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LM2917-N: LM2917-N

Drew Smith
Prodigy 20 points
Part Number: LM2917-N

Hi. Page 15 of the datasheet shows the output response time to a change in Freq at around 250ms with the stated component values. Is it possible to decrease the response time to below 30ms? I calculate Vripple to be 10mVp-p @ Fin = 3kHz, C1 = 500pF, R1 = 11k, C2 = 100nF but I do not see a formula for calculating output response time. Will I be close to 25mS using those values? Also is there a TINA model for this chip? I looked but did not see one.

Thank you

  • +1
    TI__Guru* 93105 points

    Hi Drew,

    We don't have much information about the LM2917N response time and that example shown on Pg. 15 of the datasheet is one of the best illustrations of the parameter that I have come across. 

    I do know that the response time is affected by the values for R1 and C2 seen in Figure 13. It is stated in the Applications section 10.2.1.1, Design Requirements, "C2: This capacitor filters the ripple produced by the current pulses sourced by the charge pump. Large values reduce the output voltage ripple but increase the output’s response time to changes in input frequency." It might be to possible to reduce the ripple down to 25 ms by significantly decreasing R1's capacitance, but the output ripple level may become unacceptably high. That is something you would need to experiment with and determine if the ripple is low enough for your application.

    The LM2917N was originally developed at National Semiconductor quite some time ago and no National or TI TINA model was developed to my knowledge. Nonetheless, there has been a need for one not only for my use but others as well. 

    I did some snooping around on the web a few years ago and found other people besides me were also looking for an LM2917N simulation model. In a circuit's chatroom someone listed someone provided an LM2917N Spice model syntax so I went ahead and built a TINA model using that syntax. I must say it seems to play correctly and I have used it myself to test the LM2917N in different arrangements.

    Please see the attached TINA circuit file. Do understand that this LM2917N model was not developed by National or TI. If you have problems with the model, or have questions about it, we probably won't be able to help you with them i.e apply the model at your own risk.

    I hope this helps.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

    LM2917-14_Test_01.TSC

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points

    Hello Thomas,

    Thank you for the detailed response and "use at your own risk" model. I'll check it out, much appreciated!

    Drew