LM2907-N: Does the LM2907M-8 / NOPB work with 0-5V 50% duty cycle square waves as FVC?

Hello-Fujii-san,

The LM2907M-8 requires that the signal applied to the pin 1 tachometer input (TACH+) have an input signal that passes below 0 V. That is because the other input of the comparator, pin 8 (TACH-), is connected internally to ground. If the TACH+ signal doesn't pass to voltage level below ground the input comparator will not change state.

The LM2917, 14-pin device can be used with a 0 to 5 V input square wave because the (TACH-) pin is not connected internally to ground. Therefore, it can be set to a different voltage level. Figures 20 through 22 in the datasheet show how a small-signal diode, biased by a 10 k resistor connected to B+, biases the pin to about +0.7 V. That is certainly between 0 V, on the way to +5 V, and the input comparator is able to change state in response to a 0 to +5 V square wave signal applied to the TACH+ input.

Regards, Thomas
Precision Amplifiers Applications Engineering

Hello Thomas

Thanks for your reply.

I understood if 0-5V square wave use for input voltage, I have to use LM2917.

Then, do you have any SOPIC-8 package FVC device which thay can can be set to a different input voltage level?

I mean, LM2917 is too large for our cercuit. so I am looking for SOPIC-8 package device.

Regards, Takahito Fujii

Hello Fujii-san,

If you can ac couple the input to the LM2907 a differentiator can be created by the input coupling capacitor and an internal input resistor. The differentiated square wave should trigger the comparator. We recently recommended this approach to another LM2907 user and it worked for them. See the following e2e inquiry:

e2e.ti.com/.../771554

Regards, Thomas
Precision Amplifiers Applications Engineering