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# TIDA-00528: Power Loss of P-FET

Part Number: TIDA-00528
Other Parts Discussed in Thread: INA229-Q1, INA229

Hi,

i have 2 questions:

1- i was planning to use INA current sensor for 400V output but in Low-Side configuration due to common mode voltage issue. TIDA-00528 uses a more complex method and measures on the high-side. now i am a little bit confused. whats are the benefit of this method (other than offsetting  our ground a little bit and short circuit problem in the case of different ground)? why not putting it on Low-Side?

2- i wanna measure currents of maximum 3A. if using Low-Side configuration, would that be a problem to measure very low current? i was thinking of adding a charge pump of -1V to the GND of INA to improve this low current measurement. is that possible? (is it a good idea?)

3- also in capital 4.4 transistor power loss is calculated. Shouldn't that be P=Rds-on*I^2=  46µW? why 400V is used?

Best Regards

Toktam

• Hi Toktam

1) Yes, the main benefit of sensing on the high-side is that you are able to sense load shorts to ground. If sensing on the high-side, you can implement some type of over-current protection method when the current rapidly increases due to the short to ground. The current shorted to ground through the load can not be monitored at the low-side. If you plan to use low-side sensing (VCM ~=0) , this reference design does not apply.

2) What is the minimum current to be measured? This depends on the minimum current, the INA device, and shunt resistance used.

3) Rds_on is not constant in this scenario because the PFET is in the linear region, so VBUS is used. Majority of the 400V is dissipated through the PFET, a very small amount of power is dissipated through the resistors, so the whole 400V is assumed for this calculation.

Cierra

• Hi Cierra,

thanks for the answer. also a good question. minimum measured current is not specified till now. but how do you calculate what is reasonable minimum current for INA family with external shunt resistors. i wanna use INA229-Q1. it has a shunt offset of 1µV and let us say for ADCRANGE=0 and low-Side configuration.

Also some other Questions:

- i was trying to use Power Monitor Tool. it also suggest to use INA229 (i want SPI interface). first of all there is a problem and it cannot calculate the shunt value (consequently other parameters too). secondly, there it is written "Lowest Meaningful Vsense" -> what does it mean and what do you chose for the calculation normally?

- Supply voltage is between 2.7 and 5.5V. is there a difference if i chose 3.3V or 5V? if yes, which supply is better?

Best Regards

Toktam

• Hi Toktam,

This document is meant to supplement the Power Monitor Tool: Getting Started with Digital Power Monitors

Try following the instructions highlighted in this guide, and if problems persist, please provide a screenshot of the shunt value calculation error. I tried filling out all inputs in the 'Design Constraints' section, and the calculated Rshunt value populated after clicking 'Reset'.

Lowest Meaningful Vsense is the lowest voltage across the shunt resistor that can be measured by the INA with reasonable accuracy. The Power Monitor Tool should provide you this value based on your inputs, you do not have to choose this value.

Supply voltage of 3.3V seems to have lower error according to our error tool (possibly due to improved PSRR): CS-AMPLIFIER-ERROR_TOOL

I recommend you download this error tool and try adjusting the minimum current and Rshunt value parameters to see what accuracy can be obtained for your current measurement range. You will likely need to decrease the axis range as it defaults up to 300A (this can be done by simply double clicking the axis numbers).

Let me know if you have any further questions.

Cierra

• Hi Cierra,

It took a little time but now we know what the problem was. it is because of Excel separator setting. i am from germany and here we use ',' for decimal separator. thanks and have a nice day.

Best Regards

Toktam Khani