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LMR14050-Q1: SS pin left floating

Art Mecina
Genius 16575 points
Part Number: LMR14050-Q1
Other Parts Discussed in Thread: LMR14050

Hello Guys,

Goo day.

Our customer is building a power supply using a LMR14050.  What is the result if the SoftStart capacitor is left off and the SS pin floats? 

The power supply is mains AC fed through a transformer, bridge rectifier, and 5400uF of bulk filter capacitance with no possibility of any regulator or load in-rush current at power up.  It would take about 5 msec for Vin to come up to spec.

Thanks and regards,

Art

  • 0
    TI__Mastermind 23825 points

    Hi,

    If the SS pin floats, the converter behavior during startup becomes unpredictable. Two scenarios may happen, 

    1) The converter may not startup since there is no ramp being generated to control the rising of the output voltage

    2) The converter startup as fast as it can since the internal current source will charge a high impedance at the SS pin. 

    Therefore it is crucial to add the soft-start capacitor in the SS pin. You can refer to the equation on the datasheet in here to determine the size of the soft-start capacitor

    Thanks

    -Arief