LM5121: About switching frequency

Hide

Guru 21045 points

Part Number: LM5121

Hi Team,

We would like to use LM5121 at the following conditions.

Because, our customer would like to reduce the total BOM cost.

However, we can’t simulate it at Fsw=around 1Mhz.

(we can simulate it at Fsw=around 400kHz.)

Could you please improve this problem?

[Conditions]

VIN=12V

VOUT=24V

IOUT=4A

Fsw=around 1MHz

Regards,

Kanemaru

over 6 years ago

0 Youhao Xi over 6 years ago

TI__Guru 68675 points

I don't think the LM5121 can support your application at 1MHz. Note that the min off time can be 550ns, leading to an max duty cycle of 45%, while you need about 50% duty cycle (slighter bigger than 50% owing to sync FET voltage drop) to produce 24Vout. To make it work, reduce your switching frequency to be less than 800kHz, to leave some margin.

One point to note: I do not recommend >500kHz for your application even that it is doable. The LM5121 is a hard switching controller, and the switching losses can become excessive at higher switching frequency. Low efficiency and high cooling requirement can easily defeat your effort to minimize the solution size by running at high frequency.

0 Hide over 6 years ago in reply to Youhao Xi

Guru 21045 points

Hi Youhao-san,

Thank you for the information and your advice.

We understand that a suitable switching frequency is less than 800kHz and your recommended frequency is less than 500kHz.

We would like to simulate LM5121 at switching frequency from 500kHz to 800kHz.

Could you please improve WEBENCH?

Regards,

Kanemaru

0 Youhao Xi over 6 years ago in reply to Hide

TI__Guru 68675 points

I have to forward this to the attention of our Webench expert.

0 Hide over 6 years ago in reply to Youhao Xi

Guru 21045 points

Hi Youhao-san,

Thank you for your prompt reply.

I would appreciate it if you could contact Webench expert.

Regards,

Kanemaru

0 Youhao Xi over 6 years ago in reply to Hide

TI__Guru 68675 points

It seems you haven't gotten an answer yet. I will ask them again today.

0 Hide over 6 years ago in reply to Youhao Xi

Guru 21045 points

Hi Youhao-san,

Thank you for your kind support.

I’m looking forward to hearing from you.

Regards,

Kanemaru

0 Praveen Badiger over 6 years ago in reply to Hide

TI__Expert 8365 points

Hi Kanemaru,

We are working on this.We will get back to you on this very soon.

Regards,

Praveen Badiger

0 Hide over 6 years ago in reply to Praveen Badiger

Guru 21045 points

Hi Praveen Badiger-san,

Thank you for your kind support.

I understand. Thank you very much.

Regards,

Kanemaru

0 Praveen Badiger over 6 years ago in reply to Hide

TI__Expert 8365 points

Hi Kanemaru,

Sorry for making you hold for a long time.We have fixed the issue and design can be created at 1 MHz frequency using custom FETs. Due to non-availability of suitable FETs in our database, custom FETs are being used to create the design.Please replace the suitable FET in the design as per your need.

The fix will be available at http://www.ti.com/product/LM5121 within one or two days.

Please let me know if there are any queries.

Thanks & Regards,

Praveen B

0 Hide over 6 years ago in reply to Praveen Badiger

Guru 21045 points

Hi Praveen-san,

Thank you for your response.

I greatly appreciate your cooperation.

Regards,

Kanemaru

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LM5121: About switching frequency