# Trying to determine resistor value for current sense in high sided switch

#### Turby

Dec 14, 2011
2
Hi, not sure if this is in the right section, but here goes anyway...

I've done a design using ST's VNQ600AP-E quad high side driver. This device has four independent channels and four analog sense outputs which deliver currents proportional to the outputs currents.

I trying to establish what value for RSENSE I should be using for the analogue sense. I'm using a 10K resistor for RPROT on the same output going into the ADC on a PIC.

Here is the data sheet http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00044281.pdf

I have played with various resistors from 5K down to 1K, I'd like to use 1K to get a nice range for the ADC but am unsure what effect this will have on the PIC ADC input.

Any help in deciphering the data sheet would be greatly appreciated!

Last edited:

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
What are the range of output currents?

What is your Vcc for this chip?

I presume a 5V PIC?

Ideally you want the range of currents to be represented by 0 to 5V as this will give you the best resolution and reduce the impact of any ADC noise.

However with this device you also need to consider the situation when the device goes into thermal shutdown mode.

You also may want to provide some low pass filtering and input protection for the PIC (in case the voltage rises above the maximum for the PIC input (its supply rail))

#### Turby

Dec 14, 2011
2
Sorry for lack of details!

Ok, it's running on a 5v pic in an automotive environment so it will be switching 12-14 volts. Each channel will switch up to around 8 amps and 8 bit adc provides adequate resolution. I'm aiming for 8 amps to give me around 4 volts which leaves plenty of room to distinguish over temp / current as the chip gives 4.5 (IIRC).

Am I right in thinking of adding a 5.1 zenner from adc input to ground for each adc input?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Normally you can use a series resistor and then a diode to the +ve (and negative if required) supply rail. A simple series resistor may be sufficient though.

I can't quickly see how to calculate the appropriate sense resistor, but I expect it''s in the datasheet somewhere. Sorry I have to run right now.

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