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LM5012-Q1: External components designing

Part Number: LM5012-Q1
Other Parts Discussed in Thread: LM5012, LM5102, LM5013

Tool/software:

Hello team,

Customer wants to know how to design the external components parameters, work conditions: 48V battery, Vout = 12V, Iout = 1.5A.

1. Non-sync Buck, for the output diode, how to choose the rated current parameter? For RC filtering of diodes connected in parallel, how to choose the resistance value, capacitance, withstand voltage, and power of R and C?

2. The inductance value is calculated to be 68uH according to 30% ripple current, is there a problem with the actual use of 100uH?

The actual output is 1.5A, how to choose the rated current of the inductor? Do I have to follow the manual recommendation of at least 4.5A?

Related E2E: (15) LM5012-Q1: Schematic review - Interface - INTERNAL forum - Interface - INTERNAL - TI E2E support forums

Regards,

Daniel Wang

  • Hi Daniel,

    1. The rated approximate current across the async diode can be calculated as 

    I_diode = (1-duty)*Iout_max

    Where duty is Vout/Vin.

    In this application of 48Vin and 12 Vout, 1.5A rated current; the current through diode is ~1.125A

    So consider the enough margin across the rated current, you can choose ~1.5A to 1.75A rated Async Diode.

    2. You need to choose at least 4.5A as mentioned in the datasheet. This is required because during short circuit condition, the inductor current can reach upto peak current limit of the LM5012 and 4.5A is chosen considering some margin.

    Thanks,

    Nitya

  • Hello Nitya,

        This is question is from me and Daniel help me put it in e2e 

  • I still have some questions about LM5012,  could you help me ?

    1. about selecting the async diode rated current, should we consider short circuit condition like the inductor current selection?

    2.What's the function of RA/CA/CB in the 1st page circuit of the datasheet?

    3.About BUCK working mode, what's the difference of DCM and CCM? which mode do you suggest us to use?

    Thank you. 

    Lei

  • Hi Lei,

    1. The current rating of the diode must exceed the maximum DC output current and support the peak current limit (IPEAK current limit) for the best reliability.

    2. LM5102 device works based on COT mode control . In COT mode control you need to have appropriate voltage ripple at the feedback node. There 3 types of ripple generation circuit for LM5012 device. 

    • Type-1 ripple generation method uses a single resistor, RESR, in series with the output capacitor. 
    • Type-2 ripple generation uses a CFF capacitor in addition to the series resistor as shown below.
    • Type-3 ripple generation uses an RC network consisting of RA and CA, and the switch node voltage to generate a triangular ramp that is in-phase with the inductor current. This triangular wave is the AC-coupled into the feedback node with capacitor CB. Because this circuit does not use output voltage ripple, it is suited for applications where low output voltage ripple is critical.

    For more information refer, Selecting an Ideal Ripple Generation Network for Your COT Buck Converter App note.

    3. In CCM mode, the inductor current is allowed to go negative. CCM operation provides constant frequency operation at all loads. However, the CCM mode of operation gives lower efficiency at light loads compared to DCM modes of operation. In DCM mode, the inductor current doesn't go negative. For Async converters like LM5012, the inductor current doesn't go to negative due to diode polarity and hence efficiency is better under light load condition. 

    Thanks,

    Nitya

  • Hello Nitya,

        1、about point 3, in DCM mode, the inductor current is not continuous, so the load of buck will  be power off? or in this condition ,the load will be powered by capacitor of Buck output.

        2、About Junction-to-ambient thermal resistance in datasheet, what's the size of heatsink or just use footprint without additional heatsink?

    Thank you.

    Lei

  • Hi Lei,

    1. The load is powered by the output cap. This could happen only under low load conditions.

    2. It is not with additional heatsink. It is just measured on the LM5013 EVM.

    Thanks,

    Nitya