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LM2907 Problems

David Herbert
Prodigy 20 points

Other Parts Discussed in Thread: LM2907-N

Hello everyone. I am using the LM2907 for a speed switch application (figure 20). I have for my load a 10k resistor connected to 5V, which is my vcc. For my input signal, I am using the square pulses from the function generator. C is 50uF, and R is 1k with a 1uF connected next to the 1k. However, when I tried varying the frequencies, it stays low. Keep in mind that this was a national semiconductor part, but since they were brought by TI, I thought I could ask my question here.

Can anyone provide any suggestions on how can I resolve this issue? 

  • 0
    TI__Guru 64806 points

    The device you are using is now known as the LM2907-N, for National under TI.

    Your "C" capacitor values may be too large to charge and your "R" value is too low. Try a 0.5uF and 100K resistor instead (scale cap down and resistor up proportionally). The parallel cap can stay at 1uF. Look at some of the other circuits and see what the typical RC vales are...you'll see they are generally < 0.1uF. Please read the "Choosing R1 and C1" section in the Apps Info section.

    Also, what is the level of of your input signal? The input was designed for use with bipolar inputs from magnetic sensors, so the signal must pass below ground for the input to trigger. The 2907 has a rare input stage that can take ±28V inputs.

    Either shift the signal generator so that it is is generating a minimum of ±100mVpp, or, AC couple the input through a capacitor.

    Regards,

  • 0
    TI__Guru 64806 points

    OOPS... That should be "Try a 0.5uF" capacitor

    Corrected above...

  • 0 in reply to Paul Grohe
    Prodigy 20 points

    Sorry for the slow reply everyone, but I tried the values from figure 19 of the datasheet and it seems like I'm getting a great response. However,  for my application, I need it to toggle when fin is 10hz. once again, I tried messing around with the values (this time c1=1uF, c2=1uf,, R1=100K, and a 10k pull up resistor) and I do get it to toggle, however, around 3-6hz, it goes into an unstable state which it cannot determine if its high or low.

  • 0 in reply to David Herbert
    TI__Guru 64806 points

    Hello David,

    You need to increase the size of C2 - above 10uF as a start. You are running the 2907 way below the typical operating frequencies, so the default recommended values are too low for your application.

    R1 and C2 create a filter that smooths out the ripple going into the internal comparator. The time constant of the filter needs to be longer than the period of the operating frequency. If you have a scope, look at the voltage on pin 3 and you will see it starts to have more ripple as you decrease the frequency.

    Of course, increasing the filter cap size increases the response time, as it will take a few more cycles to switch. So you have to pick a tradeoff between response time and reliable triggering.

    Regards,