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Excessive current draw on LM8261 when inputs are 3+V apart?

CurtisHx
Expert 1225 points
Other Parts Discussed in Thread: LM8261

I've got an LM8261 that's driving a high resistance, low capacitive load (1megohm+, less than a few nF).  Essentially, the output current draw is negligible.  Supply rail is 18v, and the LM8261 is the only thing hanging off of it.

The positive input sees a digital square wave (50% duty cycle) that swings between 0 and 3.6v.  The negative input sees the same square wave 90 degrees later.  So when the positive input is at 3.6v, the negative input is a 0v.  When the negative input is at 3.6v, the positive input is at 0.

According to the datasheet, the LM8261 should draw around 1mA without a load (essentially what I have).  However, I'm measuring the current draw as being 7-8mA.  Does the LM8261 draw more current as the inputs spread further apart?

  • 0
    TI__Guru 61996 points

    Hi Curtis,

    Yes. Bipolar output devices will draw more current when the outputs are railed, due to internal base drive current being dumped into the bases of the output transistors trying to turn them on hard. These are relatively high current output devices, so the gate drive current will be higher than 'normal" op amps.

    Also note that the inputs have diode clamps across them with 1K series resistance. See the Simplified Schematic Diagram (Fig 49, Page 19). The input diodes (D1 thru D4) will start conducting about 1.4V.

    You are basically using the device in a "comparator" mode, which is not ideal for this device. If used in a comparator mode, you will need to limit the current into the inputs (adding series resistance).

    Regards,