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

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.

  • 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

  • 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.

  • 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.

  • 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.

  • 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