TPS563257: Feedforward Capacitor Design

Howard Chen

Part Number: TPS563257

Hi TI Team,

Our application is 12V to 3.3V. We want to know whether feedforward capacitor is necessary because it's default DNP on your reference design.

If it's necessary in some application, we also want to know how to choose its value because its range in datasheet is very wide (10 - 470pF).

Thanks.

over 2 years ago

0 Lucia Gao over 2 years ago

TI__Mastermind 30761 points

Hi Howard,

Thanks for reaching out! Let me clarify it.

1. For our reference design which is shown on EVM, it's for 1.05Vout which means that for 1.05Vout, it does not need a feedforward capacitor.

2. If the output voltage is higher, for example, 2.2V/ 3.3V/ 5V or higher, feedfoward capacitor is required for loop stability. In the datasheet Cff shows a range and it does not mean that every value in this range can work for every combination of L and C.

3. Regard to how to choose Cff: please kindly find introduction of how to select Cff in this application note How to select Cff for DCAP2 and DCAP3.

4. For a typical design of 3.3Vout, you can also follow below typical BOM:

Thanks! Kindly let me know any of your concern! We can also help review your schematic.

BRs

Lucia

0 Lucia Gao over 2 years ago in reply to Lucia Gao

TI__Mastermind 30761 points

Hi Howard,

Also add a comment of the above recommend table that Cout is output capacitor norminal value and typical Cout category is MLCC, 0805, 10V.

Thanks!

BRs

Lucia

0 Howard Chen over 2 years ago

Prodigy 60 points

Hi Lucia,

I appreciate the quick turnaround. Attached is our schematic for your reference. BTW, where do we find the typical BOM you provided? It seems that it's different from its datasheet.

0 Lucia Gao over 2 years ago in reply to Howard Chen

TI__Mastermind 30761 points

Hi Howard,

Your welcome. Actually the datasheet is advance information status and not the final release version and we will give an update for the final release version datasheet in around May'2023. You will find the updated information in next month. Thanks for your understanding!

I will review the schematic and get back to you soon!

BRs

Lucia

0 Lucia Gao over 2 years ago in reply to Howard Chen

TI__Mastermind 30761 points

Hi Howard,

Please kindly find my comments:

(1) Since I am not sure of the actual effective value of output capacitor, I take a capacitor from Murata as an example which is GRM186R60J226ME15 22uF/6.3V/0603. Its effective value of 3.3Vout is around 7uF. The phase margin from simulation is around 45°. If you want to better phase margin, there are two ways: <1> You can remove one Cout and use 2 output capacitors, it can help increase the Fcross and increase phase margin; <2> Choose a larger Cff, for example, 68pF. It's also recommend to verify the loop performance on your board.

(2) Inductor: Since I don't have Part number of the inductor, please kindly check about the inductor rated current and saturation current. Saturation current must be greater than the calculated peak current and the RMS or heating current rating must be greater than the calculated RMS current.

Thanks! Kindly let me know any of your more concern!

BRs

Lucia

0 Howard Chen over 2 years ago in reply to Lucia Gao

Prodigy 60 points

Sorry for late reply. Is there any easier way to check whether phase margin is enough? Regarding to inductor selection, our selection is fine with your suggestion. Thanks.

0 Shuai Fan over 2 years ago in reply to Howard Chen

TI__Genius 13451 points

Hi Howard!

We are in vacation, will give reply the day after tomorrow!

Shuai

+1 Lucia Gao over 2 years ago in reply to Howard Chen

TI__Mastermind 30761 points

Hi Howard,

No worries and thanks for your feedback. Actullay, as a best practice and easiest way, suggest to start your design based on L, Cout and Cff recommended table in the datasheet. It has been validated.

BRs

Lucia