LMZ31506: About slow start charging current of LMZ31506

hisashi yoda

Part Number: LMZ31506

I designed 9.16 Slow Start (SS/TR) in the LMZ31506 data sheet with Css=4700pF (±10%) and SS time=2.8ms, but the power startup result is 2.2041ms. (See attached waveform)

Even taking into account the error of the capacitor, the result is outside the specified range.

So, could you please tell me about the Soft-start charging current range of LMZ31506?

over 1 year ago

0 Eileen Hernandez Hernandez over 1 year ago

TI__Expert 5208 points

Hi Hisashi,

Correct if wrong, but the capture looks like VOUT. If so, that wouldn't exactly capture the soft-start time. A soft start time of 2.8ms refers to the internal SS ramp than can be measured at SS pin, referred to as Vss in the datasheet figures:

Please reopen if clarification is needed.

Thank you,

Eileen

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Eileen:

Thank you for answering.

Is it possible to design the slow start time to include an error?

2.8ms is the time that can be measured with the SS pin, but please tell me how to calculate the time that Vout outputs.

For reference, I obtained the Vss waveform when Css=4700pF (±10%). (See attached waveform, green line).

It was 5.8ms, which was different from 2.8ms.

The measurement point is the SS/TR terminal.

Thank you,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

On average it looks like TVOUT is 1/3 of TVSS, you may have to tune Vout accordingly through measurements.

It's also important to note that this module contains an internal Css which allows to Tss=1.1ms when left open. My guess is that this cap has a 10% tolerance as well, so this may need to be accounted for in overall tolerance.

Lastly, make sure to have STSEL pin connected to AGND to enable SS cap.

Thank you,

Eileen

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Eileen:

Thank you for answering.

STSEL pin is connected to AGND.

Table 6. Please tell me about Slow-Start Capacitor Values and Slow-Start Time.

Does SS Time indicate the startup time of Vout or Vss?

Thank you,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

From the datasheet figures it looks like SS time refers to Vss. I have ordered an EVM to test this out. I should get you an answer by Monday or Tuesday the latest.

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

Thank you for answering.

Please confirm using EVM.

By the way, could you please tell me how to calculate the Vout time?

We have a design that controls the output time of Vout.

Thank you,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

I have tested SS time with the EVM at the same conditions as your design (Css=4700pF, Vout=3.3V) and got similar times as you for Vss and Vout, which would make your times correct. I need to do some more digging to figure out where the SS time is starting at. I'll try to get back to you on it as soon as I have an answer, ideally tomorrow.

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

Thank you for confirming.

We look forward to your reply.

Thank you,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

HI Yoda,

I will be taking some more SS measurements and get back to you on your questions. Please give me a day or two.

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

Thank you for contacting me.

I would appreciate it if you could share your measurement results.

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

I'm still collecting the SS and VOUT data. Will get back to you tomorrow with the findings.

Thank you,

Eileen

0 Eileen Hernandez Hernandez over 1 year ago in reply to Eileen Hernandez Hernandez

TI__Expert 5208 points

Hi Yoda,

I have taken measurements for different SS values. You can use the results as reference along with interpolation to achieve the desired SS value. Please see chart below:

CSS(pF)	TVOUT(ms)
Open	0.9
2200	1.5
4700	2.2
10000	N/A
15000	5.2
22000	7.6
25000	N/A

Do keep in mind that other aspects of the capacitor can affect the actual capacitance such as tolerance, derating and temperature so soft start time may not be exact.

Please mark as resolved if this answered your question.

Thank you,

Eileen

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

Thank you for answering.

We will consider it as a reference.

I have a question about the information on the datasheet.

In 9.16 Slow Start (SS/TR) of the datasheet, Regarding Table 6. Slow-Start Capacitor Values and Slow-Start Time, until now I could not understand that it was the startup time of Vss.

Please let me know if there is a description on the data sheet that explains that it is Vss.

If it is not listed, please consider revising the datasheet.

When designing for soft start time, are there any causes of variation other than the capacitor?

For example, factors inherent in the parts of LMZ31506, etc.

Thank you,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

Based on my measurements, table 6 doesn't seem to be referring to Vss. Typically in this part, Tvss is about 2.8x of Tvout.

For soft start the main variation will be the capacitor, to my knowledge no other component should make a significant difference.

Regards,

Eileen

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

Thank you for answering.

We found that Tvss is approximately 2.8 times larger than Tvout.

I have acquired the waveform when Css=open, so I will attach it.This value is also close to 2.8 times.

I understand that the cause of the variation is the change in the capacitor.

By the way, could you please tell me what the values in Table 6. Slow-Start Capacitor Values and Slow-Start Time in 9.16 Slow Start (SS/TR) of the data sheet indicate?

I no longer know what information to refer to when designing the TVout soft start time.

