DRV8350R: if buck regulator output resistor must needed?

Bruno Duan

Prodigy 225 points

Part Number: DRV8350R
Other Parts Discussed in Thread: LM5008, LM5008A, TIDA-010056

Hi Ti team,

I would like to know if the R-out must needed? If do not have it, will there have any risks?

Thanks!

Best regards,

Bruno Duan

over 5 years ago

0 Adam Sidelsky over 5 years ago

TI__Guru 65450 points

Bruno,

The Buck inside the DRV8350R is the LM5008A as mentioned above. I will forward this post to the team which supports the LM5008.

Please note that the post will not move from this location.

Regards,

-Adam

0 Ethan T over 5 years ago in reply to Adam Sidelsky

TI__Expert 3835 points

Hi Bruno,

The LM5008 is a COT device which relies on voltage ripple to properly regulate. Since ceramic capacitors had very low ESR a series resistor is required to cause more ripple and proper regulation.

Regards,

Ethan

0 Bruno Duan over 5 years ago in reply to Ethan T

Prodigy 225 points

Ethan,

Thanks for the reply!

If the value of resistor too small, feedback pin could not get 25mV ripple, will the regulator work like a regular BUCK convertor?

Best regards,

Bruno Duan

0 Ethan T over 5 years ago in reply to Bruno Duan

TI__Expert 3835 points

Hi Bruno,

25mV ripple is hard recommendation to ensure stability of the device. The problem with hysteric control is that if the ripple is not large enough, the device can oscillate in between states and cause instability.

There are other ways of ripple injection such as type-3 where the output voltage ripple will be minimal (http://www.ti.com/lit/an/snva874/snva874.pdf). This does have the highest BOM cost solution, but the lowest voltage ripple at the output.

Hope this helps,

Ethan

0 Bruno Duan over 5 years ago in reply to Ethan T

Prodigy 225 points

Hi Ethan,

THanks for the explaination!

Will oscilation and instablity damge the part permanently? or just cause bigger ripple?

Best Regards,

Bruno Duan

0 Ethan T over 5 years ago in reply to Bruno Duan

TI__Expert 3835 points

Hi Bruno,

It is possible in load transient conditions that oscillations can exceed voltage limits on devices it is connected to.

Regards,

Ethan

0 Bruno Duan over 5 years ago in reply to Ethan T

Prodigy 225 points

Ethan,

Got it, Thanks very much!

Beat regards,

Bruno Duan

0 Bruno Duan over 5 years ago in reply to Ethan T

Prodigy 225 points

I checked Ti reference design "TIDA-010056", found below design on the BUCK convertor:

Cause C38 is in parallel with C26 and R31 through a 0ohm resistor R28, will it cause any problem on ripple generation?

Best regars,

Bruno Duan

0 Ethan T over 5 years ago in reply to Bruno Duan

TI__Expert 3835 points

Hi Bruno,

The parallel capacitance will affect the ripple. The article below gets more into detail.

https://www.powerelectronictips.com/how-mixed-type-output-capacitors-affect-dcdc-converter-stability/

However, I am not worried since you have a feed-forward capacitor which makes this a type-2 ripple generation. The capacitor is AC-coupled with the voltage ripple to the FB node so the actually vout ripple does not need to be significant. Follow this app report below for more information:

http://www.ti.com/lit/an/snva776a/snva776a.pdf

Regards,

Ethan

0 Bruno Duan over 5 years ago in reply to Ethan T

Prodigy 225 points

Ethan,

Thanks very much for the explaination!

Best regards,

Bruno Duan

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DRV8350R: if buck regulator output resistor must needed?