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UCD3138064: DPWM behavior from power up to manually modify the status

Johnny Chen
Expert 4980 points
Part Number: UCD3138064
Other Parts Discussed in Thread: UCD3138

Hi Champ,

I am asking for my customer.

What would be the DPWM behavior from power up to manually modify the status (set H/L output) by user if there is no pull up to 5V and pull low to GND resistor on external circuit ?

During that time, would we see the pin as a floating status or similar like a high impedance status that depends on the external circuit connection ?

Besides, is there a suggestion written in TRM that tell about this ?

Thanks and regards,

Johnny  

  • +1
    TI__Genius 17090 points

    Johnny, I don't think there is anything specific in the Technical Reference except for the reset value of the control bits for the DPWM.  They are the only I/O pins that are made outputs on reset, and they pull low.  This is great assuming that low means off for the external power supply circuitry, and that's why they were designed that way.  

  • 0 in reply to Ian Bower
    TI__Expert 4980 points

    Ian,

    Thanks for the timely reply.

    While I still not get the direct answer of the first question about what would be the DPWM behavior from power up to manually modify the status (set H/L output) by user if there is no pull up or pull low resistor on external circuit ? Is it able to tell ?

    Cause the customer has seen about a floating voltage (max around 1.8~2V) during the time from power on to user manually output DPWM, and the timing for the voltage shows up and how long the voltage will stay on DPWM pin are random and not fixed at all. While the voltage must gone before the user manually modify the status (set H/L output). This is why I would like to confirm from power on to manually output the DPWM status, what would be the behavior ? And any comment on the floating voltage seen over here ? 

    Thanks and regards,

    Johnny

  • 0 in reply to Johnny Chen
    TI__Genius 17090 points

    When the power is below the minimum logic enable voltage, which is somewhere a bit below 3 volts, the logic is not in control yet, so at that level it may be floating.  On some devices it also waits for the BP18 to be high enough, which is why those devices recommend a bootstrap capacitor between 3.3 volts and the BP18 pin to pull it up quickly.  On later devices which don't recommend this bootstrap, I believe that the pins are pulled low by the 3.3 volt logic, so as soon as 3.3 volts is up then DPWM pins should be pulled low.  

    Check the UCD3138 Practical Design Guideline app not for which devices require the bootstrap cap.  

    When you said power on, I assumed you meant that the power was up to operating level.