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TLC556: Does the TRIG input have protection diodes? If so, what is the maximum allowed input current?

Mike Andrews
Prodigy 10 points
Part Number: TLC556

Hi,

I actually used a TLC556 in a new design. Never thought I'd have the chance again. Go figure. :-)

Because the TRIG input is edge-sensitive I used the one-shot output of the first-half of the dual-timer to generate an RC pulse on the falling edge of the TIMER output. I'm using the 2nd-half of the TLC556 as a latch to enable circuitry.

The issue is when TIMER goes high at the start of the one-shot period. The input to TRIG momentarily goes to +24V. R17 can be adjusted to handle the max allowed current into the TRIG protection diodes ... if they exist.

As you see VDD is +12V for this circuit.

The TLC556 spec says the absolute max input voltage range (Vi) is -0.3V to VDD, with VDD(max) = 18V. I took that to mean there are protection diodes on the TLC556 inputs because of the -0.3V part of the range.

Questions:

1) Is the TRIG input diode protected to VDD and ground?

2) If TRIG is protected by a diode to VDD and a diode from ground, what is the maximum current thru those diodes?

Thanks for your help.

  • 0
    TI__Mastermind 20060 points

    Hello,

    Looking at the layout there is a diode network to ground. The layout does not indicate a diode structure to the positive supply. We suspect that the reverse breakdown of the diode to GND is used to protect the input to the positive rail. This applies all input pins. The max current is 10mA. 

    Best Regards,

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Prodigy 10 points

    Thanks, Chris. I appreciate you looking into this.

    So the diode protecting the input against negative voltages is essentially an 18V zener protecting against the input going too high, with both directions limited to 10mA.

    And here I thought that since the 555 timer has around since the rock wheel was invented there wasn't anything new to learn about it. Proof that God has a sense of humor.