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LM2903: Design check

Guru 11545 points

Replies: 15

Views: 345

Part Number: LM2903

Hi Sirs,

Sorry to bother you.
We measure the impedance of R0507 is 9.137Kohm.

We think that because it's paralle OUT of the OPA
The OPA out is the ideal voltage VCC.

So the direct measurement of the impedance of R0507 will be 10Kohm parallel (5.9Kohm with100Kohm) to calculate, get the impedance of 9.137Kohm? Am i wrong?


We also like to know are there other better design for us?

  • In reply to Shu-Cheng LIN:

    Using a mosfet to invert the output of the comparator would be a clever trick.

    Is there a reason why you would like to use the LM2903? Would you be open to other options?

    It seems like you are not using the second channel on the device. Also, I noticed that all of your supply rails in your schematic are the same, so a push-pull device would be able to achieve the same result. You could use that pullup resistor as your drain load for your inverter instead :) 

    I would recommend taking a look at the TLV7031, which is our low voltage nanopower push-pull comparator. It comes in leaded or space-saving leadless options, and could allow you to conserve more power than the LM2903. There is also an open drain option (TLV7041) if you would like to stick with an open drain part.TLV7031_ThermistorApp_Inverter.TSC

    I've attached a schematic using the TLV7031 to meet your Vtl/Vth specifications. Please let me know what you think of this possible solution.

    Regards,

    Jonny

  • In reply to Jonathan Nguyen:

    Hi Sirs,

    Thanks for your reply

    Please refer our reply as red word.

    So did you have more suggestion for us?

    Is there a reason why you would like to use the LM2903?

    Actually, we don’t have a special reason for using LM2903.

    We just consider below:

    (1)LM2903 is implemented of our rev_1.0 design, and we can rework it for verification before next phase.

    (2)Material preparation.

    (3)Does it already exist PEGATRON part number?

    (4)We have to explain to our customer why change LM2903 to TLV7031

     

    It seems like you are not using the second channel on the device.

    Yes.

     

    By the way, 

    Do you recommend us to change using TLV7031 because below reasons?

    (1)Low voltage nanopower push-pull comparator

    (2)We only use 1 channel. Using TLV7031 to save spacing and cost.

  • In reply to Shu-Cheng LIN:

    Yes,

    I believe your design suggestion should work for the application.

    We created this document to help design inverting hysteresis: SNOA997_TLV7031.pdf

    Feel free to use it to help with your design.

    Regards,

    Jonny

  • In reply to Jonathan Nguyen:

    Hi Sirs,

    Thanks for your reply.

    Sorry to bother you again.

    We used LM2903DGKR and rework it to Inverting Comparator with Hystersis. Please refer below sch.

    We tested it and get below result.

    When Vin(ACDC_PSU_TEMP)<VTL, OPA out is high. The output of OPA is only about 0.7V. Is it because LM2903DGKR not a  push-pull comparator?

    I add/stuff R1313(please see below blue mark).

    When Vin(ACDC_PSU_TEMP)<VTL, OPA out is high. The output of OPA is about 3.3V.

    Our original issue:

    Using non inverting comparator with hystersis and  the output is the ideal voltage VCC due to the characteristics of OPA.

    Therefore, the equivalent impedance of ADC_PSU_TEMP to DK_3V3 is R0507//(R0512+R0513), so it is 9.137Kohm. It’s not 10K Ohm

    When we use below inverting comparator with hystersis design, The equivalent impedance of ADC_PSU_TEMP to DK_3V3 is R0507:10K Ohm.

    We just want someone can help us check and confirm it.

    Many thanks!!

  • In reply to Shu-Cheng LIN:

    Shu-Cheng,

    Yes, in your first image you provided, the output only reached 0.7V because the comparator has open drain output and requires a pullup resistor to operate correctly. Be sure to note that having a pullup resistor will affect your hysteresis network slightly. Take a look at this newest cookbook for designing inverting comparator hysteresis: http://www.ti.com/lit/an/snoa997a/snoa997a.pdf

    Referring to your previous post, you could absolutely use the TLV7031 in this application for its space saving and cost. Sorry I missed that previously.

    Regards,

    Jonny

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