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TPS63700: Schematic review

Tomoya Kawamura
Prodigy 10 points

Part Number: TPS63700

Tool/software:

Hi 

Would you help review my schematics,

My requirements, 

Vin = 5V

Vout = -3V

Iout = about 50mA

question 1, Can you please tell me the calculations required for the selection of inductor L100. I currently have 10uH attached. Do you have any suggestions?

question 2, I would like to put a current limiting resistor in the front stage of +5Vin to prevent sudden latch-up. I would like to put in a resistor of about 15Ohm, but in that case, I am concerned about oscillation concerns, etc. by putting in the resistor. Could you please comment on this? Would it be possible to confirm this by simulation?

  • 0
    TI__Mastermind 20870 points

    Hi Tomoya,

    Thank you for reaching out. Expert for this part is out of office until 10/8. Please expect a delayed reply.

    Regards,

    Mulin

  • 0 in reply to Mulin Yuan
    Prodigy 10 points

    Hi Mulin

    Thank you for your response. I appreciate you letting me know about the situation.

    Best regards,

    Tomoya

  • 0
    TI__Guru 72325 points

    Hello Tomoya Kawamura,

    1. Please find the calculation formula for the inductor in the application section of the datasheet.

    2. I would not recommend adding a 15 ohm resistor in series to the VIN. You will have about 1.5W loss on this resistor in normal operation. If you want to make sure that VIN is not collapsing, I would use a supervisor to control the EN pin of the device: Supervisor & reset ICs | TI.com

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 10 points

    Hi Brigitte

    1. I found the formula from the datasheet. As for the requirement for inductor selection, do you mean the required value of ripple current? Since the required value is not clear, if I were to make the ripple as small as possible, would 10uH be acceptable?

    2. The customer has requested that a current-limiting resistor be added, so we would like to include one. I understand that this will cause a voltage drop in Vin. I believe a slight drop in normal operation won't affect the circuit's operation, but was that not the case?

    Best regards,

    Tomoya

  • 0 in reply to Tomoya Kawamura
    TI__Guru 72325 points

    Hello Tomoya,

    1. I do not understand your question here. If you calculate the formula as given in the datasheet, the value should be ok.

    2. To limit the current, I would recommend using some circuit like this one: LMP8646 data sheet, product information and support | TI.com 

    If you are using a 15 ohm resistor and the load is changing suddenly from lowest to highest value, your input voltage will suddenly change at the same time. This can cause instability.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 10 points

    Hi Brigitte,

    1. I was able to calculate using the formula from the datasheet. I understand that the ripple current value and inductor value are OK. Thank you for your advice. I apologize for asking unclear questions.

    2. Thank you for your suggestions on the current limit circuit and IC. I also understand your concern about instability due to load changes when using a 15Ohm resistor. Thank you again.

    Best regards,

    Tomoya