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TPS3808: TPS3808G50 CT connecting to VDD

Guru 16770 points
Part Number: TPS3808

Hi

If Ct is connected to VDD directly, how does TPS3808G50 behave?

is there possibility to damage the device?

BestRegards

  • Hi

    I would provide additional information.

    -> VDD=5V.

    The absolute maximum rating shows VDD+0.3V, so I think connecting Vct to VDD directly would be accepted under normal device operation.
    Of course, I understand the margin of 0.3V should be avoided if possible.

    But I don't know how much Vct current could be accepted without device damage.

    BestRegards
  • If connecting Ct to Vdd, a resistor from 40kohm to 200kohm must be used. The supply current isn't affect by choice of resistor but there needs to be a resistor to prevent direct shorting of Vdd to Ct pin. This configuration sets the delay time to 300 ms. If you leave Ct unconnected, the delay will be 20ms. You can also use a capacitor at this pin to program the delay between 1.25ms and 10s. I hope this answers your question!

    -Michael
  • Hi Michael

    Thank you for your answer.

    Sorry, I shall inform the IC name correctly. It is TPS3808G50-Q1.

    For FMEA, my customer wants to confirm the behavior when CT pin is shorted to VDD directly.

    Could you please tell me why the resistor is needed between VDD and CT pin?

    Isn't it the current limit?

    I would take the external circuit to avoid damaging device if shorting CT and VDD leads to device damage.



    BestRegards

  • Hi Michael

    Let me explain the problem case more detail, and please check.

    Normally, the device works like the following left figure.

    And the situation we should avoid is the right one.

    As workaround, we are considering inserting resistor like as the following.

    We believe that the device won't be damaged if assuming short factor using this way.

    However, it will connect 40Kohm-200Kohm resistor between SENSE and VDD.

    Do you think there is any bad effect for SENSE monitoring by inserting resistor like this?

    BestRegards

  • That will not work. Please use a configuration shown below:

    Notice that CT isn't directly connected to VDD or SENSE in any of the configurations. CT only has 3 possible configurations as shown above. The SENSE pin can't be directly connected to CT because the SENSE is monitoring the voltage on that pin and will trigger reset if the voltage drops below Vit but CT is being charged and discharged to create the internal delay for the reset delay timing.

    Why can't you use configuration (a)?

    -Michael

  • Hi Michael

    Thank you for your reply.

    >Why can't you use configuration (a)?
    Now, VDD pin and SENSE pin are connected to VDD.
    So, I think short between CT and VDD is unavoidable if short factor is assumed, even if we use configuration (a).

    That's why customer’s idea is to use (I) configuration (following figure).

    The customer wants to avoid device damage.

    So I want to ask you if the system would work in (I) configuration, without impact.

    (II) is the unusual situation when problem occurs.

    Of course, I think it won’t work well when (II) condition.

    However, I think the device will avoid the situation of damaging device in short condition
    of CT and SENSE (CT and VDD in otherwords).

    BestRegards

  • In the above (1) case which is a valid configuration and will work fine, they will get 20ms typical delay after SENSE pin cross the ~5V threshold (slightly less than the 4.65 threshold found in the datasheet for the 5V version device due to the small voltage drop across the resistor).

    In the (2) configuration where CT is tied to SENSE and pulled to VDD through 40k to 200k, SENSE pin will get shorted to ground through CT pin for 200us to 300us at the start (this is the operation of CT each cycle to check to see if there is capacitor, open, or pull-up).

    My guess, this short will make the internal SENSE comparator to retrigger and timing delay cycle may not be completed to release the open drain RESET pin.

    Part will not get damage as long as CT has 40k – 200k to VDD but (2) configuration may not work consistently for 200ms delay.

    So to summarize, (1) normal operation will work and the device will reset when VDD drops slightly under 4.65V and the reset delay will be ~20ms. For (2) issued situation, the device won't become damaged but might not work consistently and the reset delay will be 200ms when/if it does work.

    I hope this helps answer your question!

    -Michael