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LP2980IM5-3.3 Regulator clarification

Tim Anderson1
Prodigy 120 points

Thank you in advance for your assistance. Following is a breif description of our application:

This LDO will be part of a circuit we use that will include a PIC Microcontroller (10F202) . We program the PIC serially after the circuit board is fully assembled. During normal operation the PIC supply voltage can be between 2.0 and 5.5 VDC. This fits perfectly with your LP2980 3.3v unit, however… during programming, the PIC Vdd must be at a minimum of 4.5 VDC. We would incorporate a 5-volt regulator in our programming fixture to supply 5 volts to the PIC Vdd and this would also put the 5 volts at the output pin of the LP2980 (only during programming). Our concern is the we need to put a Schottky diode across the output and input pins of the LP2980 as shown on page 11 of the data sheet to protect the LP2980. The input pin of the LP2980 will not be connect to supply voltage during this programming operation, but the LP2980 ground (-DC) pin will be connected to circuit ground. Our plan is to use a BAT54 or BAT750 (lower Vf) for this Schottky diode, but our concern (possible unfounded) is that with the Schottky diode anode on the LP2980 output (at 5-volts) and the cathode at the input, this will allow 5-volts (minus the Vf od the diode) to be at the LP2980 input. As a result this input voltage will cause the LP2980 to act as a regulator and put 3.3-volts on the output (which would already have 5-volts there) We're concerned that forcing the regulator output to a higher voltage than it is designed to provide could damage the regulator. Is this concern valid?

We don't want to have a secondary soldering operation to add a jumper, etc. after the board is assembled and the PIC is programmed.

Again, thank you for your time and any assistance you can provide.

Best regards,

Tim

  • 0
    TI__Mastermind 22610 points

    " ... this input voltage will cause the LP2980 to act as a regulator and put 3.3-volts on the output (which would already have 5-volts there) ..."

    No, that will not happen.

    If the output is set for 3.3V, the feedback loop will shutdown the pass transitor if you pull the output above 3.3V . Since the output stage is designed to source current, not to sink current, the output pin will rise with the applied voltage.

    As described here I don't see that there would be any problem.

     

  • 0 in reply to Donald Jones
    Prodigy 120 points

    Excellent! That is great news. Thank you for your very speedy response.

    I assume however, that we will still need the Schottky diode across the output and input of the regulator during our programming mode (+5V on regulator output). If so, we were planning to use a BAT750 which has Vf of 225mv at 50ma (much more current than will be drawing during programming), a collector reverse voltage (Vr) of 40V, and an If of 750ma. Would this be acceptable?

    Thank you again for your assistance.

    Tim Anderson

  • 0 in reply to Tim Anderson1
    TI__Mastermind 22610 points

    " ... I assume however, that we will still need the Schottky diode across the output and input ..."

    My answer was on the presumption that the diode would be there so the input voltage would be close to Vout value without much reverse current through flowing the device to bias the input and control stages.

    I would need to go to the bench to determine what the behavior would be without the diode.

     

  • 0 in reply to Donald Jones
    Prodigy 120 points

    Hi Don,

    Thank you again for your lightning responses. No need for any benchwork, we can put the Schottly diode in our programming fixture so it is there during the programming process only and not on the finished circuit board. Do the parameters for the BAT750 Schottky diode look like it would do what's needed?

    You have been an enormous help.

    Best regards

    Tim Anderson

  • 0 in reply to Tim Anderson1
    TI__Mastermind 22610 points

    Across the LP2980 input to output? BAT750 is more than adequate.

     

  • 0 in reply to Donald Jones
    Prodigy 120 points

    Excellent!

    Again, thank you for all of your help.

    Best regards,

    Tim Anderson