TLC555: Schematic Check

user4811888

Intellectual 530 points

Part Number: TLC555

Hi TI expert,

Would like to generate ~1k 8% duty PWM, use schematic below, please help check if it works.

BTW, do we need to add load resistor 8.06k between OUT to GND?

Reference circuit from TPS53681-EVM, page 11:

Using the TPS53681EVM (ti.com)

over 4 years ago

+1 Chris Featherstone over 4 years ago

TI__Mastermind 25405 points

Hello,

I have simulated the circuit as shown in the bottom image you provided. The top one is missing the timing capacitor and is needed for operation. I have attached the Tina simulation at the bottom of this post.

You can see that the circuit provides a duty cycle of 8.9% at 1.3kHz.

The 8.06k resistor is used for the next stage, specifically the AND gates. It is acting as a pull down resistor. If the TLC555 is off then it provides a predictable known voltage or logic 0 to the input of the AND gate. There isn't anything special though about the resistor being 8.06k. As long it isn't loading the output of the TLC555 too much. Values such as 8.06k, 10k or higher is fine to choose.

slfm002b (8).TSC

0 user4811888 over 4 years ago in reply to Chris Featherstone

Intellectual 530 points

Hi, Chris,

thanks very much for your simulation, I get the results now.

However, what do you mean by "The top one is missing the timing capacitor"? I don't find any differences between two images. Both three capacitors in total.

0 Chris Featherstone over 4 years ago in reply to user4811888

TI__Mastermind 25405 points

Hello,

Please see the images side by side below.

0 user4811888 over 4 years ago in reply to Chris Featherstone

Intellectual 530 points

Oh, please check C221

Clock & timing

Clock & timing forum

TLC555: Schematic Check