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LM5111: low side output transistor

Dylan
Intellectual 1255 points
Other Parts Discussed in Thread: LM5111

Hey Team,

The specification says that we can sink 5A, and that was tested for a 200nS period.  We have a matrix circuit that is being driven by a MOSFET that’s driven by the LM5111.  The MOSFET is driving a 4V capacitive bus, and that bus remains charged-up when the MOSFET is turned-OFF.  We are adding a switch that senses that the MOSFET is OFF, and then routing the charged bus through the switch to the same driver as drives the MOSFET through a 1 Ohm limiting resistor.  This configuration discharges the charged bus with an approximately 1.5 uS time constant, >> 200 nS.

The question is how much more current can we pump into the output sink transistor of the LM5111 in order to discharge the charged bus more quickly.  Keep in mind the discharge is an exponential decay, and we want to be as aggressive as possible slashing the value of the 1 Ohm series resistor.

Thanks!
Dylan Ashley

  • 0
    TI__Mastermind 39800 points
    Team,

    What we need most of all is some off-spec. information that we can use for experiments. We would like to know how long we can run at 5A or higher. Periods of high 100’s of nS or microseconds are anticipated. We are thinking that there’s data somewhere from testing to establish the 5A max that shows how the part reacts to longer periods at 5A or higher current. We are looking to develop some statistics on this operation to understand where to move forward. Do you have any information on this? I have already referenced page 10 of the LM5111 datasheet mentioning the pulsed source/sink currents to capacitive loads.

    Thanks,
    Evan
  • 0 in reply to Evan W
    TI__Mastermind 31580 points
    Hi Dylan,

    I looks like you are discharging a 4V cap bus through 1R with a driver that is rated at 5A peak and you would like to drop the 1R resistor to as low a value as possible.

    The output resistance of the driver when low is 2.5R max, so with a 4V bus cap the peak current is 1.6A, the typical Rol is 1.4R which gives 2.85A peak.

    What is the frequency of operation as this will affect the power dissipation within the driver? also be aware that as the value of the external resistor is reduced the proportion of power dissipated in the driver increases.

    Correct me if I have missed something but I cant see that you are going to be limited by the peak current value, my only concern is the power dissipation in the driver which will be a function of the operating frequency.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Mastermind 39800 points

    Peter,

    Thanks for the reply and information. Can you comment on my question above your reply regarding the specifications of the part and operating above 5A for the defined periods of time? I will work to see if we can get the frequency of operation. 

    Thanks,
    Evan

  • 0 in reply to Peter Meaney
    TI__Mastermind 39800 points
    Peter,

    The switching frequency in this particular version is approximately 1,724Hz, with a duty cycle of less than 1% depending on the resistor values used and the time it takes to discharge the 4V bus caps. We are also using the same driver pin to turn off the MOSFET faster using a Schottky diode in series with a 5 ohm resistor, in parallel with the 1k gate resistor. This is to prevent the possibility of both devices conducting at the same time, resulting in a shoot-through condition. These 2 loads are being sunk through the driver at the same time, so they combine to determine the total peak current in the circuit.

    Thanks,
    Evan