Fujii-san,
The LM2907-N doesn't use a lot of current; especially, with the Minimum Component Tachometer circuit which has a high value, 10 kiloohm resistor in the emitter follower output stage.
The datasheet on Pg. 7 provides Figure 3, which is a graph of total supply current vs supply voltage. The Minimum Component Tachometer circuit is powered by a single, +15 V supply. Applying the graph it is seen that the operating current is about 3.5 to 4 mA, depending on the temperature. Certainly there will be some variation between devices so the current may be a little more, or less, for a particular device.
Since the LM2907-N is a frequency to voltage converter the output at +Vout will be a voltage level that is dependent upon the input frequency and the device operating conditions. Since the output load is the 10 kilohm resistor the output current through it will be s function of the voltage level. For example, if the output voltage is 10 V, the output current will be 1 mA. That current would be added to the operating current.
Overall it looks like the power supply current for the tach circuit is likely to be around 5 mA. Accounting for the maximum output voltage level, temperature range and component variation a 10 mA supply should cover the variances.
Regards, Thomas
Precision Amplifiers Applications Engineering
