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LM70880-Q1: EMC test fail

Chase Jeong
Mastermind 7555 points
Part Number: LM70880-Q1
Other Parts Discussed in Thread: LM70880

Tool/software:

Dear TI experts,

My customer did EMC test of their product using LM70880-Q1.

In that test, wide range of frequency (from 40MHz to 150MHz) fail situation is observed. Please see the file below ;

LM70880-Q1_EMC_250627.xlsx

And my customer have a question to improve this EMC situation.

1) In section 6.3.6 of datasheet, I can see Dual Random Spread Spectrum.

It seems that 41.2Kohm is effective for 150KHz~30MHz, 71.5Kohm is effecttive for 30MHz~108MHz.

1-1) Do you have any special reason for this resistor value? (formula, equation, etc..)

1-2) Should I use only between these 2 values? or can I use other values for specific frequency? (for example, 40MHz~150MHz)

2) About CBOOT pin, My customer added additional 1ohm because Vin is 7V.

2-1) Is there any possibility of resonance in the device because of 1ohm external resistor?

2-2) Can my customer remove 1ohm resistor if Vin is 7V?

3) About BIAS pin, My customer connected it to GND based on EVM schematic and WEBENCH.

But datasheet says that there are some options for BIAS pin.

Is there any example for reducing EMC using this BIAS pin?

Please check this issue. Thanks.

Best regards,

Chase

  • 0
    TI__Genius 11605 points

    Hi Chase,

    Please see answers to your questions:

    Chase Jeong said:

    1) In section 6.3.6 of datasheet, I can see Dual Random Spread Spectrum.

    It seems that 41.2Kohm is effective for 150KHz~30MHz, 71.5Kohm is effecttive for 30MHz~108MHz.

    1-1) Do you have any special reason for this resistor value? (formula, equation, etc..)

    1-2) Should I use only between these 2 values? or can I use other values for specific frequency? (for example, 40MHz~150MHz)

    DRSS improves EMI performance in BOTH - low frequency and high frequency band. Please read this app note: https://www.ti.com/lit/pdf/snva974

    The device understands what mode to configure in based on the resistor connected to the CONFIG pin. The resistor values are just there to configure the device in one of the configuration modes as given in the Table 7-2:

    The DRSS in the device is fixed and not configurable using a resistor. 

    Chase Jeong said:

    2) About CBOOT pin, My customer added additional 1ohm because Vin is 7V.

    2-1) Is there any possibility of resonance in the device because of 1ohm external resistor?

    No, I doubt there will be resonance here. The 1 Ohm Boot Resistor should help in the EMI performance since this slows down the SW node slightly. 

    Chase Jeong said:

    2-2) Can my customer remove 1ohm resistor if Vin is 7V?

    I am not sure what the design specs are. Can you share the schematic and the design specs.

    Chase Jeong said:

    3) About BIAS pin, My customer connected it to GND based on EVM schematic and WEBENCH.

    But datasheet says that there are some options for BIAS pin.

    Is there any example for reducing EMC using this BIAS pin?

    The BIAS pin gives an option to the customer to maximize efficiency in high input voltage applications. This will not do much to improve efficiency. 

    EMI is highly dependent on the layout of the design. Ensure all datasheet recommendations are followed while making the layout. You can also refer to the EVM User Guide https://www.ti.com/lit/ug/snvu872a/snvu872a.pdf. The EVM meets CISPR 25 Class 5 and UNECE Reg 10 EMI standards.

    I can help review the design too if you can share the schematic and the layout files with me. 

    Thanks,

    Best Regards.

    Taru