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CSD13381F4: Vds too high thus very large voltage drop

Christelle Saliba
Prodigy 40 points
Part Number: CSD13381F4

Hello, 

I am using one of your mosfets CSD13381F4 to create a current sink in accordance with application note SLAA868. The circuit works perfectly, and I do get the current I need. However, I am using this same mosfet as well in order to switch on 5V on a resistor. You can find attached the design in the following request. The mosfet is introducing a very high voltage drop even though my current is 100mA. According to the datasheet, at 100mA, VDS should be much smaller however I am getting at least 1.5 V drop. I am not sure what is the problem.

Thank you.

Best,
ChristelleCircuitTI.TSC

  • 0
    TI__Mastermind 29525 points

    Hi Christelle,

    Thanks for your interest in TI FETs. Based on the simulation schematic, I don't believe you're getting enough gate-source voltage to fully enhance the FETs, T1 and T4. The schematic shows a supply voltage of 5V at the drain of T1 and 5V connected to its gate. In order to ensure that the FET is fully on, you need to drive the gate to a voltage higher than the drain by at least 1.8V which is the minimum VGS where the on resistance is specified in the datasheet. I ran a DC simulation of your circuit and VGS of T1 is 1.37V and VGS of T4 is 1.45V. These are both less than the minimum VGS where on resistance is specified and the FETs are operating in the saturation region (current flowing with high VDS). If you can, increase the gate drive voltage to at least 6.8V. You should see a much smaller VDS voltage drop on these devices. TI cannot guarantee on resistance when VGS < 1.8V.

  • 0 in reply to John Wallace1
    Prodigy 40 points

    Hello John,

    I guess you meant that the gate voltage has to be greater than the source by 1.8. I tried it as you said and yes, i reached the 310mOhm of Rds when i increased the Vg to 1.8, getting a lower drop.

    Thank you a lot for your assistance!! 

    Regards,

    Christelle