LM2907-N: Electrical characteristics questions

Hi,

Regarding your LM2907N-8 questions:

1. What is the minimum supply voltage for the 2907 8-pin?  I saw a post on this forum saying 6 volts, but I cannot find that in the data sheet (page 5 says -28 to 28V.

The datasheet really doesn't specify the minimum supply voltage for the LM2907N-8, but the Section 7.6 Characteristics curves include Figure 3. Total Supply Current and Figure 7. Tachometer Currents I2and I3 vs Supply Voltage. Both of the graphs show the minimum supply voltage as 6 V. SInce we don't have any additional information indicating that it will operate with a lower supply voltage we go by the graphs.

2. What is the minimum and maximum input voltage of the frequency generator for the 2907 to register it?  Values are listed on the data sheet, but they are under very specific test conditions (on page 5).

By register I assume you are referring to the tachometer input comparator? The Section 7, Absolute Maximum Ratings lists the maximums as +/-28 V. That seems high, but the LM2907N-8 has a 10 k series input resistor at the tach input and ESD diodes. I would suggest keeping the input voltage to a positive maximum of the V+ supply voltage and not much more than about -0.5 V below ground. That keeps the ESD diodes from being turned unnecessarily. That is the way that I've tested it on the bench and it operated nicely. Remember the LM2907N-8 tach input needs to drop just below 0 V on the low end because the other input of the comparator is internally connected to ground. 

3. What does the low level and high level output voltage mean on page 6?

The datasheet states in the notes below the Section 7.5 Electrical Characteristics Table, note 2, "(2) VOH = 0.75 × VCC – 1 VBE and VOL = 0.25 × VCC – 1 VBE, therefore VOH – VOL = VCC / 2. The difference (VOH – VOL) and the mirror gain (I2 / I3) are the two factors that cause the tachometer gain constant to vary from 1." VOH and VOL appear to be output limits for the LM2907-N NPN output transistor in the emitter follower configuration. 

I find the AN-162 Applications Note for the LM2907/LM2917 to be useful for additional information.

www.ti.com/.../snaa088.pdf

Regards, Thomas

Precision Amplifiers Applications Engineering