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TLC555-Q1: CONT specification

Simon Jagla
Expert 8415 points
Part Number: TLC555-Q1
Other Parts Discussed in Thread: TLC555

Hi Experts,

could you clarify how the voltage on the CONT pin affects the output voltages/pulses? There seems to be no specification in the datasheet. Customer is looking to use the pin to design a Pulse-Position Modulation.

  • 0
    TI__Mastermind 42013 points

    Hi Simon,

    By default the trigger voltage level is (1/3)VDD and the threshold voltage level is (2/3)VDD due to the internal resistor divider. As you can see in the simplified schematic I attached, applying a voltage to cont changes the threshold level to cont, and changes the trigger level to (1/2)cont. Changing this value changes the duty cycle of the timer.

    This plot shows the change in cont voltage (the modulation waveform), an the corresponding change in duty cycle on the output.

  • 0 in reply to Katlynne Jones
    Prodigy 50 points

    Hello Katlynne,

    thanks for your answer!

    Your answer ist quite clear for applying a voltage to the CONT pin with a very low impedance.

    In my application as a Pulse Position Modulation according to Figure 16 in der datasheet, the modulation voltage is fed in via a resistor network. In this case the impedances of resistors R do have an impact on the resulting threshold levels.

    Is any specification for the CONT pin impedance available? Typical and min./max. values are needed for total application specification.

    Thanks in advance!

    Best regards,

    Rafael Lindner

  • 0 in reply to Rafael Lindner
    TI__Mastermind 42013 points

    Hi Rafael,

    The TLC555-Q1 is the automotive version of the TLC555. It looks like the TLC555 datasheet gives a typical characteristics table for control impedance vs temperature at a few supply voltages. Unfortunately we don't have a min/max spec for the control pin impedance. 

    Another option for you could be to buffer the output of your resistor network to get a low impedance input for the control pin.