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Original question:

TPS923650: the Buck & Buck-Boost mode

TPS922051: Help with Buck/Boost Configuration parameters

Mike Jones
Prodigy 10 points
Part Number: TPS922051
Other Parts Discussed in Thread: TPS923650

Tool/software:

Hello E2E,

I have the TPS922051/0 that allows a buck/boost configuration. When in this configuration, it will not boost when the Vout-Vin is more than 5V. Are there certain restraints with this configuration that does not allow a larger boost voltage? 

Example - I have the eval kit TPS923650D2DRLRE VM with these parameters

Vin - 12VDC

VLED - 28.6VDC

ILED - 0.5A

With these parameters, I can only get 350mA output but when the Vin voltage is increased to 23VDC, the current is 508mA. The Vref @ 350mA is 130mV so it looks like the reference voltage is being affected by the large Vout-Vin difference. 

Another question about the calculations on the datasheet. What is IL (Inductor Max) referring to? Is this MAX current that the inductor will see? Is it ILED with a 30% ripple? I cannot get my inductor calculations correct to try and troubleshoot the above issue. 

  • 0
    TI__Mastermind 25850 points

    Dear Sir,

    TPS922051/0 is designed to work as Buck configuration. Just want to double confirm, not sure if you are referring to TPS923650/1?

    Mike Jones said:
    I have the TPS922051/0 that allows a buck/boost configuration. When in this configuration, it will not boost when the Vout-Vin is more than 5V. Are there certain restraints with this configuration that does not allow a larger boost voltage? 

    Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 10 points

    I have the TPS923650D2DRLR with the TPS923650D2DRLRE VM Eval Kit.

  • 0 in reply to Mike Jones
    TI__Mastermind 25850 points

    Hi Mike,

    I got you. Pls kindly let me check and get back to you before end of this week. Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    TI__Mastermind 25850 points

    Dear Mike,

    From my understanding, you are setting the TPS923650D2DRLREVM as boost configuration

    Pls kindly find FET current limit as shown below:

    For boost configuration of TPS923650, pls make sure the inductor peak current not exceed Ipeak_min which is 1.4A. 

    IL refers to maximum inductor DC current. You can also use above equation for a boost topology inductor calculation. A good rule is to make K in the range of 20%-60%.

    Mike Jones said:
    What is IL (Inductor Max) referring to? Is this MAX current that the inductor will see? Is it ILED with a 30% ripple? I cannot get my inductor calculations correct to try and troubleshoot the above issue. 

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 10 points

    Hello,

    I am not using Boost. I am using the Buck/Boost configuration, and it will not boost. Please see the attached picture.

    With this configuration, I have a 12VDC Input with 500mA output. I am using 2 LEDs that I need to use interchangeability. LED1 has a 28.3VDC drop. LED 2 has a 11.7 drop. LED1 will not operate at full current output @ 12VDC input. The driver only outputs 365mA and the Vref of the current sense resistor is 130mV. It will only operate at full current when the input voltage is increased to 23VDC.  

  • 0 in reply to Mike Jones
    TI__Mastermind 25850 points

    Dear Mike,

    Thanks for your feedback. I will check your question and get back to you during these days. Thank you!

    BRs

    Lucia

  • +1 in reply to Lucia Gao
    TI__Mastermind 25850 points

    Dear Mike,

    If you are using buck-boost configuration of TPS923650 as below application condition:

    Vin - 12VDC

    VLED - 28.3VDC

    ILED - 0.5A

    With this configuration, suppose the efficiency is 100%, D=1-Vin/(VLED+VIN), the inductor average current IL(DC)=Iout/(1-D)=Iout/ (Vin/(VLED+VIN))=0.5*40.3/12=1.68A. Actually the inductor peak current will be equal to IL(peak) = IL(DC)+0.5*Iripple. And IL(peak) should be lower than the minimum value of below switching FET current limit.

    If you increase the input voltage, it means that the IL(DC) will decrease and if it not trigger FET current limit, you can get 0.5A. 

    We have an app note which discusses about how to calculate maximum switch current for boost which will be similar as this case.

    Basic Calculation of a Boost Converter's Power Stage (Rev. D)

    Thank you!

    BRs

    Lucia