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LP38855: LP38855 Bias Pin and Enable Pin, Analogue Ground

shadders
Expert 1065 points
Part Number: LP38855

Hi,
I wish to use the LP38855 voltage regulator to provide 1.2volts for the DSP Core. The datasheet mentions that the Bias pin needs to be connected to 3volts to 5.5volts, and other forum posts state that you can connect it to the 3.3volts input voltage.

Therefore, is it ok to connect the Enable pin to the Bias pin, which is also connected to the 3.3volts input voltage supply ?

Is it prudent to implement the reverse voltage diodes from output pin to ground and output to input pins ?

The datasheet states on page 9 that "The output capacitor must be located less than 1 cm from the output pin of the IC and returned to the device ground pin with a clean analog ground."

Since the product can be used for digital IC's as per the datasheet application list page 1, then why must the ground pin be connected to a clean analog ground ?

Will a digital ground plane suffice ?

Thanks and regards,
Shadders.

  • 0
    TI__Mastermind 29260 points

    Hi Shadders,

    Please find my answers to your questions below.

    1. As per the maximum ratings table on page 2, you can connect up to 6V to the enable pin.  It does not matter if this is from Vin, Vbias, or another voltage supply.

    2. One nice feature on the LP38855 is the lack of a body diode in the pass element.  The only way you can have reverse current is if the pass element is enabled, which happens when the EN signal is high or if Vbias is above the UVLO threshold.  Unless you are operating in these conditions you will not need reverse voltage diodes.  Many customers do not need the reverse voltage diodes as their operating conditions will never have this issue crop up.  You can always place them into the design and list them as "do not populate" so if you reuse the design in the future you can easily populate them into the board if the new design needs them.  Lastly the datasheet states that reverse current should be avoided.  This is because when reverse current is flowing it is often under conditions which remove current limiting and thermal limit circuitry (as they are often unpowered).  So continuous reverse current should be avoided as conditions which cause reverse current often turn off the internal protection circuitry. 

    3. If the LDO is being used to power digital electronics, then a digital ground plane will suffice.  You will want a large, fat ground plane (typically layer 2 of the PCB is entirely GND) so that the loop area of power to ground is small (which provides the smallest inductance and best layout for digital signals).

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Expert 1065 points

    Hi Stephen,

    Thanks for the reply and confirming the solutions. Much appreciated.

    Regards,

    Shadders.

  • 0 in reply to shadders
    TI__Mastermind 29260 points

    No problem, let us know if you need anything else.

    Thanks,

    - Stephen