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LM5146-Q1: Converter is making ticking sound and output voltage is varying from 3V to 35V

Part Number: LM5146-Q1
Other Parts Discussed in Thread: LM5146,

Hi,

The converter is designed for an input voltage of 50V-84V and an output current of 10A.

When we give a supply of 70V, the converter is making a ticking sound, and output voltage varies between 4V to 35V.

  • Hello Amit,

    The LM5146 is a buck converter and will not regulate 70V out with a voltage less than ~70V in. When the input voltage is less than or equal to the set output voltage [70V] the converter will operate in "dropout" mode. Please refer to the link below which provides you with guidance with how the maximum duty cycle is determined. 

    https://www.ti.com/lit/an/slyt293/slyt293.pdf?ts=1623706493004&ref_url=https%253A%252F%252Fwww.google.com%252F

    The condition you are describing appears to be the controller operating in a max duty cycle condition. The LM5146 is not recommended for the application you are describing above. If you need a regulated output voltage where the input voltage is above and below the output voltage you will need to use a buck-boost converter. 

    Regards,

    John Donaghey

  • Hello John,

    The converter is designed to provide 12V Constant output with a maximum load current of 10A.

    We are applying around 70V for testing and the converter is making a ticking sound with output varying from 3V to 35V. I'd request you to kindly review the schematic posted previously.

    Thanks in advance,

    Amit B.

  • Amit,

    I checked the schematic and did not see anything fundamentally wrong. Please be aware of the DC bias derating of the output capacitors, I recommend a minimum output capacitance to be 3 x 22uF to take into account the DC bias derating, you may require more capacitance depending on your output ripple voltage specification, please refer to the quick start calculator in the product folder for additional guidance.

    Regarding the sounds you are hearing, curious to know is the output voltage regulating? Additionally do you hear clicking when you are not adjusting the output voltage i.e output voltage setpoint equals 12V. Or do you only hear clicking when adjusting the output voltage above or below 12V? Your schematic does not show how you are adjusting the voltage, can you please describe? 

    Should you need further support it would be very helpful if you can provide scope captures of the switch node. 

    Regards,

    John Donaghey

  • Hi Amit,

    Sorry we can't internally access your google drive files. Please use the LM5146-Q1 quickstart calculator to check compensation, considering the derating of ceramic capacitance as mentioned above.

    Note that the EVM is a good reference here as it's configured for 12V/8A/400kHz.

    Regards,

    Tim