LM25149-Q1: Design Suggestions/Recommendations for using LM25149 for Synchronous Buck Converter

Prem Ankitkumar Shah

Part Number: LM25149-Q1
Other Parts Discussed in Thread: LM25149, TMS320F28027, LM25141

Hi,

I am designing a synchronous Buck Converter with ZVS turn-on of High-side switch(basically using the negative current of inductor to facilitate ZVS switching of high-side switch). I am considering using LM25149 as a design solution as I believe it fits the Criteria.
Here are my design parameters :
Vin : 9-12 V, Vout =5 V, fs=200 kHz, L=2uH, COUT=390 uF, Peak inductor current : 3.7A, Minimum inductor current : -0.7 A(to facilitate ZVS turn on for High side switch)

I am following a combined Digital-Analog control technique in which the error amplifier and Type-II compensator will be implemented externally in TMS320F28027 and the output of this digital control is being fed to EXCOMP pin of LM25149.

I am looking for recommendations on how this would be possible.

PS : I think since the error amplifier is OTA, it would be possible to override it by using combination of Controllers' GPIO pin with buffer in needed.

Thanks,
Prem Shah.

over 3 years ago

+1 Timothy Hegarty over 3 years ago

TI__Guru 52702 points

HI Prem,

Yes, you can overdrive COMP if needed as it's the output of a transconductance error amp. The LM25141 is another option here - a similar current-mode controller without the active EMI circuit.

Regards,

Tim

+1 Prem Ankitkumar Shah over 3 years ago

Prodigy 130 points

Hi Tim,
Thanks for the response.

Do you recommended any additional pin connections for driving the EXCOMP pin, for instance recommended connection for FB pin( As I will be having Error Amplifier separately implemented in microcontroller)?

Also, just to confirm, since the Error Amplifier source and sink current are listed as 180uA and 160uA respectively, I should be able to drive EXCOMP directly from a microcontroller without any additional buffer, right?

Thanks again, I appreciate all your help!

Best,

Prem.

0 Timothy Hegarty over 3 years ago in reply to Prem Ankitkumar Shah

TI__Guru 52702 points

Yes, correct. The EA sink/source currents are low, so it should be easy to drive externally. Tie FB high or low - it doesn't really matter as EXTCOMP is forced by your control circuit. However, connect a feedback divider for the particular Vout setting if you need the PGOOD output.

0 Prem Ankitkumar Shah over 3 years ago in reply to Timothy Hegarty

Prodigy 130 points

Hi Tim,

Thank you again for the help.

I was wondering about calculation of slope compensation if I am planning to use different value of inductor given as opposed to Lo-ideal given in datasheet section 8.3.12.

According to the formula (8) in datasheet, I am getting around 6uH of inductance value for obtaining slope compensation equal to down slope of inductor current. Now, Instead if I plan to select 2uH of inductance, that would mean that slope compensation will be inductor down slope divide by 3, right? As I assumed that slope compensation inside the chip is fixed.

Please let me know if I am understanding this incorrectly.

Thanks,

Prem Shah.

+1 Timothy Hegarty over 3 years ago in reply to Prem Ankitkumar Shah

TI__Guru 52702 points

Hi Prem, please use the quickstart calculator to assist with component selection: https://www.ti.com/tool/LM5149-LM25149DESIGN-CALC

If you select an inductance 3x lower than recommended, then the internal slope comp will be 1/3 of ideal.

Regards,

Tim

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LM25149-Q1: Design Suggestions/Recommendations for using LM25149 for Synchronous Buck Converter