Please let me ask 3 questions about meaning of open circuit, calculation of pull up resistor value and selection of MOSFET.

I'm trying to understand how to use the TPS3808G33 reset IC and the followings are the questions. It might be obvious to most of people here, but as a beginner in engineering I couldn't find answer by myself..It would be very helpful for me if you could give answers.

  1. On page 4. the description of C_t pin says that 'leaving it open results in fixed delay times'. Then would it be okay to connect the C_t pin directly(or through a resistor) to GND?

  2. On page 4. the description of RESET pin says that 'A pull-up resistor from 10kΩ to 1MΩ should be used on this pin'. Could you please explain how to calculate proper value of this pull up resistor for TPS3808G33? In figure 10 they used 1MΩ for TPS3808G25, but I have no idea how to determine the value. ...Could I just use any value between 10k~1MΩ?

  3. On page 8, it's written that 'To minimize current draw, a logic-level FET can be used". But... could I use any N-channel MSFET? The data sheet didn't mention about it.

Thank you in advance.


First answer is here: Reset delay

See a picture, you can also check datasheet, to check about capacitor connection.

Second: Placing larger pull-up resistor value will reduce current, so it depends on you and where you are going to use it. But it is not wrong if use any value between 10k and 1M.

You can not use any MOSFET (it is connected to 3.3V through pull-up 90k), you need one that can be controlled by Vgs<=3.3V.

  • \$\begingroup\$ Thank you very much for the answers. But If I understood correctly the figure shows how to adjust delay time by using a capacitor. For the fixed delay time of 20ms, could I use GND? I'm sorry that I didn't understand the difference between making open circuitt and grounding in general. \$\endgroup\$ – SD11 Feb 17 '16 at 10:17
  • \$\begingroup\$ For a 20ms delay you can leave it open or use capacitor of proper value to connect to GND. But connecting directly to GND will not give you 20ms delay. \$\endgroup\$ – Haris778 Feb 17 '16 at 10:58

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