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TPS78227 Ground pin

alain rimlinger
Prodigy 30 points
Other Parts Discussed in Thread: TPS782

Hello

Datasheet of TPS78227 shows that we need to connect both ground pins together.

Part is pin to pin compatible with an On semi Ldo except on one ground pin taht is left open on the On Semi part.

PCB is already done with On Semi part, but we would like to use the Ti part instead.

Question 1 :

Can we leabve one of the ground pin open ?

Question 2 :

If not, can we tie together the ground pin with a 0.1ohm resistor made with a strap in 0603 format (datasheet of the 0 ohm strap specify a max resistance of 0.1ohm).

What would be the influence of the 0.1ohm resistor if any on the stability ?

 

Thank you very much.

With best regards, Alain

  • 0
    TI__Expert 8890 points

    Hello Alain,

    The data sheet says that you need to connect both pins together for "proper operation".  My suspicion is however that one of the grounds is dedicated to the active pull down circuit and the other to contriol/analog circuitry -  but unfortunately I don't know which pin is which.   I'll need to check on this matter and then you will need to decide whether you can use the part minus some functionality.

    I don't understand the 0.1ohm resistor.  However, if this resistor is placed in series with the analog ground I would speculate that this would act as an ESR added to the output capacitor constituting a frequency domain zero in the control loop - and this would have some, perhaps minor, effect on the ac operation/performance.  There may be other effects as well.

    Regards

    Bill

  • 0 in reply to Bill Stokes
    TI__Prodigy 570 points

    Hello Alan,

     

                  What On-Semi part are you referring to? and also, what package for TPS782 are you planning to use?

  • 0 in reply to Victor Ceron
    Prodigy 30 points

    Hello Victor

    Competition part is NCP561SN27T1G from OnSemi in sot23-5 package with one pin left open.

    Footprint of TPS78227 is similar but the NC pin on the OnSemi part needs to be connected to the ground on TPS78227 part.

    Idea is to connect the ground through a 0 ohm strap in 0603 smd format, but this strap has a 0.1ohm resistance in the real world.

    Question is what would be the influence of this resistance on the noise and stability of the regulation ?

    As the current is very low, I would expect no difference, but I would like to get an official confirmation.

     

    Regards, Alain

  • 0 in reply to alain rimlinger
    TI__Expert 8890 points

    Hello Alain,

    It seems to me that the N/C pin for the ON Semi part has no internal connection (see Pin Function Description table, page 2)  and so pin 4 can be connected to ground with no implications. 

    Bill

  • 0 in reply to Bill Stokes
    TI__Intellectual 1080 points

    Hi Bill,

    This is a re-visit but did we ever find out the internals of the TPS782xx GNDs?

    I have a customer that wants to leave the "second" GND (the one that is normally the FB pin) floating in order to dual footprint with an adjustable device without having to stuff a zero-ohm to GND.

     

    Regards,

    wes.

    Wes Wirth
    AFA - Redmond, WA

  • 0 in reply to Wes Wirth
    TI__Expert 3625 points

    Hi Wes,

    I checked with the Project Manager, Peng Wu, and both grounds (pin 3 and pin 5) on the DRV package must be grounded for the 2.7 and 2.8V TPS782 parts. Otherwise, the output voltage could float between two different values. The TPS782 2.5V and 3.0V parts however, are not known to have this problem. However, leaving either of the ground pins floating will increase the Iq of the part. Also, leaving the ground pins floating could alter the performance of the part, which we cannot guarantee.

    Regards,

    Fern