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LSF0204: Questions Regarding Design Requirements

user446310155
Intellectual 460 points

Part Number: LSF0204

Hi there supporter,

I've read SLVA675B, the application note for LSF family, and have following questions about LSF0204,

1. Seems the threshold voltage of the reference passgate and the channel passgate are the same. So, since the gate voltage will be applied as Vref_A+Vth, a little less than Vref_A present on either side of the channel will make the passgate turned on, right? Why is it necessary to request the input low shall be below Vref_A-0.5V?

2. The on resistance of the passgate is proportional to temperature. However, I cannot find any data on the datasheet talking about this. Do you have any figures about this?

3. Finally, why Vref_A less than Vref_B-1V is necessary?

Regards,

S.P. Lin 

  • 0
    TI__Mastermind 25235 points
    Hello S.P.

    The correct applications contact will get back to you shortly on this.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Intellectual 460 points
    Hi Michael,

    Thanks. So, I can just wait and see and no need to put this question to other forum, right?

    Regards,
    S.P. Lin
  • 0
    TI__Intellectual 2100 points

    Hey S.P.,

    S.P. Lin said:
    Why is it necessary to request the input low shall be below Vref_A-0.5V?

    Your description is correct on how the part operates. The reason for this is to take internal differences in the chip into account and to make sure that you give yourself enough headroom for proper operation. Relying on a few millivolts on high speed switching signal to make sure that you switch when you should is not the best practice. Giving this bit of headroom will make sure that you can always operate your part properly and that it won't interrupt the signal path.

    S.P. Lin said:
    However, I cannot find any data on the datasheet talking about this. Do you have any figures about this?

    I have not seen anything that specifically mentions these figures. I will take another look through just in case I missed it in my searching.

    S.P. Lin said:
    Finally, why Vref_A less than Vref_B-1V is necessary

    B needs to be greater than A so that the switches can be biased properly and actually switch. Without that extra volt they might not be able to overcome stray voltages, silicon imperfections, and diode or gate voltage drops.

    If this answered you question please hit the green "Verify Answer" button below. If not, just let us know and we'll help you out!

    Best Regards,

    Nick

  • 0 in reply to Nicholas Mariani9
    Intellectual 460 points
    Hi Nick,

    Thanks for your replies and I am looking forward to any data regarding turned-on resistance v.s. temperature.

    Regards,
    Lin, S.P.
  • 0 in reply to user446310155
    TI__Mastermind 25235 points

    Hello S.P.,

    Please see typical values in the datasheet for Ron at 25C
    We were able to test the device on the bench and we can provide the following data regarding 3.3V to 1.8V at 15mA pass current:
    CMOS Rdson will increase with increasing temperature.
    -40C typical value is 4.3Ohms
    25C typical is about 5.7 Ohms
    125C typical is 8 Ohms

    Additionally, if you are interested in evaluating this device further yourself, we have an EVM available to test any LSF device including the LSF0204:

    http://www.ti.com/tool/LSF-EVM

    I hope that this helps.
    Best,
    Michael