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LV3842: EMI/EMC certification

Rohan
Prodigy 20 points
Part Number: LV3842
Other Parts Discussed in Thread: TPSM33615

Tool/software:

Hi Team,

I'm designing a new circuit. My Input is 24VDC & Output is 5V/350mA. I selected LV3842 as my switching controller. I need CE certification for my product. What care I need to take to get EMI/EMC certification for the same?

Since this is new design, I can even change controller as long as it fits my budget.

  • +1
    TI__Genius 12435 points

    Hi Rohan

    Thank you for your question. When it comes to conducted EMI, your schematic and layout are important in your design.

    • You want to be following all the datasheet recommendations for your application. A handy guide in the datasheet begins in section 9 of the datasheet.
    • A Pi filter is also recommended for designing a good EMI solution. Please see this application note for designing input filters.
    • You also want to follow the layout guidelines present in section 9.4 of the datasheet.

    As for new parts, we do have other converters that provide extra features such as spread spectrum and smaller packages that are conducive to good EMI performance. For your application I can recommend the LMR43606 as a converter or the TPSM33615 as a module. For these devices, the same recommendations exist as above, but as these parts are newer, they may be easier to work with to pass EMI certifications.

    Thank you,

    Joshua Austria

  • 0 in reply to Joshua Austria
    Prodigy 20 points

    Dear Sir,

    Thank you for guidelines. LMR43606 & TPSM33615 both will go out of my budget. So as of now I will stick with LV3842 only.
    I have one more question. What will happen if I use higher value of output capacitor & output inductor (than specified by design formula) to output performance (output ripple & peak current) as well EMI/EMC interference?

  • 0 in reply to Rohan
    TI__Genius 12435 points

    Hi Rohan,

    For a higher output inductor, you will decrease the current ripple through the inductor which will decrease switching losses in the device, improving your efficiency and EMC. The caveat is that if inductance is increased too low, the device's internal control loop may have trouble regulating properly as it relies on this signal to regulate output voltage. The signal to noise ratio may be too low for the comparator, which is why TI recommends an inductor value that puts current ripple within the range of 20-40% of the maximum current the device is capable of. 

    For a higher output capacitor, you will get better transient results, but your loop response may slow down considerably as a result. This would harm your load regulation and you would see greater variance than what is in figure 7-4 of the datasheet. TI recommends that your crossover frequency remain to around 10% of your switching frequency, to ensure proper regulation. As for EMC, you should not see too large of an effect if any. The main factor in EMI performance is going to be your PI filter and following layout guidelines. 

    Thank you,

    Joshua Austria

  • 0 in reply to Joshua Austria
    Prodigy 20 points

    Dear Joshua,

    Thank you for your response. In my design I'll try to keep value as much near to recommended values as possible. Probably keep inductance on higher side as circuit may not always draw full-load current and as per design formula I found lower current requires higher value of inductance. 

  • 0 in reply to Rohan
    TI__Genius 12435 points

    Hi Rohan,

    Sounds good. A note, is that the formula is designed to be used with Iout max (600 mA for this device). The pk-pk ripple current equation can be used to determine the current ripple you will observe when using a certain inductor. You can definitely use a larger inductor to decrease your ripple, but a good rule of thumb is to make sure that the inductor current ripple is between 20 - 40 % of 600 mA. 

    Thank you.

    Joshua Austria