LM2917 Maximum Voltage Output?

[StudentSA]

Hi

I am building a simple F-V based on the application circuit in the datasheet which I modified resistor/capacitor values to suit my application.

I want my max frequency of 300Hz to give an output of 10V. My Vcc is 12V.

See attached schematic


The problem I am having is that the output voltage level reaches a maximum of 6.22V and refuses to go to 10V. I have tried playing with C and R but with no luck.

Please advise.
Kind Regards,
Student SA

 

[crutschow]

If you look at the data sheet you will see that the zener regulator at the power input to the output op amp has a voltage of 7.56V. Thus the op amp output can never go above this value. Due to drops in the op amp and the base-emitter voltage of the output transistor, the maximum output is the 6.22V you measured. If you need 10V you can add a non-inverting op amp with gain at the LM2917 output.

 

[StudentSA]

Thanks for the quick response.

So this problem can be avoided using the LM2907 non zener regulated IC?

I never even considered the Zener to have any effect but will definatly look into it.

 

[crutschow]

I know data sheets can be confusing, but they do contain a lot of info about how a device works.

You could use the LM2907 non-zener version but that would make the output voltage a function of the supply voltage. This is not desirable if the voltage varies, as the battery voltage does in a vehicle.

If the voltage source is a battery then you could use the non-zener version with a low-dropout 12V voltage regulator on the battery supply to provide a stable output voltage.

 

[StudentSA]

100% correct

Using the LM2907 has allowed me to attain a output voltage of 9.2V with the exact same circuit configuration. My voltage supply of 12V will be regulated so this IC should be fine.

Once again. Thank you.

Regards,
StudentSA