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LM3409HV: About the switching frequency and other of LM3409HV

Zhun Liu
Intellectual 290 points
Part Number: LM3409HV
Other Parts Discussed in Thread: LM3409

Hi,there are some questions about LM3409HV that need help.

1. What is the frequency range of the PWM signal for pin EN?

2. Does the PWM dimming frequency affect the range of the switching frequency?

3.When Vin=70V and Vout is about 56V, in order not to exceed the Absolute Maximum power, what is the output switching frequency range for MOSFET?

4.When Vin=70V, What is the possible range of Vo? (the maximum and minimum voltage drop from input to output)

  • 0
    TI__Mastermind 25500 points

    Hello Zhun,

    To answer your questions:

    1 & 2) There isn't really a limit on the frequency. It depends on the requirements for your application. EN dimming is only limited by the rise time of the current (v=Ldi/dt) as shown in the datasheet plots. So it works regardless of frequency, but linearity will be affected by frequency. The lower the frequency the better the linearity. On that note the lower the PWM frequency versus switching frequency the better the linearity will be also.

    3) The switching frequency would be limited by the VCC current. VCC will need to supply I = Fsw*Qg which is switching frequency times total gate charge. At such high voltages you will need to consider AC losses which will be specific to your application.

    4) The LM3409 is capable of 100% duty cycle, so it depends on the Rds(on) of your FET. But realistically you will get the best performance if you have at least 4V to 6V or more between Vin and Vout.

    Regards,

    Clint