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LM5150-Q1: Low switching frequency issues for LM5150-Q1

Bonggyun Kim
Prodigy 10 points
Part Number: LM5150-Q1

Dear TI colleagues,

We are using LM5150-Q1 for electronic brake system application.

For this application, I have designed switching frequency 499.34kHz using RT=44.1kOhm(1% tolerance).

But measured frequency is slightly lower than calculated value in real circuit.

   -Measured switching frequency : 484.2~484.94kHz in tri temperature conditions(-40, 25, 120 degree celsius)

   -Measured voltage on RT pin : 1.133V

   -Measured resistance of RT in real circuit : 44.1kOhm(Nearly)

My questions are

1) Measured frequency(484.2~484.94kHz) is no problem considering RT value and RT voltage?

2) When RT value is fixed, is there switching frequency tolerance? If yes, please let me know how much tolerance exists.

3) Voltage on RT pin is related to switching frequency? and there is any voltage tolerance on RT pin?

  • 0
    TI__Guru 64450 points

    Hello Bonggyun,

    Let me try to answer your questions.

    1. The switching frequency is within the tolerance as we are in the range of 10% or more for a given RT resistor, see page 9 of the datasheet for min and max values of 2 resistor values. For the voltage, you need to measure it in reference to the AGND pin of the device.

    2. Please review page 9 of the datasheet about the frequency variation with a fixed resistor on RT.

    3. We specify the frequency variation for 2 different resistors. In addition, you can find the typical curve in the datasheet on page 10. We do not specify the RT voltage variation.

    The internal oscillator is not accurate enough to exactly stay at the value you calculated. If the frequency range is too wide for your needs, the device can be synchronized to an external frequency.

    Best regards,
    Brigitte