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TL7702B: TL7702B Questions

Part Number: TL7702B
Other Parts Discussed in Thread: TPS3808, TPS3890

Hello

I have a circuit where I'm using a TL7702B to monitor the main supply rail (12V), and startup a sequence or other rails which are powered from this.  VCC is connected to 12V and I have a potential divider connected to SENSE such that the device will go out of reset when the supply reaches 9V.  I have a 10uF capacitor connected between CT and GND.  I have a few questions about this circuit:

*  With the 10uF cap, I'm seeing a delay between #RESIN going high and #RESET going high of about 150mS which seems much shorter than the equation in the data sheet suggests.  Can you suggest why this might be?

*  Is there a practical limit to the size that this capacitor should be?  I'd ideally like about 300mS of delay.

* On page 13 of the data sheet, it talks about a resistor in series with the capacitor if SENSE is monitoring VCC.  Is this required in my case as SENSE is connected to VCC via a potential divider (tens of K ohms), I read this as meaning it's only required if the connection is direct.

*  I'm pulling #RESET up to 3.3V and RESET down to GND via 10K resistors.  Is this OK, or should #RESET be pulled up to VCC only?

Thanks

Martin

  • Martin,

    The datasheet mentions the current source to charge the timing capacitor can vary +/- 15% so that is one source of error. Also, the reference voltage varies approx. +/- 5%. And once the capacitor charges, it discharges to about 0.6V not completely to 0V. So it's possible that the capacitor isn't actually discharging to 0.6V or charging up completely because of the capacitor size. The typical cap value for this device is 0.1uF.

    Also your pull-up and pull-down resistors are correct and properly sized.

    I recommend using a device such as TPS3890 (programmable from 40us to 30s) or TPS3808 (programmable from 1.25ms to 10 s) which are much better and higher accuracy devices with a tighter window on the programmable delay. TPS3890 being slightly more accurate than TPS3808.

    Please let me know if you are open to trying a different device or have any other questions or need any additional support. Thanks!

    -Michael