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LMG3411R070: Switching frequency limit

Part Number: LMG3411R070
Other Parts Discussed in Thread: LMG34XX-BB-EVM, LMG3411EVM-029, LMG3410R070, LMG5200, LMG1210

Hello I am using the LMG3411EVM-029 together with LMG34XX-BB-EVM.

In my application I need to go very high with switching frequency, and I need to exploit the limit of the LMG3411.

At no load condition, I tried to give a 50% duty cycle PWM; I left the default dead time of the EVM untouched.

When I raise the PWM frequency, as I get around ~1900kHz I get a fault condition (still at no load conditions).

If I look at the two complementary HI and LO pwm inputs, it seems there is still room to increase the frequency, and I see the correct figure with dead times at the two inputs.

In fact I expected to be able to go higher.

What could cause the fault condition, if there is no load? How can I make a check of what is going bad?

  • Hello Atrac,

    The 1.9MHz is already over the limit of the maximum switching frequency of the LMG341x FET. The maximum switching frequency is around 1MHz, and beyond this point, a fault could appear. The reason for the fault is that there is an internal buck-boost converter operating in DCM inside the package to generate the negative gate voltage required to turn off the GaN FET. As the switching frequency keeps going up, the inductor couldn't supply the negative voltage required, and thus triggers UVLO. You can look at this by probing the BBSW and VNEG pin.

    By the way, may I know the application? Why it requires to go around 2MHz? 


  • Hello Yichi,

    Ok, thank you for the reply, it is very clear now why it cannot be used over that frequency.

    I could not find a detailed explanation of the UVLO in the datasheet, maybe there is some application note?

    Reson for going so high in frequency is that I want to use the mosfets in the final stage of a class D amplifier (ultrasound application, so I have to go high in frequency), and I need to increase sampling frequency.

    1MHz should be enough, but as I need to do some oversampling, 2-3MHz would have been better.

    Is there any other GaN device that can go higher in frequency?

    Thank you

  • Hello Atrac,

    For UVLO description, we don't have an app note dedicated for it specifically. In datasheet of LMG3410R070, it has detailed description of what will be disabled when UVLO is triggered. Also on page 7, there are threshold for the UVLO level.

    For switching frequency beyond 1MHz, current solution will be LMG5200 which is two FET in a half bridge configuration in single package. However, please note that the voltage is 80V for this device. 

    Also another option is to use LMG1210 which is the driver for GaN FET plus a discrete GaN. 

    Hope this helps and please let us know if there are any additional questions.