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TPS92511: How to understand floating buck topology

Gabriel Xu
Intellectual 2145 points
Part Number: TPS92511
Other Parts Discussed in Thread: LM3414, , TPS92513

Hi teams.

I have some question about floating buck topology (Such as TPS92511 or LM3414).

I wondering to know what's the major advantage of floating buck topology compare to conventional buck topology? (besides it don't need high side FET charge pump driver?) 

In what LED voltage/ILED/Vin should we choose floating buck topology? (For example, if we need 1.5A LED driver should we choose TPS92513 or LM3414?)

The second question is if we using a LED driver for 100mA LED current, will TPS92511 be a more proper part to apply than TPS92513? (I found TPS92511 need much larger inductor)

The third question is if the customer wants to use extremely small volume capacitor about 10nF-100nF, do we have any solution to achieve this with reasonable current ripple?

  • 0
    TI__Mastermind 25500 points

    Hello Gabriel,

    As you mentioned having no need to bootstrap to drive the FET is one advantage. That also allows 100% duty cycle if Vin is very close to the LED string voltage which is another advantage. It also allows you to connect the LED string directly to Vin which is advantageous in certain applications.

    As for what to choose it really depends and I would need more information about the application requirements. The control methods are different between a lot of devices as well. For example the TPS92511 will have much faster current edges if PWM dimming where the TPS92513 will have much better analog dimming range since it can run in DCM (which is why the 511 requires a higher inductor value).

    As for reducing the ripple with small caps you will basically want to run at a high switching frequency and use the largest inductor value that will fit. But that too depends on what is "reasonable" ripple in your application.

    If you want to provide more details (Vin, LED string voltage, max current, analog and/or PWM dimming needs) I'd be happy to give a recommendation. Thanks.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    TI__Intellectual 2145 points

    Hi Clint
    Thanks for your explanation, it's very clear.
    The customer is still evaluating the solution, therefore, many parameters are undetermined.
    But the reason why they want to reduce the output capacitance value is that they hear some noise when the capacitance is higher than couples hundreds nF. So they take out the capacitor to solve this problem. But the side effect is a larger ripple. (They finish this test in TPS92513 EVM) Do you have some solution or comments about this noise issue?

    About TPS92513 IADJ PIN, in datasheet page 11 last paragraph said that IADJ can apply 0V-1.8V, but it also state that less than 180mV in IADJ PIN may have "more variation between circuits". How to understand that? Are they suggest to using this part in CCM mode although it can be used in DCM mode? Will CCM mode have better current regulation accuracy than CCM mode?


    Another question, are LM3414 can use in 100mA-300mA range? Can it operate in DCM mode?
    Thanks in advance!

  • 0 in reply to Gabriel Xu
    TI__Mastermind 25500 points

    Hello Gabriel,

    I'm sorry if I missed this one.

    What it means in the 513 datasheet is that the error amplifier has a possible offset over temp of +/-10mV. It usually will not vary that much, but in any case that translates to +/-5% for a current sense of 200mV but more the lower you go. It's relatively reasonable that they will match ok down to a certain point which is what the 180mV IADJ recommendation is for. You can go lower but the matching may become an issue for certain applications.

    For the LM3414 you can analog dim so you should be able to run 100mA to 300mA, but you do need to keep it running in CCM so you will need to use a higher switching frequency and/or higher inductor value.

    Regards,

    Clint