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UC1846: Current Limiting on the VREF pin

Part Number: UC1846

What sort of current limiting, if any, is used on the internal 5.1-V reference regulator output (VREF) -- I assume this is an LDO -- is it using a brickwall or voltage foldback current limiting approach? The customer is concerned that permanent damage may occur during a short circuit condition on this pin as the datasheet specifies the Vref pin has an absolute max current of -30mA, but has a short circuit current of -45mA typical which exceeds that. Given the fact that this rail isn't used internally, would the device continuously operate during this condition or is there an internal thermal limit that would shut down the device?

  • Hi Sanel,

    You are correct, the VREF output is driven by an LDO connected to the VIN pin.  The datasheet shows an ABS MAX current of (only) 30mA for the VREF pin.  I'm sure there is some design margin on the 30mA specification, for example 2x.  The ABS MAX is defined in the datasheet as "...Exposure to absolute-maximum-rated conditions for extended periods of time may affect device reliability."  The key points here are "extended periods" and "device reliability".

    Shorting the VREF pin to ground will not instantly damage the controller.  Damage may eventually occur if VREF were shorted to ground for a very long time at a high junction temperature.  It's impossible to predict if damage would occur after hours, days, or even weeks.

    If the VREF pin is shorted to ground it will deliver 10mA to 45mA of current, per the datasheet.

    Why do you think the VREF pin is not used internally?  Even though it's not shown in the block diagram, the VREF pin is used throughout the controller.  At the very least, consider the typical schematic from the datasheet (shown below) - VREF is connected to the (+) pin of the Error Amplifier.  VREF is the reference voltage for the control loop.  So, if VREF = 0V the error amplifier output will be extremely low and PWM switching will stop.