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TLC77: Maximum capacitance value for CT pin

MESH
Intellectual 880 points
Part Number: TLC77
Other Parts Discussed in Thread: TPS3808E

Tool/software:

Hello,

I will use TLC7701 for reset signal circuit.

The reset duration is approximetry 5sec.

I will connect 47uF x 5 to CT pin.

I would like to confirm limitation for maximum capacitance value for connecting CT pin.

Regards, MESH

  • +1
    TI__Mastermind 19100 points

    Hi MESH,

    ~250uF is way above the typical maximum capacitance for our devices, I will have to double check with our design team to confirm, but typically, for timing capacitor, the limit is usually 20uF to 10uF. 

    If you are looking for a device can do 5 sec of reset time delay, I can recommend the TPS3808E. This is a newer device with higher accuracy and lower power consumption.

    https://www.ti.com/product/TPS3808E?

    Please let me know if the TPS3808E would not work for you.

    Jesse 

  • 0 in reply to Jesse Villanueva
    Intellectual 880 points

    Hi Jesse,

    Thank you for your reply.

    Now I check TPS3808E.

    If I have any concerns for TPS3808E, I will ask.

    Regards, MESH

  • 0 in reply to Jesse Villanueva
    Intellectual 880 points

    Hi Jesse,

    I am considering a reset circuit using TPS3808E with the content shown below.
    If you notice anything about the circuit, please let me know.


    (1) I will use TPS3808EG01 for reset circuit.

    (2) TPS3808E VDD pin is supplied with 5V generated from 24V by LM7805.

    (3) Connect three 330nF capacitors to the CT pin of TPS3808E.(Reset time delay is approx 5.7sec)

    (4) In this reset circuit, manual reset function is not used.
    Connect a 1kohm pull-up resistor to the /MR pin of TPS3808E and pull it up to 5V.
    The /MR pin is pulled up at 90kohm in the internal circuit, but it is also pulled up externally just in case.

    (5) TPS3808E SENSE pin receives a voltage obtained by dividing 24V using a voltage dividing resistor.
    The resistance values ​​of the voltage dividing resistor are 24kohm and 499ohm.
    Connect a voltage divider resistor directly to the SENSE PIN of TPS3808E.
    Also connect a 10nF capacitor to the SENSE pin.
    When the 24V supply voltage reaches 20V, the input voltage at the SENSE PIN of the TPS3808E will be 0.405V.

    (6) Connect the base of npn transistor and 1.3kohm pull-up resistor to the /RESET pin of TPS3808E.
    This pull-up resistor is connected to 5V.
    The current flowing into the TPS3808E /RESET pin during reset duration is approx 4mA.

    Regards, MESH

  • +1 in reply to MESH
    TI__Mastermind 19100 points

    Hi MESH,

    Everything sounds good to me except (4) and (6).

    For (4), it is fine to have an external resistor, but the value chosen is too low. A parallel 90k and 1k only gives a 990ohm pull up. At least 10k is needed. 

    For (6), the pull up resistor is also too low, the recommended value for a pull up resistor for this device is 10k-1M.

    Jesse 

  • 0 in reply to Jesse Villanueva
    Intellectual 880 points

    Hi Jesse,

    Thank you for your reply.

    I will incorporate your recomendations into my circuit design.

    Regards, MESH