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LMC555: Output current affects timing?

Andy Russell
Prodigy 40 points
Part Number: LMC555


I have a circuit wired up in a 50% duty cycle oscillator configuration as shown in section 9.6 (page 16) of the datasheet. As I vary the output load to ground from unloaded to 100 ohms, the output frequency and voltage change drastically. V+ is holding a constant 3.3V +/- 1mV regardless of the output load or voltage, indicating my power supply is doing what it's supposed to. When unloaded, the output frequency is measured to be very close to the desired frequency (<0.5% error). This behavior has been seen on two boards of the same design.

My best guess is that the LMC555 isn't able to supply the current that the design requires. However, the datasheet shows that the absolute max output current to be 100mA and that the chip was tested to 50mA. I see a significant change in output frequency and voltage at about 2mA. With the output inverter inside the chip (as shown on page 3 of the datasheet), I'm not sure why I'm seeing any change at all.

Thoughts?

  • 0
    TI__Guru**** 169086 points
    Andy,

    For a 50% duty cycle VCC - VOH must be equal to VOL - GND. Load to ground will reduce VOH only
    Output current maximum and on channel resistance will vary with VCC; 3.3V is on the low side of range.
    VOH and VOL test in data sheet have 4X the current for VOL as VOH, therefore ther VOL transistor is more powerful than VOH transistor.
  • 0 in reply to Ron Michallick
    Prodigy 40 points
    Hey Ronald,

    Thanks for the reply. I guess I didn't see that info in the datasheet before. Oh well. I only did rough simulations and didn't breadboard this circuit before I bought the prototype PCBs, so maybe this is the best I could have hoped for.

    Sounds like my best option is to drop in a MOSFET or two.

    Thanks again,

    Andy