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Minimum rail risetime on UCD9240 (TON_RISE)

EDGAR BROWN
Intellectual 450 points
Other Parts Discussed in Thread: UCD9240

If I interpret the TI UCD92xx errata correctly the maximum slew rate for a specific rail on an UCD9240 would be given by:

  • Maximum EADC Slew-rate of  0.156V/ms.
  • AFE gain setting
  • EAp/EAn divider
  • Rail voltage

That is, if I have a 1.8V rail that uses a divider of 2/3 (resulting in a control voltage of 1.2V), and an AFE gain of 4x.

  1. The maximum slew rate for this rail would be given by: 156mV/ms / 4 * 3/2 = 58.5mV/ms
  2. The minimum rise time for this rail would be given by: 1200mV / 156mV/ms * 4 = 30.8ms

Is this interpretation correct?

What are the consequences of violating these conditions?

  • 0
    Intellectual 450 points

    This should be an easy question. Anybody?

  • 0 in reply to EDGAR BROWN
    TI__Intellectual 1755 points

    Edgar,

    The max slew rate during the soft start and soft stop ramps are based on the following table.  

    AFE gain

    dynamic range

    max step/100 usec

    slew rate

    1X

    ±256

    192 mV

    1920 mV/ms

    2X

    ±128

    96 mV

    960 mV/ms

    4X (default)

    ±64

    48 mV

    480 mV/ms

    8X

    ±32

    24 mV

    240 mV/ms

    During the soft start ramp, the controller is calculating a value and updating the setpoint reference DAC every 100 usec.  The digital control loop then uses this new value to regulate the output voltage to.  To keep the sensed error voltage within the dynamic range of the sampling ADC, each step in the ramp is limited to 75% of the dynamic range.

     

    So, for this example, the fastest the voltage that can be ramped during a soft start or soft stop is
                (480 mV/msec)*(3/2) = 720mV/msec

    Then a 1.8V rail can be brought up no faster than

                (1800 mV)*(1 msec / 720 mV) = 2.5 msec

    This can be seen by setting the TON_RISE value to 0 and watching a start ramp of Vout on the scope.  

     

    Note that trying to ramp any power supply fast means that you are asking it to charge the output capacitance fast, which demands lots of current.  The faster the soft start ramp, the more potential there is to generate an over-current fault.