TLC555C source current

Does the moderator or anyone know why my original post was moved from timer and clocks forum to precision amplifier forum?

My post is about the TLC555C, which is a clock.

Anthony,

There are many different product groups in TI and the TLC555 happens to be "owned" and supported by an amplifier group. I won't attempt to explain why this is so. The moderator moved your question so that it could be seen by the proper support engineers.

The TLC555 is not capable of directly driving an LED to 10 mA on a +3V supply. At this lower supply voltage there is not sufficient sufficient gate drive voltage to the output CMOS transistor to support 10 mA output current. A circuit such as shown below may work for you. T1 inverts the output polarity so you may need to adjust the duty cycle of your astable circuit.

Regards, Bruce.

Thanks, Bruce!

My duty cycle is 50%, so I think I am okay.

If I decide to go with using the transistor, do you see any problems with connect a resistor and LED in series from +3V supply to output of TLC555, such that the LEDs are alternately blinking?

Also, I might find another way to just deal with whatever source current is available with 3V.  With 3V as Vdd, how do I determine what the maximum source current the TLC555 can deliver?  The most I measured with 13Ohm load was 5mA.

Anthony,

I think to get sufficient drive, you would need to use a second transistor to invert the output of T1. Alternatively, there are various CMOS inverter ICs that may be able to do all this in a single package without resistors. You would need to check carefully for drive capability at 3V supply.

Regards,  Bruce.

Bruce,

If I decide to accept the limitations of output source current of TLC555, and not use inverters, how do I determine the maximum source current at 3V?

I measured about 5mA when output is under a 13ohm load.

Anthony,

It's difficult to say what current output you could get in this type of operation. Not much characterization information is provided in the data sheet.

If you are measuring current drive with only a 13-ohm resistor (no LED in series) you are likely to get significantly more current. the forward voltage drop of the LED will significantly reduce the available output current. The P-channel output (sourcing current) will be weaker than the N-channel (sinking current). There will be significant differences in the output current with production variation and over temperature.

My advice:  If you are building just one... try it and see if you get acceptable results. Use a soldering iron and cold spray to check variation with temperature. Include the LED in temperature changes as its forward voltage will change. If you need a robust, reliable, production-worthy design, use transistor or inverter drivers that will have more predictable results.

Regards, Bruce.

Hi Bruce,

I want to try to use the inverters.  It seems that inverter package will take less space.

In looking at the following dual inverter package, SN74LVC2G04,  I see that the high level output current for Vcc=3V is -16mA and -24mA.

I also see that the low level output current is 16 and 24 mA.

Why are there two values for the high level and low level output currents?

Also, I am planning to do this with the inverters and the TLC555C.  It seems that high level will be use to turn on LED, and the current should be enough...what do you think?

Thanks Bruce!

I want to breadboard the circuit using the inverter, but the ones I ultimately want to use are too small to breadboard.

Can you suggest an larger size package inverter that is similar to  SN74LVC2G04 I can use just to breadboard and convince myself the circuit will work?

Anthony,

The drive capability of your circuit is important. I strongly recommend that you verity using the actual chip that you intend to use. Other chips in a larger package may not have the same drive behavior.

The blog in the link below provides some suggestions on breadboarding with micropackages. Though it is intended for op amps, you may be able to use the adapter system mentioned in this blog to check your circuit. Get creative. I'm sure you can find a way to do it.

Breadboarding with Micro-Packages—Ouch!

Regards, Bruce.