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TPS3836: CT Tied to VCC Delay

Maksym Kloka
Prodigy 20 points
Part Number: TPS3836
Other Parts Discussed in Thread: TPS3840, TLV803E

Hello,

I am using TPS3836 in my design. Unfortunately something else caused a requirement change that puts me in a difficult situation. I am comfortable with 200msec of delay (CT attached to VCC=3.3V); however, the specifications on this delay range from 100msec to 300msec from 200msec (nominal). This is prohibitive tolerance range. Am I reading these values correct? I need something in the 150ms to 250msec range. Does TI have a similar pinout part that can achieve better tolerancing on delay? 

Sincerely,

MK. 

  • 0
    TI__Genius 9375 points

    MK,

    Yes, this variation includes device variation, temperature, and conditions which can all affect the internal charging of the CT block in the circuit. You might be better off with an adjustable timing device such as the TPS3840, or using a newer non-adjustable device such as the TLV803E. With the TPS3840 you can program your own timing using the CT pin, and with the TLV803E, the timing is tighter at 130ms - 270ms. Across the industry, however, I think you'll find that 25% range across all variables is hard to achieve. TPS3840 will give you best of class variation in this spec.

    Thanks,
    Abhinav.

  • 0 in reply to Abhinav.Sharma
    Prodigy 20 points

    Hi Abhinav,

    thank you for the quick reply. I have a potential solution to this dilemma ... if I were to use TPS3840 in my design, could I parallel a tight tolerance resistor (1% from VCC to CT pin), such that equivalent resistance is Rext||Rct. Lets say I choose 68.1k ... now, my overall resistance tolerance is dictated more by the Rext, verses Rct. Looking at the internal block diagram of TPS3840 (Page 16), there seems to be a PMOS and NMOS between Rct ... would this invalidate my suggestion in some way? ... or is my recommendation a potential solution. Thank you.

    Sincerely,

    Maksym Kloka

    269.762.1294

  • 0 in reply to Maksym Kloka
    TI__Genius 9375 points

    Hi Maksym,

    This suggestion, although clever, would not work. Connected a resistor from Vdd to CT would cause essentially a pullup.on CT to Vdd. This would not allow a capacitor to charge properly and most likely give you a time delay as short as the device allows. 

    There is no practical way to externally improve the variation of the CT delay block.

    Thanks,
    Abhinav.