LM3900: High Input current observed in LM3900N part

MATHI AZHAGAN

Part Number: LM3900

Hi Sir/Mam,

I am using LM3900N part in my design. Circuit image attached.

1.) 5V input given to Inverting input pin, Input current taking 0.25A. why this too much current taking for this 1channel input ? pls clarify. ( Attached figure 1)

Figure 1 - Inverting Input Configuration.

2.) 5V input given to Non Inverting input pin, Input current taking 0.3A. why this too much current taking for this 1channel input ? pls clarify. ( Attached figure 2)

Figure 2 - Non Inverting Input Configuration.

Regards,

Mathiazhagan K.

+91 9626461996.

10 months ago

0 Clemens Ladisch 10 months ago

Guru 260510 points

The LM3900 is a Norton amplifier, so it has low-impedance current inputs. See p. 7 of the datasheet.

0 Art Kay 10 months ago

TI__Mastermind 18067 points

Mathiazhagan,

What is the objective of your design. The LM3900 is an unusual device (released 1979). As Clemens mentioned this is a Norton amplifier (current input and voltage output - or transimpedance difference amplifier). https://en.wikipedia.org/wiki/Norton_amplifier Normally these amplifiers are used with feedback. Without feedback they act like current input comparators. Do you want an op amp or comparator?

Best regards,

Art

0 MATHI AZHAGAN 10 months ago in reply to Art Kay

Prodigy 10 points

We are redesigning the existing board design. This design don't have feed back path. We could not able to toggle the output of this device to check its working operation. We tried giving 5V input but output LED brightness is very less. Please let me know how to test this configuration.

0 Clemens Ladisch 10 months ago in reply to MATHI AZHAGAN

Guru 260510 points

Do you want to test the LM3900 itself, or your entire design?

To toggle the LM3900's output, the two inputs must have different currents. The easiest way to force this is to have two different resistors from V_CC to the inputs.

0 Art Kay 10 months ago in reply to MATHI AZHAGAN

TI__Mastermind 18067 points

Mathi,

If you are re-designing the PCB, and your goal is to turn on and off an LED I suggest a different approach would be a good choice. Let me know what your input signal is and I can suggest an alternative.

Art

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LM3900: High Input current observed in LM3900N part