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LM2907-N: lm2917

Shai Finkman
Prodigy 50 points
Part Number: LM2907-N

Hi 

I'm using the LM2907 14 pins. In page 4 of AN162 it says:  "The voltage appearing at pin 2 will swing between two values which are approximately 1/4 (VCC) - VBE and 3/4 (VCC) - VBE" my first question is how can I figure out VBE value  and my second is that in my application VCC is equel to 24V and regardless of what frequency is applied on pin 1 ( 0-2.3Khz, 0-5V) I'm measuring  17V on pin 2, (C1=5nF) and the voltage on pin 4 is stuck on 1.3V and going down when the frequency is rising. Do you have any explanation for this behviour ? Is there any evaluation board I can purcahse ? 

Thanks 

Shai

  • 0
    Guru 140200 points

    Hi Shai,

    the interesting question is what you do with pin 11 of LM2917. If your input signal is toggling between 0V and 5V, the input comparator will never change its output state when pin 11 is connected to signal ground. You would need to apply an auxiliary voltage between 0V and 5V to pin 11 to allow the comparator to toggle. Figure 20 of datasheet shows a suited method with a diode and a current limiting resistor.

    Can you show a schematic of your circuit?

    Kai

  • 0
    TI__Guru* 93105 points

    Hi Shai,

    Regarding your LM2907 questions:

    my first question is how can I figure out VBE value - 

    The Vbe is not specified and will vary a little between LM2907 devices. Likely, it will be the in the typical 0.6 V to 0.7 V for nearly all silicon transistors operating at low current levels.

    and my second is that in my application VCC is equel to 24V and regardless of what frequency is applied on pin 1 ( 0-2.3Khz, 0-5V) I'm measuring  17V on pin 2, (C1=5nF) and the voltage on pin 4 is stuck on 1.3V and going down when the frequency is rising. - 

    The LM2907 requires that the tach input drops below 0 V to trigger the charge pump's input comparator. Make sure that is the case. As Kai mentioned a schematic for your circuit is needed for us to assist you further.

    Do you have any explanation for this behviour ? Is there any evaluation board I can purcahse ?  -

    TI does not offer an evaluation module for the LM2907. Your fequency range is low enough that you can test it in a commonly available plug-in white board.

    Regards, Thomas

    Precision Amplifiers Applications Engineering