Regards,

Yoda

0 Athos Zhao over 1 year ago in reply to hisashi yoda

TI__Expert 7320 points

Hi Yoda,

Our US team will reply you tomorrow.

0 hisashi yoda over 1 year ago in reply to Athos Zhao

Intellectual 640 points

Hi Athos:

Thank you for your response.

We look forward to reply.

Regards,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

Your measurement for SS=open is very similar to my measurement. I measured 0.9ms for Vout and and 2.6ms for SS.

Values for table 6 are meant to display the time it takes for Vout to ramp up. I noticed some difference between the measured values and the table, however those changes are due to capacitor tolerance, derating etc. I believe table 6 was meant to show actual capacitance not capacitor value.

Regards,

Eileen

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

Thank you for answering.

I understand that Table 6 is not the value of the capacitor, but the actual capacitance.

Using LMZ31506, the startup time of TVout is determined by the capacitance of the capacitor.

What should I consider when deciding the capacitor constant?

If I had a calculation formula, I could calculate the capacitor's tolerance and derating, but I'm having trouble because I don't have any information other than Table 6.

Please let me know if you have any information.

Regards,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

Unfortunately we don't have an SS equation for this device in particular which is why table 6 is provided.

One way you could try to get an estimate is by plotting the measured time (data provided earlier in the forum) vs Css, and extracting the equation from it. Typically this can be tuned through excel or similar program.

Using that method, I get equation "y = 0.0003x + 0.7671", where x would be Css and y would be the time. However do keep in mind the equation through this method is an approximation and may not 100% accurate in all cases.

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

Thank you for your suggestion.

Is it possible for you to provide documentation regarding the calculation method?

Table 6 is the actual capacitance, so

Could you please tell me the capacitance caused by the internal parts of the LMZ31506?

Regards,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

Yes this is the calculation based on my measured Vout time:

The graph below generated the linear equation mentioned earlier as approximate.

Regards,

Eileen

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

Thank you for the information about the calculation method.

Can I use this result as the design value for LMZ31506?

Also, could you please include this information in the TI documentation to be used not only for this E2E but also for the design of the LMZ31506?

Regards,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

I can't confirm at this moment the inclusion into TI documentation, however your suggestion is valuable and will be considered in future updates. Please feel free to proceed with the calculation method above, do keep in mind that it may not be the exact same due to capacitor variances.

Regards,

Eileen

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

I understand the variation in capacitors.

I think it is necessary to show the calculation formula to include the variation in the calculation.

The information you showed (y=0.0003x+0.7671) is the result of measurement using an evaluation board owned by your company.

I don't think it is possible to design based on this information.

I would like to hear your opinion on the design concept.

Regards,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

Table 6 is intended to represent the timing of the VOUT ramp up period, I may have mixed this earlier on the thread with the time at SS pin.

There's difference between the data measured vs the table which can be attributed to capacitance effects. The device incorporates an internal Css cap with a tolerance range of +/-10%. This variance should be considered when calculating soft start and adding external capacitance which comes with it's own tolerance as well.

The table I provided earlier are from my own measurements are meant as approximation for you to use as reference and therefore not concrete data to add into TI documentation. Also the variation can't be shown because each capacitor is different, which is why Table 6 only describes the capacitance and not the capacitor.

Lastly, the values measured make sense with Table 6 when the internal cap tolerance is accounted for. Hope this answered your question.

Regards,

Eileen

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

It was agreed that Table 6 is the start-up time of VOUT.

I think the internal cap tolerance is the capacitor (red frame) shown in the block diagram in the data sheet.

What is the capacity and tolerance of the internal cap?

Also, could you please tell me the values for VREF and current value (blue frame) shown in the block diagram?

If you know this information, I think you can calculate the formula.

Regards,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

Internal cap is 3300pF with 10% tolerance.

Based on a part that has same SS and Vout circuitry, the requested values should be:

- Vref be 0.6V

- Iss=2.3uA

You can use SS formula below:

Regards,

Eileen

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

Thank you for answering.

The unit of Css is nF, but isn't it pF?

Please tell me the variation of Vref and Iss.

Regards,

Yoda

0 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

That is correct, you will need to convert Css to nF to use that formula.

Vref typically has about 1% variance and Iss can range between 2.26uA-2.32uA (rough estimate).

Regards,

Eileen

0 hisashi yoda over 1 year ago in reply to Eileen Hernandez Hernandez

Intellectual 640 points

Hi Eileen,

Thank you for answering.

Regarding units, I forgot to convert.

The design value can now be determined.

Regards,

Yoda

+1 Eileen Hernandez Hernandez over 1 year ago in reply to hisashi yoda

TI__Expert 5208 points

Hi Yoda,

Glad to hear the the issue is resolved.

Regards,

Eileen

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LMZ31506: About slow start charging current of LMZ31506