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TPS734: Soft Start Circuit as seen in Figure 6 of SLVA156

steve wenner
Expert 2355 points
Part Number: TPS734
Other Parts Discussed in Thread: TPS732,

I would like to know if the TPS73401 can use the soft start circuit as in SLVA156 figure 6 (app note says there are certain regulators that this circuit will NOT work with)?  

My second question is does TI have a pin for pin 2.5V 250mA LDO with a soft start mechanism?  The configurator shows the TPS732 as being soft start but NO where in the datasheet does it mention 'soft start'.

Sincerely.

Steve

  • 0
    TI__Guru 55940 points
    Hi Steve,

    We have not tested TPS734 with that particular circuit; however, you can size your feedforward capacitor (Cfb in the datasheet) in order to provide a soft-start.

    Any pin to pin device will also not have a NR/SS pin which is the pin that is dedicated to a soft-start; however, similar to TPS734, when using an adjustable device, a larger feedforward capacitor will provide some softstart functionality.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Expert 2355 points

    Hi Ryan....

    Thanks for your response.  I am a little confused however.  Do you have a spice model for the TPS734 that you could send me?  I am running some linear regulators through LTSPICE and varying the Cfb does NOT seem to change the start up time of the output waveform.  Currently my Cfb is 5pF for fast response.  


    I have a TPS734 designed onto my product (a pulsed wireless sensor).  I enable it, collect analog data for a very brief period then RF transmit.  The act of turning on the sensor creates a very high current pulse to supply the capacitors on the analog section of the board.  With LiMn source technology this pulse current causes a minor voltage dip in VCC.  Of course the downside of this technology is operating temperature.  I would like to support high temp markets so I am looking at a Lithium Sulfuryl chemistry (not without its own problems 3.9V nominal) as well as Thionyl Chloride.  My concern with these technologies are pulse current capabilities.  In particular I have witnessed Thionyl Chloride voltages dips of 1.3V at high temperatures because they cannot deliver the energy fast enough (0.5A for a 50us duration).  

    As a add on:  from TPS734 datasheet (doesn't look like you get much soft start value from Cnr when this variant of product is used only 5us of value from 1nF to 47nF)

    CNR = none 45 µs Startup time, V CNR = 0.001µF 45 µs OUT = 0 ~ 90%, TSTR VOUT = 2.85V, CNR = 0.01µF 50 µs RL = 14Ω, COUT = 2.2µF CNR = 0.047µF 50 µs

    Thx

    Steve

  • 0 in reply to steve wenner
    TI__Guru 55940 points

    Hi Steve,

    We do not currently have a model for TPS734.  For this device an EVM is available to help prototype a board.  

    Please note that there is not an NR pin on the adjustable version of TPS734.  If you are switching to a fixed version of the device, it is important to note that the fixed versions of TPS734 have a quickstart feature.  The quickstart is used to charge the NR capacitor faster for customers who wish to use a larger NR capacitor for noise considerations but do not wish to incur a longer startup time.  I did not mention the quickstart in my previous response as you had indicated that you were using the adjustable version of the device (TPS73401) which does not have a quickstart.

    Very Respectfully,

    Ryan