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LMR33640: Questions when configuring LMR33640 with IBB topology

Conor
Guru 10455 points
Part Number: LMR33640
Other Parts Discussed in Thread: LMR14020, LMR14030, TPSM265R1

Hi,

I have questions about the following documents:

------

Working with Inverting Buck-Boost Converters

https://www.ti.com/lit/an/snva856a/snva856a.pdf

------

(1) There are examples of LMR33640 and LMR14020 in "8 Design Examples". The LMR14020 has a Schottky diode inserted on the output side. On the other hand, LMR33640 does not have a Schottky diode inserted. Is there any reason?

(2) I have a question about using a step-down DCDC as an IBB topology.
Is it possible to theoretically configure an IBB topology on all devices in a device with a wide input voltage range? Or is it limited to a specific device due to some restrictions?

Thanks,

Koki

  • 0
    TI__Mastermind 27470 points

    HI

    1. LMr33640 is synchronous buck convertor, it integrated the low side fet instead of external diode for LMR14030.

    2. Ideally all the buck converter can be configure to IBB, the restriction is mentioned in the apps notes that the device's abs max need to higher than Vin+Abs(-Vo), and current limit will related to Io and Iin both. 

    Thanks

  • 0 in reply to Daniel Li14
    Guru 10455 points

    Hi, Daniel

    Thank you for your reply.

    1. LMR33640 is synchronous rectification. On the other hand, the LMR14030 is a diode rectification, so a Schottky diode is required. Is this understanding correct?

    　

    2.It is an item about "2.1 Additional Bypass Capacitor and Schottky Diode" of the following URL.

    https://www.ti.com.cn/cn/lit/an/snva882/snva882.pdf

    Although the TPSM265R1 integrated a low side FET, it is recommended to place the Diode externally in this document. What do you think about this?

    If Diode is recommended for TPSM265R1, I think that Diode should be placed for LMR33640 as well.

    　

    3.　Please tell me the details of the following quote.

    Daniel Li14 said:
    current limit will related to Io and Iin both. 

    Thanks

  • 0 in reply to Conor
    TI__Mastermind 27470 points

    HI Koki:

    1. yes, that's right

    2. for the Schottky diode , it is recommend to added at output side .

    3. for inverting buck boost, the average current in inductor equal to Iin+Io. as LMR33640 limited peak current of inductor, so it is related to Iin and Io. and lower Vin will have higher Iin. so the max output power is limited by the min input voltage

    Thanks

  • 0 in reply to Daniel Li14
    Guru 10455 points

    Hi, Daniel

    Thank you for your accurate advice.
    I read the following documentation and have additional questions.
    Working with Inverting Buck-Boost Converters (Rev. A)
    https://www.ti.com/lit/an/snva856a/snva856a.pdf

    　

    1. On P.5, there is a description "In the IBB topology, both the input and output currents are" chopped "."
    I couldn't understand the implication that the output current was chopped. What current is defined as output current?

    　

    2. The selection method of the capacitor on P.15 is decided based on the data sheet. There is also a statement that it is recommended to increase the input / output capacitors for the IBB topology.
     - A. Try the capacitor values ​​listed in the data sheet.
     - B. If the noise ripple is large, increase the input / output capacitor.
    Is it okay to understand the development flow of "A" to "B"?
    I was worried because the description of "B" was not in the design example.

    　

    3.The LMR33640 data sheet Table 9-2 lists the capacitor values in typical settings.

    https://www.ti.com/lit/ds/symlink/lmr33640.pdf

    If I set a Vout that is not in this table, how do I need to calculate the capacitance value for each capacitor?

    Thanks,

  • 0 in reply to Conor
    TI__Mastermind 23845 points

    Hi Koki,

    In terms of your question, 

    1) Output current is defined as the current that flows through the load. In an inverting buck boost, it is the average current that goes through the diode/synchronous rectifier. It looks similar as below

    2) Due to the discontinuity of the output current above, the output voltage may have a larger ripple compared to a buck converter. 

    Hence the suggestion to increase the output capacitor to reduce the fundamental frequency ripple

    3) You may try webench for a start in terms of finding out the necessary output capacitor needed for your voltage requirement. We also have PSPICE for TI in order to simulate and double check the component selection for your design. 

    Thank you

    -Arief

  • 0 in reply to Arief Hernadi
    Guru 10455 points

    Hi, Arief

    Thank you for your reply. I have additional questions to ask you.

    According to the following E2E blog,

    https://e2e.ti.com/blogs_/b/powerhouse/posts/laying-out-an-inverting-buck-boost-converter-for-success

    it said that CIN(capacitor between VIN and GND) should be placed as close as possible to VIN pin for optimizing the switching-current loop area of inverting buck boost circuit.

    I’d like to ask about where CIO(capacitor between VIN and -VOUT) should be placed.

    CIO should be placed closer to VIN than CIN? or we should give top priority to CIN position?

    Thanks,

    Koki

  • 0 in reply to Conor
    TI__Mastermind 27470 points

    Hi

    CIO will be top priority , it will help to suppress the noise

    Thanks

  • 0 in reply to Daniel Li14
    Guru 10455 points

    Hi, Daniel

    Thank you for your reply. It was helpful. Both are good near the VIN pin, but the top priority is CIO, right?

    Thanks,

    Koki

  • 0 in reply to Conor
    TI__Mastermind 27470 points

    HI Koki:

    Yes, that's right. 

    Daniel Li

  • 0 in reply to Daniel Li14
    Guru 10455 points

    Hi, Daniel

    The documentation below describes CIN as follows:
    "CIN can be a small aluminum modulus of 47 µF to 100 µF."

    Given the layout above, which is the recommended CIN, aluminum electrolytic or ceramics?

    https://www.ti.com/lit/an/snva856a/snva856a.pdf

    Thanks,

    Koki

  • 0 in reply to Daniel Li14
    Guru 10455 points

    Hi,Daniel

    Do you have any update?

    Thanks,

    Koki

  • 0 in reply to Conor
    TI__Mastermind 27470 points

    both is ok, I suggest use both capacitor to decrese the input ripple. the input rippple is related to ESR and input power, so low ESR capacitor is recommend.

    and in the same time, it also need to provide power, so we need put 47 uF to 100 uF capacitor

    Thanks