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Part Number: TPS25982

Hi,

・Could you please tell me about the Output voltage ramp time, TdVdt in Table 8 (Page 31)?

①Is 20ms a typical value?

If it is a typical value, could you please tell me the maximum value?

②If the values of CL and RL change, will the value of 20ms also change?

Could you please let me know if there are any other elements that change the value?

③If the value changes depending on CL, RL, etc., could you please tell me the formula for calculating TdVdt?

Thanks.

• Hi Shindo san,

1. No, 20ms is not the typical value. Our device provides adjustable slew rate (soft start) using an external capacitor at dvdt pin. This shown in 9.2.1 section is an example. You can refer to section 8.3.3.1 to see the dvdt setting and I am attaching section 7.7 table which show some examples. Please refer this section 7.7 also.
2. CL & RL won't change the start up slew rate but will determine the current drawn. You have to ensure that very high current is not drawn which results in thermal shutdown. We have a design calculator to help with that. It is excel based with a row about glitch free start up. I am attaching the link to it. SLVC779 Calculation tool | TI.com

Best Regards,
Arush

• Hi Arush,

①Could you please tell me the maximum value of Ton when the external capacitor on the dVdt pin is 10nF?
②Could you please tell me the formula to convert the SR calculated in 8.3.3.1 to Ton?
③Is TdVdt not Tr but Ton?
④Is CdVdt the only element that changes the slew rate?
⑤Thank you for letting me know about the calculation tool.
Are there any materials written on how to use the calculation tools?

Thanks.

• Shindo San,

1. SR you get with 10nF is 4600/(10*1000) which is 0.46V/ms. You did not mention nominal operating voltage in your application. I will assume it to be 12V. So time required will be 12/0.46 = 26ms.
2. point #1 show the steps to calculate. Find SR using 4600/Cdvdt (pF) and use Slew Rate to find the time as T = V/SR.
3. Section 7.7 has a figure showing the difference between Tr and Ton. Basically, Ton includes some time delay + rise time.
4. Yes, if the device is in dvdt start up, Cdvdt will control the slew rate. If the current drawn is very high, then the device will be in Active current limit mode startup which will be different. It is not recommended to start the device in ACL mode as it can cause startup issues.
5. I am not aware of any guide. You can try to understand it. You basically only enter your design requirements in few cells which are marked with different color. Please try it and if any specific issue comes, you can ask it.

Best Regards,
Arush

• Hi Arush,

①I would like to know the Min and Max of T.
・Is 4600 of CdVdt=4600/SR a typical value?
If it is a typical value, could you please tell me the Min and Max?

②T=V/SR, but is it Rise time (Tr)?
In the case of Tr, could you please tell me how to find Turn on delay (Td,on)?

★I ultimately want to know the maximum value of Ton under the following conditions.
・CdVdt=10nF
・Vin=12V

Thanks.

• Hi Shindo San,

I left early due to some reason and hence missed your reply. Here are my replies.

1. Yes, this formula gives you typical value. As such we don't give min-max formula for slew rate calculation. If I use characterization of dvdt charging current from the figure 20 in section 7.8. You will get the formula range as SR = 4450/Cdvdt to 4650/Cdvdt.
2. Rise time is calculated from 10% voltage level to 90% voltage level. If you multiply 0.8 with the time calculated using this formula, you will get rise time. Again Td,on delay is not characterized for the 10nF 12V value. Based on the table, we can expect delay to be around 4-5ms.
3. I understand what you require. it should be around 25.8ms~27ms + Td,on (which should be around 5ms), so you can expect maximum value of Ton will be around 32ms. Add some buffer if the timing constraints are very tight.

I have one example of this device in startup. Here if you observe, we are getting around 5ms Td,on delay. This test was done for 15nF dvdt 12V. We are not characterizing this spec and I am just sharing a test which I did for some other reason as an example.

Please feel free to reply back if something is not clear or any follow up question. Click resolved if no other questions.

Best Regards,
Arush

• Hi Arush,

Thank you for providing an answer to my question. I really appreciate it.

1.Why is the value of Idvdt in Figure 20 of Section 7.8 and the table so different?

Thanks.

• Hi Shindo San,

This is very valid point and I think I made some error here.

Figure is typical value for different voltage and different temperature. Table is done by characterizing multiple devices so include process variations also.

So, if we use the table values, then we will get 19ms to 60ms. In reality, most of the devices will remain near the time range mentioned in previous answer (typical time range for 12V at various temperature) but this range will guarantee for corner cases also.

My apologies for creating this confusion and thank you for pointing this out.

Best Regards,
Arush

• Hi Arush,

Could you please let me know if you have any information on how often special cases may occur?

Thanks.

• Hi Shindo San,

So, I found the data, but I will first have to double check this with Validation team. I will get back to you tomorrow EOD.

Best Regards,
Arush

• Hi Shindo-san,

The data which I got is for something else. I am trying to get the statistical data to find the distribution.

Meanwhile can you please share the end application, operating temperature and other conditions. If you don't wish to share it here, you can send me direct message or mail me.

Best Regards,

Arush

• Hi Arush,

What information is required regarding the end application?

The operating temperature will be 45℃.

Are the other conditions the VIN, VEN/UVLO, RILIM, and Cdvdt values?

Thanks.

• Hi Shindo-san,

I have answered using the private message feature.