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TPS61230EVM-089: Maximum soft-start time

Part Number: TPS61230EVM-089
Other Parts Discussed in Thread: TPS61232

Hi,

I'm trying different size SS capacitors with my eval board and the soft-start time seems to max out around 700-800us.  Is there a maximum soft-start time (or capacitor value) for this part?

Best,

Matt

  • Hi Matt:

    The soft start time could be calculated with Equation 1 in datasheet. Please note the value of the Css should be the effective value with DC bias considered.

    I have noticed that you get the maximum time around 700-800us. If there is no difference when you try much larger Css, please share below information with us:

    1. The test condition: Vin range, Vout target, Load. And the value of the Feedback resistance.

    2. The effective value of Css you use.

    3. The waveform for the Vout, SS, FB, Vin pin. 

  • Hi Minqiu,

    Thanks for the assistance, I have been using Equation 1.  I have resolved my issue in another matter, but I did some testing on SS and believe I can provide some more info to help.

    1. Vin = 3-4.2V, Vout = 5V, Load = 200mA, FB = none (I'm using a TPS61232)

    2. Started with 15nF, tried several values (100nF, 1uF, 10uF) with what looks like little result

    3. See below.

    What I've noticed is that VOUT ramps most of the way within 700-800us BUT then takes the full SS time to reach the final output.  This is confirmed by looking at PG.  Below is a waveform of VOUT (yellow) and PG (blue) with cursors marking the approximate 10% and 90% VOUT points.

    This would be fine if I was monitoring PG, but I'm not.  Instead, the load uses an LDO which starts almost immediately, since the VOUT voltage reaches a point that the LDO can operate.  See image below for VOUT (yellow) and the LDO on the load starting almost immediately (blue):
    As I said, I was able to workaround this issue.  Instead of using SS, I added an RC circuit to the EN pin of the TPS61232 tuned to reach the minimum VEN value after the required delay I needed.  So far this is working wonderfully, but I'd say that TI might want to review this part family as this SS behavior is a bit unusual (I'd expect a steady ramp) and might need to be reviewed for a future design.
    Best,
    Matt
  • Hi Matt:

    It's glad to see you have find a solution. Yes, it's really a good idea and easy to do.

    I guess the result "700us soft start time" you test is for Vout rise from 0V to around 3.xV, am I right? Actually, it's not the soft start time mentioned in datasheet.

    There are several phases during the startup. And before the Soft Start, there is a pre-charge. The output capacitor is continuously charged to a value close to the input voltage.  The details could be seen in Datasheet 8.3.1 "Startup"

  • Hi Matt,

    If you look at the datasheet carefully, in part 8.3.1, the datasheet says the whole soft start includes three phases. The first one is ouput capacitor will be charged to 0.5V with a 500mA current limit. Then output capacitor is charged  to close to Vin with a pre-charege current limit. After that, device starts switching until programmed Vout. So the soft start time means the time from 0V to Vout.

    In your case, the LDO would work once boost converter output voltage is up to Vin, so adding the soft start capacitor won't help. Adding the RC circuit will help.