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Original question:

LM25180: RSET Resistance

LM25180: The RSET resistance question

Ryker She
Prodigy 590 points
Part Number: LM25180

Hi Team,

If the resistance of RSET pin ca be chosen 10kΩ? And what's the result if we use 10kΩ？ I think it doesn't influence the output voltage as below equation. Thanks.

Best Regards,

Ryker.

  • +1
    TI__Guru 71625 points

    Hi Ryker,

    Thank you for posting.  You are right that the referred equation governs the output voltage setting. However, we do not recommend to choose RSET to be different from 12.1k, mainly because the IC is trimmed with RSET=12.1k, and with it you will get the most accurate Vout regulation. 

    Why do you want to change it to 10k?  Isn't 12.1k a standard resistor?

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Prodigy 590 points

    Hi Youhao,

    Thanks for your reply. Yes, customer hasn't 12.1k resistor. But I have asked them to use 12k resistor at least.

    Besides, I have another question of RTC. There isn't TCDiode of customer's Diode datasheet, so how can we determine the RTC by Equation 27 as below?

    And the datasheet of customer's Diode datasheet as below. Thanks

    HRS_35030377_E2DF-F1-S6688HF.pdf

    Best Regards,

    Ryker

  • +1 in reply to Ryker She
    TI__Guru 71625 points

    Hi Ryker,

    First, I don't think 12k is too different from 12.1k, so it should be okay to use.

    Regarding TC, the attached datasheet just have one curve for 25C.  This is not enough to get a clue.  The customer should ask the diode manufacturer to provide additional info like shown below.  At least three different temperatures, then you can estimate the TC.

    Or, they can ignore TC and implement the design without it for now.  After running test (without TC) and check the Vout vs temperature, then calculate backward to get the correction factor by TC pin.

    For instance:  At 25C, Vout=5V under a given load.  and At 85C, Vout=4.9V under the same given load, then the diode TC is about -1.67 mV/C /Nps. 

    Hope this clarifies.

    Best Regards,

    Youhao