• State TI Thinks Resolved
  • Locked Locked
  • Replies 8 replies
  • Answers 2 answers
  • Subscribers 62 subscribers
  • Views 529 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LP38691: LP38691DTX-5.0/NOPB as alternative for LDFM50DT-TR

Manan Mehta
Prodigy 30 points

Part Number: LP38691

Hi,

In my current design, I am using LDFM50DT-TR LDO (Digikey link) for 5V output which acts as a source for RFID transceiver circuitry. LDFM50DT-TR is preceded by MEV3S0505SC (Digikey link) whose output goes to LDFM50DT-TR, please check the attached image for more of the circuit diagram.

I am planning to replace LDFM50DT-TR with LP38691DTX-5.0/NOPB after going through the LP38691DTX-5.0/NOPB datasheet, it seems LP38691DTX-5.0/NOPB can be a viable replacement for LDFM50DT-TR but my concern is will work with an input voltage range of 5V ~ 5.5V ? since in the datasheet it is mentioned Vin = 5.5V ~ 6V does it mean the minimum input voltage has to be 5.5V)

I have also attached a comparison table of LDFM50DT-TR & LP38691DTX-5.0/NOPB for reference.

Please provide us with your suggestion on the viability of the LP38691DTX-5.0/NOPB or TL720M05QKVURQ1 as a replacement for LDFM50DT-TR or another suitable alternative.

Thank You,

Manan Mehta

  • 0
    TI__Expert 5656 points

    Manan,

    Not sure where you got this information from. As per the datasheet for LP38691 the min. required Vin is 2.7V.

    We also have other options, please see comparison tables below.

    Thanks,

    Prakash L

  • 0 in reply to Prakash Lakhit
    Prodigy 30 points

    Hi Prakash,

    Please refer to this schematic, LDFM50DT-TR is getting its input from MEV3S0505SC (Digikey link), and MEV3S0505SC provides output in the 5V ~ 5.3V range (this I have measured) primary use of LDO is to reduce the ripple presents on the 5V ~ 5.3V output of MEV3S0505SC, for which LDFM50DT-TR was chosen since it has very low dropout voltage and I need 5V output from LDFM50DT-TR too which goes to sensitive RFID circuit.

    In the table that is why i have mentioned the minimum input voltage for LP38691DTX-5.0/NOPB as 5.5V (accounting output from MEV3S0505SC and the minimum dropout voltage of LP38691DTX-5.0/NOPB) 

    My question should have been 'Will input of 5V ~ 5.3V to LP38691DTX-5.0/NOPB or TL720M05QKVURQ1 provide output of 5V' like I am observing in the case if LDFM50DT-TR? 

    Thanks,

    Manan Mehta

  • 0 in reply to Manan Mehta
    TI__Expert 5656 points

    Manan,

    Ok, that makes more sense. Could you please share the load current here? 

    As per EC table, for 100mA the typical dropout is around 45mV (which is approx. 1% of 5V). So may be 1% is well with in the tolerance of the expected variation on 5V rail and this solution is usable.

    Thanks,

    Prakash L

  • 0 in reply to Prakash Lakhit
    Prodigy 30 points

    Hi Prakash,

    I measured the load current of the RFID circuitry which is powered by the LDO and it sinks 300mA.

    Also, the input voltage to the LDO is 5.2V 

    Let me know if there is any more data you require.

    Thank You,

    Manan Mehta

  • 0 in reply to Manan Mehta
    TI__Expert 5656 points

    Manan,

    This should work, checking with our applications engineer.

    Thanks,

    Prakash 

  • 0 in reply to Prakash Lakhit
    TI__Expert 5656 points

    Manan,

    Update from Applications engineer:

    Dropout is usually linear with load, so I think the best we could do is linearly interpolate the 2 max data points we do have for the 5-V device, which would give VDO (max) @300mA = 100mV + (300mA – 100mA)*(450mV – 100mV)/(500mA – 100mA) = 275mV. So whether it would work would depend on the operating temperature, but we don’t have sufficient data to narrow it down further. Some devices will probably work but certainly not all of them will.

    Thanks,

    Prakash L

  • 0 in reply to Prakash Lakhit
    Prodigy 30 points

    Can you please explain a bit more about this equation 

    VDO (max) @300mA = 100mV + (300mA – 100mA)*(450mV – 100mV)/(500mA – 100mA) = 275mV.

  • 0 in reply to Manan Mehta
    TI__Expert 5656 points

    Manan,

    It’s just a linear interpolation equation. I didn’t pull it from anywhere; I just constructed it. You want to interpolate between 2 data points, so you take the first point (100mV) and add to it the slope (rise/run = (450mV – 100mV)/(500mA – 100mA)) times how far into that region you are from the beginning of the region (300mA – 100mA).

    Regards,

    Prakash L