 10 points

    Hi Clemens,

    Thanks for your reply.

    I'm not sure I understand how the device does not have hysteresis.  For example, using 5V CMOS as an example case, V_IL(max) is 1.5V and V_IH(min) is 3.5V (these numbers are taken from the STATIC ELECTRICAL CHARACTERISTICS table of the CD4504 datasheet).  Therefore the highest input potential that can register a logic low output, is 1.5V, and the minimum input potential that can register a logic high output, is 3.5V.  This hysteresis is defined in the datasheet...

    At the moment, the problem in my case can be thought of as more of an issue with V_IH(min).  At the moment, my device outputs a logic high level when the input is significantly below V_IH(min).

    Kind regards,

    David

  • in reply to David Jeckells
    Guru 311760 points

    The specification "VIL(max) = 1.5 V" means that all input voltages up to 1.5 V are guaranted to be interpreted as low.
    Similarly, "VIH(min) = 3.5 V" means that all input voltages down to 3.5 V are guaranted to be interpreted as high.
    Input voltages between 1.5 V and 3.5 V are invalid, and it is not guaranteed whether you get low or high.

    There is no hysteresis. The two values VIL(max) and VIH(min) specify a range in which the single switching threshold is guaranteed to be.

    For devices that actually have hysteresis (e.g., CD4093B), the datasheet specifies separate ranges for positive-going and negative-going thresholds: