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Original question:

LMR14020-Q1: Max Css for soft start

LMR14020-Q1: WSON package soft start maximum Css value

Jingjing Zhao
Prodigy 80 points
Part Number: LMR14020-Q1
Other Parts Discussed in Thread: LMR14030-Q1, , LMR14030

Hi,

It's mentioned" For design with LMR14030-Q1 in WSON package, the maximum value of CSS is 4.7 nF." in datasheet.

WSON package is used in our application, EN pin is connected to external circuit, not to VIN.  What will be happened if Css is 22nF? How can I design the test case to verify/simulate it?

If we change EN connection like below way, is it OK to keep Css 22nF?

In order to understand the mechanism of Soft start, I find serial related questions had been answered before. Sorry still unclear about some points. It's much appreciated to have your opinion.

1. I am not clear about" If you ramp too slowly, the device will go into current limit trying to charge the output caps. " which mentioned in below link. Could you please kindly explain more?

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1101346/lmr14020-q1-max-css-for-soft-start

2. A waveform is posted about no soft start at begining, however, I don't know The mean of channel 1, 2, 4. Could you please give your comments. I assume channel 4 is output current, The peak current means no Soft start, the correct one should be like Datasheet "Figure 24 Start-up By Vin IL waveform", right?

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/554338/lmr14030-q1-wson-soft-start

3. I don't undstand why Css will be no discharge path if Vin powered and EN disabled, would you please kindly share your comments? 

 

Plus, Same desription of LMR14030 WSON package in datasheet. May I know whehter same behavior between LMR14030-Q1 and LMR14020-Q1?

Do you have any PSpice simulation file of LMR14020-Q1?

Thanks!

  • 0
    TI__Genius 15380 points

    Hi Jingjing,

    I would strongly advise not designing with a soft-start capacitor larger than what's specified in the datasheet. You are welcome to test and validate the design yourself, but just know you are operating outside datasheet conditions and could run into trouble either during validation or in the field.

    1. Here's the process flow of what happens when Css is too high.

          a. The device will attempt to charge the output caps while slowly ramping the output voltage.

          b. The device will spit out too much current and hit the internal current limit of the device.

          c. The device will turn off for a short amount of time, then attempt to turn back on again.

          d. repeats back to a.

    2. The waveforms are probably #1 VOUT, #2 PGOOD, and #4 SW.

    3. The device itself charges Css off of VCC internally generated. It uses that with respect to VCC to slowly track the output voltage. 

    The all devices should have similar soft start behavior. Check the respective datasheets for detailed information.

    Here is the PSPICE file:

    https://www.ti.com/lit/zip/snvm706

    Thanks,

    Andrew

  • 0 in reply to Andrew Kutzler
    Prodigy 80 points

    Hi Andrew,

    Thanks of your comments firstly!

    Sorry still some more questions about it.

    1. Regarding the circuit I post( EN connect to Vin PIN via resistor divider), Is it OK or not Keep Css 22nF? I noticed there some explaination in the past---Should be no problem EN connected to VIN pin directly.

    2. From my understanding, cap charge current I=C(du/dt), if U(output voltage) slowly rumping, output current should not high in general, Could you please kindly explain more?

    3. Do you mean VCC is VRef?  How to discharge of CSS when EN from Disable to Enable?

    Thanks in advance!

  • +1 in reply to Jingjing Zhao
    TI__Genius 15380 points

    Hi Jingjing,

    No problem!

    1. There's no problem connecting EN to VIN directly or through a resistor.

    2. It depends on what the load is and how it's configured. If the load is sucking current while the output is slowly ramping and charging COUT then the current may still be high.

    3. Yes, I was referring to the internal reference voltage the device generates (using an LDO) for logic operation. Your second questions is also a good point for sizing Css small. The device will have to discharge Css itself, but it should be ok if it's a small enough size. The same principle is applied to CBOOT.

    Thanks,

    Andrew