LM2907-N: Freq to Volt converter in pin 4

Issam boukabou

Part Number: LM2907-N

The output that I get at pin 4 & 7 is always 0.3 V. However, when I measured it at pin 3, I get the right result. Can You also check my values for the high and low frequency circuits.Low Tach & High Tach circuits.pdf

over 6 years ago

0 kai klaas69 over 6 years ago

Guru 140200 points

Hi Issam,

do you shortcircuit the output? What happens if you disconnect the rest circuitry from pin 4 and 7? Becomes the LM2907 hot?

Kai

0 Issam boukabou over 6 years ago in reply to kai klaas69

Prodigy 40 points

I did not test that because the chip is already populated in my board. The way I solved it is to get the output from pin 3 instead of 4.

0 kai klaas69 over 6 years ago

Guru 140200 points

Hi Issam,

is there a resistance missing from the right side of C17 to 15VCOM?

And take care of the correct polarity of polarized caps!

Kai

0 Issam boukabou over 6 years ago in reply to kai klaas69

Prodigy 40 points

I don't see C17 in my circuit! All the caps are not polarized.

0 kai klaas69 over 6 years ago in reply to Issam boukabou

Guru 140200 points

The Tach+ signal in your pdf... TP10.

0 Thomas Kuehl over 6 years ago in reply to Issam boukabou

TI__Guru* 93105 points

Hi Isaam,

The three LM2907 devices shown in the NEW OUTPUT schematic attachment have their TACH+ inputs connected together at the left side connection of C17, but there isn't any resistance from that circuit node to ground (as Kai questioned). The TACH+ input (pin 1) needs a dc path to ground. It serves as a base current path when the PNP transistor on the TACH+ input side is required to turn on. I suspect the voltage on that input is never getting to ground at any point of the input drive cycle, and the LM2907 input comparator is not flipping state. You could monitor the input voltage waveform of the three LM2907 inputs with a DSO and observe what it looks is doing.

Or - try connecting a resistor (several kilohms) from the LM2907 TACH+ inputs to ground and see if the device begins to function. Some experimentation with the resistor value may be required.

Regards, Thomas
Precision Amplifiers Applications Engineering

0 Thomas Kuehl over 6 years ago in reply to Thomas Kuehl

TI__Guru* 93105 points

Hi Isaam,

I haven’t heard back from you, I’m assuming you were able to resolve your issue. If not, just post a reply below (or create a new thread if the thread has locked due to time-out).

Regards, Thomas
Precision Amplifiers Applications Engineering

0 Issam boukabou over 6 years ago in reply to Thomas Kuehl

Prodigy 40 points

I did place a resistor between TP10 and 5VCOM but it didn't work. I am also running into an issue with my High Tach and Low Tach circuits. Thanks

0 Thomas Kuehl over 6 years ago in reply to Issam boukabou

TI__Guru* 93105 points

Hi Issam,

Is 5vCOM the at same voltage level as 15VCOM in the schematic? The resistor would be connected between the TACH+ and 15VCOM. Once the resistor is in place capture the signal on the TACH+ pin with a DSO. Post the image so we can get a look at it.

Regards, Thomas
Precision Amplifiers Applications Engineering

0 Thomas Kuehl over 6 years ago in reply to Issam boukabou

TI__Guru* 93105 points

Hi Isaam,

I haven’t heard back from you in a while regarding this LM2907 issue. We would like to close this thread if you no longer require assistance.

Regards, Thomas
Precision Amplifiers Applications Engineering

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LM2907-N: Freq to Volt converter in pin 4