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REF3433-EP: Output capacitor value vs enable time

Gregory Helton
Intellectual 320 points
Part Number: REF3433-EP
Other Parts Discussed in Thread: REF35,

This voltage reference has an output capacitor spec of 0.1 to 10uF. The time from enable signal active to stable output voltage is specified as 2.5mS typical with 10uF output cap.

Looking at Fig 13 in the datasheet, one may conclude that lowering the output capacitor value would lower the "power up" time, probably proportionally to the output cap value.

For instance, if the output cap value were lowered to 0.1uF, would the power up time be reduced by a factor of 100 (25uS) ?

I have an ultra low current requirement for an MSP430 application where I want to keep the reference off until it's needed, but it needs to "power up" quickly. The micro would wake, turn on the reference, and must be ready to sample (use the reference) in less than 100uS.

Thanks.

  • 0
    TI__Genius 9045 points

    Hello Gregory,

    The device will have reduced start up time with lower output cap. This will not be directly proportional as the device will have fixed time before internal circuits are enabled and this will then be followed by cap based variable delay.

    How about considering a new REF35 family? https://www.ti.com/product/REF35 

    This device consumes <1uA typical quiescent current. You may not need to shutdown the reference.

    Also, do you need EP rated devices only for this application?

    Thanks & Regards,

    Shridhar.

  • 0 in reply to Shridhar%20More
    Intellectual 320 points

    Hi Shridhar,

    Thank you for the reply. Yes, I do need an EP rated device for this application. We must go down to -55C. I do like the ultra low Iq of the REF35 series, but temp rating is an issue for this series. For my application, I have allocated around 3uA Iq for the regulator/reference in sleep mode (with very little load current), and around 15uA Iq while active (plus load currents as well).

  • 0 in reply to Shridhar%20More
    Intellectual 320 points

    Hi Shridhar,

    Is there a "maximum" enable time for the REF3433-EP to become stable ? The datasheet gives a typical time of 2.5mS with a 10uF output cap. However, I need a maximum time for the reference to become stable. My plan is to enable the reference 2.5mS ??? before it's needed, then take my ADC samples. As we discussed, I can use a smaller output cap to get somewhat faster startup times, but the exact times are somewhat indeterminate. Does the turn on time change with temperature, input voltage, etc ? Can you provide better turn on time numbers ?

  • 0 in reply to Gregory Helton
    TI__Genius 9045 points

    Hello Gregory,

    I will check with design and validation team for the max time estimates and share with you.

    Allow me couple of days.

    Meanwhile, REF35 is operational from -55C to 125C.

    Will this also work for you even if it is not EP rated.

    Thanks & Regards,

    Shridhar.

  • 0 in reply to Shridhar%20More
    Intellectual 320 points

    Thanks Shridhar,

    I will wait for you reply. This is for an aviation application, so I'm not sure how the customer would react to a component where the operational characteristics wasn't fully specified over the entire operating range. Another issue with the REF35 series is the maximum input voltage. For the REF34 series the maximum input voltage is 12V. However, for the REF35 the maximum input voltage is 6.5V. This won't work for us.

    Thanks again.

  • 0 in reply to Gregory Helton
    TI__Genius 9045 points

    Hello Gregory,

    Based on char data across few devices at 10uF load current we identified the fixed vs variable time contribution to the settling time.

    Scaling this to 1uF load capacitance, we expect the settling time will be ~1.1msec.

    We checked the device simulation numbers and they do match datasheet specs. Taking C load lower than 1uF will not help reduce the settling time lower.

    With 1uF load cap, we should have enough margin to settle within 2.5msec before ADC conversion can begin.

    REF35 has lower VIN which is a limitation for your application. How about having a low current Shunt as power supply to REF35 which can help keep REF turned ON always.

    Thanks & Regards,

    Shridhar.