TPS546D24EVM-856: Help measure the undershoot under heavy transient current load.

Ricardo_Li

Part Number: TPS546D24EVM-856
Other Parts Discussed in Thread: TPS546D24

Hi Teams,

Can you help measure the undershoot at 0.8V output voltage and 35A transient current load？ The distributor told us it would spend several months if we ordering EVM board now. But it may too late for our evaluating.

In our application, transient current load could get up to 35A, output voltage should cover 0.8V to 4V. If we set the switching frequency to 325KHz, using a 300nH inductor. Under a max 200mV undershoot, 12V ±10% Vin. After the calculation, the minimum output capacitor is 2300uF(6.3V). Can you help measure the undershoot using TPS546D24EVM-856?

Do you have a good solution to decrease the output capacitance? 2300uF(6.3V) is too large. How to drop the capacitor cost for mass production?

Thanks for your help in advance.

Regards,

Ricardo

over 4 years ago

0 Gerold Dhanabalan - Online Design Tools over 4 years ago

TI__Genius 14455 points

Hi Ricardo,

You can use the WEBENCH tool to design and run a quick simulation to check if the load transient would be able to meet the specifications.

Here is a link to the WEBENCH design. Please customize and simulate it based on your design requirements. It has advanced capabilities to design for custom load transient specs (available on the left side of the tool)

webench.ti.com/.../SDP.cgi

For the EVM, can you try ordering from ti.com?

https://www.ti.com/licreg/docs/swlicexportcontrol.tsp?form_id=293433&prod_no=TPS546D24EVM-856&ref_url=app_bsr_bsr-ccp

Please let us know if you have more questions.

Regards,

Gerold

0 Ricardo_Li over 4 years ago in reply to Gerold Dhanabalan - Online Design Tools

Intellectual 260 points

Hi Gerold,

Here is simulation result:

Even using a 3000uF capacitor, the undershoot is still very large.

The inductor was set to 300nH, the switching frequency is adjustable.

Do you have any idea to decrease the undershoot? 3000uF capacitor is also very large, How to reduce the capacitance to drop the cost?

Regards,

Ricardo

0 Gerold Dhanabalan - Online Design Tools over 4 years ago in reply to Ricardo_Li

TI__Genius 14455 points

Hi Ricardo,

Where are these simulations from?

regards,

Gerold

0 Anthony F over 4 years ago in reply to Gerold Dhanabalan - Online Design Tools

TI__Mastermind 35115 points

Hi Ricardo,

Can you also share the schematic used for at least one of these simulations? It looks like you may have created this design in WEBENCH. if you did can you share the design with a link?

There are some things you may be able to adjust to improve the transient response. Such as the compensation, switching frequency or inductor value.

0 Ricardo_Li over 4 years ago in reply to Anthony F

Intellectual 260 points

Hi Anthony,

Thanks for your comments.

For our application, we want get a wide output range, from 0.8V to 4V(adjustable via PMBus), the normal output voltage is 1.2V and 3.3V, the max transient current load is 35A. If using TPS546D24 in our application, inductor value should be set to a middle point to cover the wide output range. So a 300nH inductor was chose. 325KHz switching frequency was set to get 0.8V output, 1.1MHz was set to get 4V output.

The max undershoot is 200mV in our application. Can you help me adjust the compensation and switching frequency to get a better output performance? Is there any example that other person using TPS546D24 output a wide rang voltage?

Here are transient simulation files.

WBDesign16_Load Transient-4V 35A 3000uF.pdf

WBDesign12_Load Transient-0.8V 35A 3000uF.pdf

WBDesign15_Load Transient-3.3V 35A 3000uF.pdf

WBDesign14_Load Transient-1.2V 35A 3000uF.pdf

Regards,

Ricardo

0 Anthony F over 4 years ago in reply to Ricardo_Li

TI__Mastermind 35115 points

Hi Ricardo,

I've taken a look through the 4 V design. I think part of the issue may be related to the relatively high ESR of the 1 mF caps being selected on the output. In order to get <200 mV overshoot/undershoot with a 35 A step, the cap impedance must be <5.7 mΩ. The 12 mΩ ESR of the two 1 mF caps in parallel limits the impedance to 6 mΩ at the lowest.

Also you should be able to use ILOOP 7 and VLOOP of 4 for this application to maximize the loop bandwidth. If you can use PMBus to change the compensation, you can likely optimize this even more.

How about the design linked below as a starting point? This uses 3x 10uF and 3x 330uF caps. You should be able to use different bulk caps but make sure their ESR is not too high. The simulated transient response of this one is also below. It's not quite 200 mV yet but it does have a significant improvement.

Also, 35A is quite a large load step. Just to make sure, do you really need this design to support this large of a load step?

https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=306DA9DA65A8B991

0 Anthony F over 4 years ago in reply to Anthony F

TI__Mastermind 35115 points

It is also worth trying ILOOP = 6 and VLOOP = 8 with the 4V output design to further maximize the voltage loop bandwidth through pinstrapping only. However you should carefully check the phase margin and/or transient response on the real board. This setting may cause the voltage loop to crossover too close to the current loop. If this happens, it can cause low phase margin in the voltage loop.

0 Ricardo_Li over 4 years ago in reply to Anthony F

Intellectual 260 points

Hi Anthony,

Thank you for your advice.

Our application is used for burn in test. The transient current is very large sometimes, will get up to more than 30A. Low ESR capacitors would be great for our design.

I have asked some sample to evaluate. Would you mind sharing the PCB source of TPS546D24EVM-856 to us?

Regards,

Ricardo

0 Anthony F over 4 years ago in reply to Ricardo_Li

TI__Mastermind 35115 points

Hi Ricardo,

I should be able to share the PCB source. Do you have a preferred format? This EVM was designed using Altium.

0 Ricardo_Li over 4 years ago in reply to Anthony F

Intellectual 260 points

Hi Anthony,

Altium is OK. Here is my email address: Ricardo_Li2(at)ymtc.com

Regards,

Ricardo

0 Anthony F over 4 years ago in reply to Ricardo_Li

TI__Mastermind 35115 points

Altium files are attached.

PWR856A (12-10-2019 9-44-33 AM).zip

0 Ricardo_Li over 4 years ago in reply to Anthony F

Intellectual 260 points

Hi Anthony,

That's very nice. Thank you for your help.

Regards,

Ricardo

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TPS546D24EVM-856: Help measure the undershoot under heavy transient current load.