TPS54560: Variable Voltage Ouput (60V input and 5-50V output) Compensation issues.

Sagar Jaiswal

Part Number: TPS54560
Other Parts Discussed in Thread: TPS54360

Hello,

I have designed a PCB that would take a 60V input and output a voltage from 5V to 50V depending on a digital potentiometer IC.

This is my 3rd/4th iteration of the board and it would be great if I could get someone to look over the PCB and Schematic as I have doubts on the compensation circuit.

How will the compensation values work since the output is a range? I used the Webbench and for every different output voltage the compensation circuit changes.

I do not want to post the schematic and PCB layout here, so it would be awesome if I can email it to the person that helps me out!

Kind Regards

Sagar

over 3 years ago

0 Timothy Hegarty over 3 years ago

TI__Mastermind 49827 points

Hi Sagar,

If the output capacitance is ceramic, there will be some variation with voltage (you can check Murata's online Simsurfing tool to get an idea of capacitance falloff with voltage).

The effective output capacitance and oad resistance set the dominant pole, so if you compensate at max Vout it should be good across the load range.

Regards,

Tim

0 Sagar Jaiswal over 3 years ago in reply to Timothy Hegarty

Prodigy 230 points

Hello Tim,

Thank you for the reply.

The output capacitance is not ceramic, it is an electrolytic. Specifically i'm using two of the VE-101M1JTR-1010 in parallel (2 x 100uF).

So just to confirm, by using web-bench and setting the voltage to the max that we'd like to output (50V) I should calculate the compensation and so it should operate fine for the rest of the voltages?

0 Timothy Hegarty over 3 years ago in reply to Sagar Jaiswal

TI__Mastermind 49827 points

Hi Sagar,

The capacitance is constant for an electrolytic. However, the ESR is likely very high for this component and has significant variation over temperature (much higher at cold), thus complicating compensation (as it contributes an ESR zero).

Recommendation is to use a polymer electrolytic with low and stable ESR. The compensation guidelines are given in the TPS54560 datasheet.

Regards,

Tim

0 Sagar Jaiswal over 3 years ago in reply to Timothy Hegarty

Prodigy 230 points

Again, thank you Timothy Hegarty for your prompt responses.

Yes I've read the datasheet before, because we were using electrolytic we were using TYPE 3 circuit for compensation (but it's not doing the job).

As per your recommendation, I've went and found this polymer capacitor (https://www.mouser.ca/ProductDetail/United-Chemi-Con/HHSD630ELL101MJC5S?qs=sGAEpiMZZMs9%2FSpGH%2Fyc7Oi7G6YMoDvi8VTgYicAe1UU%2FBb%252BDTWekg%3D%3D)

Would something like this work? It has an ESR of 20mOhms. So calculating compensation using this for 50V should work?

0 Timothy Hegarty over 3 years ago in reply to Sagar Jaiswal

TI__Mastermind 49827 points

Yes, that cap looks better, although I see its impedance increases 1.5-2x at cold.

Generally, type-3 compensation shouldn't be required for current-mode control, but the cap across the upper feedback resistor is useful if extra phase boost is required.

Regards,

Tim

0 Sagar Jaiswal over 3 years ago in reply to Timothy Hegarty

Prodigy 230 points

Ok Awesome, I will use that one or a similar polymer capacitor and try to keep the ESR as low as possible.

So just to confirm, with a low ESR polymer capacitor, if I calculate the compensation using web-bench at 50V (the max voltage we need) it should suffice?

You've been great, thank you!

Kind Regards

Sagar

0 Sagar Jaiswal over 3 years ago in reply to Sagar Jaiswal

Prodigy 230 points

Timothy Hegarty could you please confirm above so I can set the issue resolved. Thank you :)

0 Timothy Hegarty over 3 years ago in reply to Sagar Jaiswal

TI__Mastermind 49827 points

Sounds good, Sagar. you can also use the datasheet equations to calculate the compensation components. Take a look also at the TPS54360 Excel calculator (the link is in the TPS54560 product folder).

Tim

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TPS54560: Variable Voltage Ouput (60V input and 5-50V output) Compensation issues.