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BQ24725A: Inrush current limit resistor on VCC

Howard Zou
Genius 12365 points
Part Number: BQ24725A

Hi,

my customer's schematic is as below. The inrush current limit resistor they use is 10ohm 0603 package(R906 below), but our typical application is using 1206 package(10ohm).

Now they find that it happens the resistor will be burnt.

They wonder if they could just solder a 0ohm resistor to replace the 10ohm resistor because they have inrush limit for POWER_9V.

The adaptor input schematic is as below:

  • 0
    TI__Mastermind 25800 points

    Hey Howard, 

    We do not recommend changing the inrush current filtering resistor. I would recommend the customer use a high wattage rated resistor.

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    TI__Genius 12365 points

    Joel,

    then why do we use 1206 (1/4W) resistor?

    If the input voltage is 8V, the resistor is 10ohm, at startup there is a moment that the power=8*8/10=6.4W which the resistor can't handle.

    So I want to know how to calculate the correct power rating of the resistor.

    At the same time, why we use 10ohm resistor instead of other values?

  • 0 in reply to Howard Zou
    TI__Mastermind 25800 points

    Hey Howard,

    Power rating of 1/4W is only relevant to long-term power dissipation.

    What is more relevant is the single-pulse power dissipation of a 1206 (1/4W) resistor and 0603 (1/4W) resistor. I found two example thick-film resistors from Vishay, one a 1206 10-Ohm resistor, 1/4W rating and the other a 0603 10-Ohm resistor, 1/4W rating. 

    If I assume the peak current through the 10-Ohm resistor to the 1uF capacitor is maintained for 5xRC (this is not correct, but an assumption), this is abount 50-us. Comparing the 50-us single-pulse Power Dissipation rating between the 1206 resistor and the 0603 resistor revealed a delta of ~18W of peak power dissipation for 50us. 

    Keep in mind that the above examples were THICK film resistor, not thin-film, but it identifies the fact the peak power dissipation (short term) of a larger package size is higher than smaller package. There is just more material to dissipate energy. 

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    TI__Genius 12365 points

    Joel,

    where do you find the data

    Comparing the 50-us single-pulse Power Dissipation rating between the 1206 resistor and the 0603 resistor revealed a delta of ~18W of peak power dissipation for 50us. 

    Could you please provide the link? 

    Thank you.

  • 0 in reply to Howard Zou
    TI__Mastermind 25800 points

    Hey Howard,

    I looked at two Vishay thick-film 10-Ohm resistors with the same 1/4-W rating, but different package sizes.

    You can online search and find different resistor datasheets that have a Max Power Dissipation vs. Time graph. Granted, not all resistors have this graph, but those that do can be used as a good comparison between package size. 

    Here are another two examples that I just looked up today. Keep in mind to compare the 1/4W 0603 and the 1/4W 1206. See the graphs with Pulse Power vs. Pulse Duration.

    https://www.mouser.com/datasheet/2/427/rcse3-1013761.pdf

    https://www.mouser.com/datasheet/2/427/dcrcwife3-105365.pdf

    Regards,

    Joel H