CSD19538Q3A: when using this MOSFET in current limiting application

Hello Mohamed,

Thanks for your interest in TI FETs. Let's make sure we are in agreement on terminology as detailed below. In the linear region, the FET is on (enhanced) and on resistance is constant for a given value of Vgs. In the saturation region, the FET operates in linear mode with voltage across it and current thru it. For the CSD19538Q3A, as long as Vgs ≥ 6V, the on resistance is constant and is less than the maximum specified in the datasheet. There is no need to adjust Vgs although increasing it (up to 10V) reduces the on resistance and conduction (I²R) loss. As shown in the Rds(on) vs. Vgs graph on page 1 of the datasheet, when Vgs < 6V, Rds(on) increases exponentially. When Vgs > 10V, Rds(on) is relatively flat.

To regulate the current when it exceeds the threshold, you would need a closed loop feedback system to sense the current, compare it to a reference and adjust Vgs to achieve constant current. This requires additional external circuitry. You might also consider using a high side switch controller or hot swap controller which typically includes features to limit the current during inrush when the FET turns and overcurrent protection. I hope this helps. Let me know if you have any additional questions.

• LInear region: Vds << Vgs - Vth
• Saturation region: Vds > Vgs - Vth

Best Regards,

John Wallace

TI FET Applications

Hi John, 
Thanks for your explanation,I am still confused between two concepts: 
1) when VGS is just above thresholds it means that the channel is just pinched off, at the same time at this point the RDSon did not reach it'S minimum yet, should this operating point be used when the current is within my specs or above the limit ? and why ? ( the shaded region )
2) at the same moment when we increase VGS to reach the flat RDSon, that is the region that has RDSon at its minimum, at the same time I can allow huge amount of current to flow,should this operating point be used when the current is within my specs or above the limit ?  ( the white region )

I see that VGS is the only parameter used to control MOSFET operation region, does sensing the drain voltage should not be considered in this process ?

BR,
Mohamed

Hi Mohamed,

Thanks again for your interest in TI FETs. The threshold voltage specified in the datasheet is where the FET just begins to conduct and is tested and ID = 250μA. The minimum value of VGS where Rds(on) is specified and guaranteed is 6V. The Rds(on) vs. VGS curve on page 1 of the datasheet shows at VGS values between 6V and threshold, the on resistance increases exponentially. During normal operation where the current is less than its limit, the FET should be fully turned on with VGS ≥ 6V. This will minimize the conduction (I²R) loss in the FET. When the current exceeds the limit, VGS will be adjusted down to < 6V to regulate the constant current. In this mode the FET operates like a variable resistor in the white shaded region moving up and down with VGS adjustments. While VDS will have some effect on the drain current, VGS is the primary control mechanism to regulate drain current. Sensing VDS is used to protect against over voltage/over power conditions.This is typically how hot swap controllers work. Let me know if you have additional questions. 

Thanks,

John

Hello Mohamed,

Following up to see if your issue has been resolved. Please let me know.

Thanks,

John