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LM25180: Questions about LM2518x series

Tachibana Masanori
Prodigy 100 points
Part Number: LM25180


Hello support team,

Now, I 'm studying LM2518x series for isolated DC-DC converter.
And I have some questions about them.
So could you tell me about the following?

[Questions]
1. About switching MOS FET (pin 1)

Is there any problem if the waveform of pin 1 does not exceed the following values under any conditions?
  Vds : 70V (Absolute maximum rated voltage)
  Id : Maximum value of ISW-PEAK

Is there a restriction on the drain loss (the product of the drain voltage value and the drain current value)?
Is it no necessary to pay attention to it?

2. About RSET

What is the side effect if the resistance value (12.1 kΩ) of RSET changes?
(For example, when RSET is 20% smaller etc.)
However, I understand that the output voltage changes, so please teach me about the case where the ratio of RSET and RFB is not changed.
Of course, I also understand that such way to use is not recommended.

Best regards,
M. Tachibana

  • 0
    TI__Mastermind 34875 points

    Hi Masa

    Let me check and update you later.

  • 0
    TI__Guru 74485 points

    Hello

    The absolute maximum rating of the FET is 70V.

    It is not recommended to operate the device with a SW voltage of 70V continuously.

    The maximum recommended continuous SW voltage should be limited to 65V.

    The peak FET current is limited by the current limit specification in the data sheet EC table.

    A variation of 20% in the RSET value should not cause any issue, as long as the ratio between

    RSET and RFB is kept constant.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 100 points

    Hello Frank-san,

    Thanks for teaching me.
    To make sure, can I ask you again?

    1. Is it no necessary to pay attention to drain loss of the Power MOSFET?
    Is it OK as long as the conditions of Vds <65V and Id <ISW-PEAK (for example, Vds<65V and Id <1.73A for LM25180) are maintained in all operating conditions?

    2. Why 12.1 kohm of resister is recommended?
    Is there any proble if 12 kohm or 10 kohm of resister is used for RSET?

    Best regard,
    M. Tachibana

  • 0 in reply to Tachibana Masanori
    TI__Guru 74485 points

    Hello

    If the voltage and current are kept below the specified values, then no damage will occur.

    The conduction loss in the FET can be calculated under normal conditions by using the RMS FET current and the FET Rdson.

    However, a more accurate calculation of FET loss can be found from the calculator tool on the product folder for this device.

    The value of 12.1k gives a current of about 100uA; the control circuit is optimized for this value.  However, a change of resistor value

    of +/- 20% should not cause a problem.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 100 points

    Hello Frank-san,

    Thank you for your support.
    I understand.

    Best regards,
    M. Tachibana