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LM5105: not working in continuous mode with the N channel fets at about 16VDC and above.

Lance Brady
Prodigy 10 points
Part Number: LM5105
Other Parts Discussed in Thread: SN6501

We are trying to use the LM5105 to drive two N channel FETS that will control the 50V B+ to the drain of a RF power GAN transistor.

We chose this IC, due to ability to quickly turn on and off our 50V B+ rail.

We would like CW as well as fast pulse control.

We have built the basic circuit shown in the 7.2 functional block diagram of the data sheet.

VDD pin has 12VDC.

The N channel FETS are on a 50VDC rail.

The output between the N channel FETS are loaded with 1K Ohm for testing.

The input pin is manually being switched between open (with the pin internally pulled low), and 5VDC.

What we notice, is that the circuit works fine as long as we keep our 50V rail below about 16V. Above 16V, the circuit fails to work in our CW condition.

1) will this chip perform in a CW condition? (if so what do we need to change?)

2) if not, is there a minimum pulse frequency, width and duty cycle that we need to meet?

3) if this is the wrong part for our application, can you recommend a better product?

  • 0
    TI__Genius 12990 points
    Hi Lance,

    thanks for asking about LM5105 and welcome to e2e!
    I have contacted the best engineer to solve your issue.
    I am the GAN driver engineer and I would like to know:
    Can I ask why GAN? what gate voltage is needed, you have 12V gate GAN? what switching frequency is needed?

    thanks,
  • 0
    TI__Mastermind 20950 points

    Hello Lance,

    Thank you for the interest in the LM5105. I see you referenced the circuit in section 7.2 of the datasheet.

    I do have some questions about the circuit and the operation. It sounds like you are using the LM5105 and Mosfets to switch on a 50V rail to the downstream power circuit. In the circuit in section 7.2 of the datasheet, is the high side Mosfet drain connected to 50V and the Mosfet common source and drain connection the 50V that you want to switch/control?

    I am curious when you mention CW do you mean you want to switch the high side MOSFET on at a DC constant on state? I assume this may be the case.

    The bias capacitor for the high side driver, which is the HB-HS pins connection, is charged when the HS pin switches to ground, typically when the low side FET turns on. The charge on the HB-HS capacitor needs to be recharged at a minimum repetition rate in order for the high side bias to remain above the UVLO threshold.

    If there will always be some maximum period that the high side Mosfet, and low side Mosfet with switch, you may be able to size the HB-HS capacitor to maintain the charge. Refer to the LM5105 datasheet Section 8.2 to determine the bootstrap capacitance value required.

    If the high side MOSFET needs to be switched in a DC state, there will need to be a floating bias supply for the HB to HS voltage. The SN6501 is a small simple solution for the high side floating bias that can be added for DC operation.

    In your application record waveforms with the scope of the HB voltage, HS voltage and HO voltage on the driver to confirm if the high side driver bias is above the 8V+HS minimum recommended in the datasheet.

    Please confirm if this answers your questions.

    Regards,

    Richard Herring