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CSD19536KTT: NMOS/PMOS available to disconnect ground?

David__
Genius 3585 points
Part Number: CSD19536KTT
Other Parts Discussed in Thread: OPA551

Hi, guys.

I'm using the application note

http://www.ti.com/lit/an/snoa474a/snoa474a.pdf

to create the Howland Current Pump with OPA551 with an MSP-EXP432P401R launchpad.

A periodic signal (100Hz ~ 4kHz, max amp = 20V) is fed to the OPA551 input.

For my personal reason, I wish to disconnect the GND which is connected to the load.

Like most NMOS/PMOS application, I can use 3.3V GPIOs to open or close the circuit.

However, since the OPA551 is giving high voltage output and current(also positive + and negative -), I wasn't sure whether I can apply NMOS/PMOS for my case.

Can I use NMOS or PMOS to disconnect GND when I want? If so, what part from TI can you recommend?

The power consumption, package size, or price doesn't matter. I wish a product that can handle this high voltage and current.

Also, I do prefer a package that can be populated to a breadboard right away.

Or if there is a better solution instead of using NMOS/PMOS, considering that I'm using a MSP-EXP432P401R launchpad, I wish to hear other methods, too

  • 0
    TI__Mastermind 34005 points
    Hi David,
    Thanks for your interest in TI FETs. In this application it looks like you're using the FET as a low side load switch referenced to GND. I'm assuming that you're using an N-channel FET. When the FET is ON (VGS > VTH and VGS >= minimum voltage at which rds(on) is specified in the datasheet), current can pass thru the channel in either direction. When the FET is OFF, it can only block a positive voltage from drain-to-source due to its integral body diode. If the drain-source voltage is negative, the body diode will be forward biased and it will conduct current. If that is the case, then you may have to use two MOSFETs in a back-to-back configuration to block reverse current thru the body diode. Also note, it is possible to drive the FET using a 3.3V GPIO. However, the FET must have rds(on) rated at 3V and the turn-on/turn-off times may not be very fast due to the limited current drive capability of the GPIO.

    What is the output voltage and current you need to support? Is the FET switching speed important?
  • 0 in reply to John Wallace1
    Genius 3585 points

    Thanks, John.

    John Wallace1 said:
    If the drain-source voltage is negative, the body diode will be forward biased and it will conduct current. If that is the case, then you may have to use two MOSFETs in a back-to-back configuration to block reverse current thru the body diode.

    I was afraid to encounter this type of problem. Thanks for pointing out.

    John Wallace1 said:
    However, the FET must have rds(on) rated at 3V and the turn-on/turn-off times may not be very fast due to the limited current drive capability of the GPIO.

    I see.

    John Wallace1 said:
    What is the output voltage and current you need to support? Is the FET switching speed important?

    I will be feeding a bi-phasic signal to the Howland current pump.

    Although I'm using OPA551, which can drive -30~30V, I will be using other high power Op Amps later, all the way to -100 ~ 100V.

    As a result, I wish this to support -100V ~ 100V, 100mA maximum. The signal frequency range is 50 ~ 4kHz.

    About the FET switching speed, if it is faster than 2msec, it will be fine for my system.

    One thing I want to note is that I hope the gate threshold voltage (Vgs) is low enough so that I can use the GPIOs to toggle it, lower than 2.8V maybe.

  • 0 in reply to David__
    TI__Mastermind 34005 points
    Hi David,
    TI FETs go up to a maximum BVDSS rating of 100V. In order to have adequate margin, a FET with a higher voltage rating (e.g. 120V to 150V) would be required for +/-100V application. All of our 100V FETs have rds(on) rated to a minimum VGS = 6V. We do not recommend operating the FET at VGS < 6V since we cannot guarantee rds(on). For +/-30V application, you will want to use a 40V (or greater) FET. I'm not finding any matching parts with VDS >= 40V that can be driven directly from GPIO signal of 2.8V. You would have to have an external gate drive circuit to use any TI FET. This can be a low side gate driver IC or a discrete implementation using FETs or BJTs.
  • 0 in reply to John Wallace1
    Genius 3585 points

    John Wallace1 said:
    You would have to have an external gate drive circuit to use any TI FET. This can be a low side gate driver IC or a discrete implementation using FETs or BJTs.

    Thanks, John.

    I think this might be similar to your suggestion. Then how about cascading NMOS?

    One for low gate voltage so I can control it with a microcontroller,

    the other as a switch

  • 0 in reply to David__
    TI__Mastermind 34005 points
    Hi David,
    I was thinking something simple. For example, you can use the GPIO to drive the gate of a small signal NFET with its source referenced to GND. The drain would be pulled up to a higher supply voltage (5V or 12V) thru a resistor. Then the drain would be connected to the gate of the high voltage (40V - 150V) NFET. The gate is pulled up to the supply voltage when the small signal FET is off and driven low when it is on. There are many vendors who make small signal FETs and even high voltage FETs in SOT23 packages that should do the job. Unfortunately, TI doesn't have a good fit for this relatively low current application.