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REF3330: How should the short circuit current be seen?

Frank Rothoff
Expert 6030 points
Part Number: REF3330
Other Parts Discussed in Thread: TMP103,

Hi Team,

My customer is mis-using the device as low Iq voltage source. They don't want to add a new device at this late stage in the design.

How is the current limited and to what level (max)?

The DS implies Isc being typ 35mA and Abs max mentions 180mA. As there is drop-out there is also a 'restriction', at 5mA at -40dgC to 85dgC Vdo=135mVtyp which implies an impedance/resistance of 27 Ohms.

Reason is the REF supplies the external TMP103 sensor which is located outside of the PCB or box, This might get shorted and they don't want to damage the REF3330. Which is supplied by 3V3.

  • 0
    TI__Mastermind 19370 points
    Hi Frank,

    I have seen REF3300 be used as an LDO and its fine. Most ADCs external VRef consume 1mA or less. Sometimes the ADC use the VCC pin as the VREF pin so we spec the REF3330 at 5mA to power the ADC and VREF.

    The absolute max current (done with short circuit) is 180mA, while the 35mA is a typical value. I would not go above 5mA for a continuous load if you want to keep the high accuracy. But if a short circuit happens of 35mA, the device should be fine.

    In case of a short, the current is being limited by the impedance. The impedance is typically constant with the amount of current going through the device as long as there is a load. In your calculation, the assumption was 5mA so the calculated impedance is correct for 5mA. But for short circuit it is different, the current is larger and the dropout voltage is the input voltage. I would use the calculated impedance = 5V (input voltage condition)/ 35mA (short circuit current) which calculates to give a impedance of 142Ohms for a typical short circuit.

    -MZ
  • 0 in reply to Marcoo Z
    TI__Expert 6030 points

    Hi MZ,

    Thanks for sharing your experience on powering a light load <5mA.

    So to me it is still not clear what inside the device limits the output current. Also it's hard to quantify your mentioned 35mA short-circuit current which is typ in the DS compared to abs max 180mA. The latter is confusing as the device delivers the current so it implies that the user should prevent it from going over that value?

    Regarding short circuit conditions I'm not re-assured yet. The point the customer and I have is that a short circuit condition is undefined and worst-case zero Ohms. The point I was trying to make with my calculation is that the device has an internal impedance at a certain current. This impedance we could benefit from in an undefined short-circuit condition. My assumption is that the voltage drop over the internal Zout or Rout will increase if the current through it increases (short circuit condition), from that we can benefit as that limits the short-circuit current. However this is in linear mode and I'm not sure how the 'passing element' of the REF behaves in saturation (outside the linear region).

    Best Regards,

    Frank