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LM2733: LM2733 Output Capacitance

Basri Kaya
Prodigy 130 points
Part Number: LM2733
Other Parts Discussed in Thread: TPS54060A,

Tool/software:

Hello everyone,

I have an 5V output from TPS54060A and I am using 5V to switch 12V with LM2733. You can see my design below.


I am driving NCP81071B with 12V output and output of NCP is driving 260ohm coil.


Here is my questions:

1) When I check the datasheet of LM2733, it says that you may use 4.7nF to 10uF and bigger capacitance is also fine. I have twelve paralel 10uF capacitor. Is this required? May I use only 10uF?

2) What is minimum capacitance value for lowest ripple?

3) How can I optimize my design?

Thank you for your help.

Best Regards.

Basri KAYA

  • 0
    TI__Guru 65170 points

    Hello Basri Kaya,

    Please be aware that the expert for this device is out of the office until Wednesday, but let me try to answer some of your questions:

    1. If you are speaking about the output capacitance, this depends more on your load than on the LM2733, but a minimum of 4.7uF is recommended.

    2. For minimum ripple you need more capacitance typically, so you need to check in your application how low you can go.

    3. For optimization, I think you need to check the system during startup, normal operation and shutdown.

    Best regards,
    Brigitte

  • 0
    TI__Mastermind 18440 points

    Hi Basri,

    Please refer to Brigitte's comments for the Cout optimization. 

    Here are just two more comments:

    • A 51kΩ resistor is recommended to be added between Vin and SHDN pin.
    • What's the LM2733 version you used? "X" or "Y"? I need to know the work frequency to estimate the relationship between effective output capacitance and Vout ripples.

    Regards

    Lei

  • 0 in reply to Brigitte
    Prodigy 130 points

    Hello Brigitte, 

    I tested my circuit before send this thread but the result was not clear enoguh to re-design.

    First Test:





    Second test:




    Yellow signal: Output 12V of LM2733

    Pink signal: Input 5V of LM2733

    Green signal: NCP output

    Blue signal: NCP input
      

    Best Regards,

    Basri

  • 0 in reply to Lei Zhong
    Prodigy 130 points

    Hi Lei,

    I will look to datasheet for 51kΩ. Thank you.

    My ordering code is "LM2733XMF/NOPB".

    Best Regards,

    Basri

  • 0 in reply to Basri Kaya
    TI__Mastermind 18440 points

    Hi Basri,

    I think your measurement setup may not be appropriate.

    Could you let me know where is the test point for the LM2733 in and output test? And what's the used bandwidth of the scope? 

    In your application, here is the rough estimation for Vout ripple:

    Vout ripple (mV) = 420 * Iout (A) / effective Cout (μF).

    For example, suppose the total continuous input for NCP, also the output for LM2733, is 0.2A, and effective Cout is 5μF, then

    Vout ripple (mV) = 420 * 0.2 / 5 mV = 16.8mV.

    You may decide the total effective Cout based on the required Vout ripple.

    Regards

    Lei