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LM5118-Q1: For Cold Crank Product Recommendations.

lee jaewook
Prodigy 75 points
Part Number: LM5118-Q1
Other Parts Discussed in Thread: LM70880, , LM5177

Hi TEAM 

Our customers want the boards we develop to work well in 12V, 24V, and 48V vehicle battery environments.

At first, I tried to use a buck boost product (LM70880), but considering the cold crank test of the 12V system, I think I should use a buck-boost product.

So I think LM5118-Q1 is good for our development board. Is it right?

We need below specification for our development board.

Vin ->  4V~60V   (12V, 24V or 48V)
            : I think the Cold Crank test requires Min. 4V
Vout -> 10V   (Max Current-> 6.5A, Normal Current-> 4.5A)

If LM5118-Q1 is not good, Can you recommend other product? 

thank you.

  • 0
    TI__Guru 59845 points

    Hello lee jaewook,

    Thank you very much for using the E2E forum.

    You are right, for this application a buck-boost converter is the right solution. But as you mention an output current of 4.5A typically, the current in the boost diode could get as high as 11A or more at 4V. This is too much power dissipation for a diode. Therefore I recommend having a look at LM5177.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 75 points

    I reviewed the datasheet for the LM5177. And I am worried about the load dump test.

    I was thinking that TVS diode should be on the 48V input to prepare road dump test.

    1. TVS diode's Vbr should under 60V for long term voltage test (The input voltage is maintained at 60V for 60 minutes.).ISO 21780:2020 TEST-06
    2. A TVS diode with a Vbr of 60V seems to have a Vcl of at least 75V.

    For the above two reasons, the input voltage of buck-boost should be at least 4V~80V.

    Am I right? So, can you recommend a design method that satisfies both conditions?

    thank you.

  • 0 in reply to lee jaewook
    TI__Guru 59845 points

    Hello lee jaewook,

    Why are you afraid of the load dump? The VIN voltage has abs max of 85V. You can apply 60V DC and the converter will still survive transients to some higher voltage.

    I think LM5177 satisfies both conditions.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 75 points

    Does this mean that voltage transients (Max. 85V) will not affect the output voltage?

    I think that If Transicent (Max.85V) go to input pin of LM5177, LM5177 will don't damage but
    I don't think the LM5177's output power will be maintained. Isn't it?

  • +1 in reply to lee jaewook
    TI__Guru 59845 points

    Hello Lee,

    Voltage transients will always affect the output voltage somehow as the device is not able to react instantaneously on an input voltage change. Voltage transients up to the abs max rating could affect long term reliability, but in general the device will still regulate to the set output voltage. There is nothing included that stops the device from operating if the input voltage rises.

    Output power is only maintained, if you limit the average current on the output, as then the average current loop limits the power on the output together with the target output voltage.

    If there is no average output current limit, the output power is defined by the peak current limit you have and the input voltage level, so increasing the input voltage will increase the available output power on a buck-boost device, if in buck-boost or boost mode. In buck mode, the output power is defined by the peak current limit and the target output voltage.

    You need to make sure that the power stage is able to handle the maximum power that can happen in your system.

    You could as well check your case out by getting an EVM and having a look on how the device behaves at these high input voltage levels, but then you need to exchange some components as the EVM does not have the external components defined for the abs max ratings.

    Best regards,
    Brigitte