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LM46002 help

bernat feixas
Prodigy 30 points
Other Parts Discussed in Thread: LM46002, LM46000

Hello, 

We are using for a low power application (50mA max) the LM46002 converter with the following schematic. 

Vout= 5.0V Vin = from 12V to 20V  Iout max=50mA

But it doesn't work properly. It raises until 5V then stops and don't restart again until it reaches 3,2V. The switcher just works in the raise ramp then it stops.

Could anyone help?

Thanks in advance.

Bernat Feixas

  • 0
    TI__Genius 15810 points

    Hello,

    Your BOM doesn't look problematic. At light loads like 50mA, the device would work in Pulsed Frequency Mode and not in Continuous Conduction Mode. But the device shouldn't be off until Vout reaches 3V. Would you please probe the SW node, the current through the inductor and also share your board layout with us? Have you made sure that your input power supply is not current limited?

    Another side note: If you are using the device for a max load of 50mA, then you might want to consider using LM46000 which is a 500mA device. 

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Prodigy 30 points

    Thanks for your answer, 

    We suppose that our problem is in the layout which is not efficient enough as the print circuit is of just 2 layers.    

    In order to verify this point, before we order new circuits, we would like to make some manual arrangements.

    Could you tell us which are the more important layout rules we must follow in this chip?

    Regards

    Bernat

  • 0 in reply to bernat feixas
    TI__Genius 16025 points

    Hi Bernat,

    Please refer to page 44 of the LM46002 datasheet that shows the layout example. This example has everything you need for a good layout.

    EDIT: Please increase the voltage rating of your input ceramic to at least 35V from the present 10V.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 30 points

    Thank you for your answers,

    At the end we have solved it by adding a resistor of 220kOhm connected to the pin RT and ground.

    Is it recomended for the serial production to change this chip for the  LM46000 ?

    And in this case we would have to change any component from our actual BOM?

  • 0 in reply to bernat feixas
    TI__Genius 15810 points

    Hello,

    If you're happy with the solution then its alright. The reason we suggested LM46000 is because if your max load is only 50mA, then the LM46002 is oversized. As such the devices are pin to pin compatible and your existing BOM should work. As one of the earlier replies suggested, we'd suggest that for 5V out, if you are using ceramic caps, then use a voltage rating of more than 10V. The ceramic capacitance reduces a lot with applied voltage across it.

    Regards,
    Akshay