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LM5105 Half Bridge Gate Driver

Scott McElroy1
Intellectual 2155 points
Other Parts Discussed in Thread: LM5105, LM5101

I am looking for a Half bridge gate driver with the following specs.

-Operate at 28VDC.

-Output 500mA

-Can be configured as high impedance.

-Switching speed less then 200ns.

I found the LM5105 that will meet these specs, but I have a few questions about this IC.

1. Will the LM5105 operate at DC (0Hz) if I provide a power source to keep the high side gate driver bootstrap capacitor charged?

2. Will a charge pump work to generate a high voltage (maybe 45V) along with a current limiter and zener diode across the bootstrap capacitor in order to prevent the bootstrap capacitor from discharging during DC operation?

Any feedback would be much appreciated. Thanks for the help.

 

  • 0
    TI__Expert 3545 points

    Scott,

    1. Yes a floating high side bias supply should work.

    2. I'm not sure I understand question 2? Can you draw the circuit and send it to me?

    Terry

  • 0 in reply to Terry Allinder
    TI__Intellectual 2155 points

    The question was basically should a charge pump across the bootstrap capacitor be used to keep the bootstrap capacitor from discharging in DC operation. The chargepump would be configured as as described.

    Also, what do you mean by a floating high side bias supply? Do you mean a supply across the bypass cap or bias on the MOSFET? I am not very familiar with how this part functions so forgive me if this seems redundant, I do know that the bypass cap gives the high side MOSFET the drive power.

    Thanks for the help!

  • 0 in reply to Scott McElroy1
    TI__Expert 3545 points

    Scott,

    If you look at the simplified block diagram, Figure 1, in the data sheet.  The bootstrap capacitor is charged when the low side FET is turned on.  The high side drive output (HO) is referenced to the center point in the half bridge.  So if your going to connect a high side bias supply to run the HO output at DC, then the bias power needs to be connected from VDD (+)  to HS (-).  A floating high side bias, not referenced to ground. 

    Terry

  • 0 in reply to Terry Allinder
    TI__Intellectual 2155 points

    Thanks Terry,

    That was a huge help on understanding the gate driver. I appreciate your support.

  • 0 in reply to Terry Allinder
    Intellectual 775 points

    I have a very similar question about the same device, so I figured I would use this thread, since it is so recent.  I am also trying to build a high voltage discrete output that works from ~100kHz down to DC.  I need a driver that works at DC, so I need it to work with bootstrap power supplied by an external source.  I have included a circuit schematic of the LM5105.  When operating, the input ports will be supplied (by external circuitry) as follows:

    • Vdd=12V
    • Enable=0-12V
    • Signal In=0-12V, 0-100kHz
    • Switch Drive=Bus Power + 12
    • Bus Power = 12-60V

    So my question is: am I doing it right?

  • 0 in reply to Nick Wernicke
    TI__Expert 3545 points

    Nick,

    Look at the simplified block  diagram on the front sheet of the LM5101 data sheet.  The HS pin is the high side driver ground pin, the HS pin needs to be connected to the center point of the two FETs (OUTPUT).

    Terry