what is the min overhead voltage needed to output 4mA @ 5Vout?
Cannot find it spec'd in the datasheet.
I already emailed the product owner of the AD8280 regarding your question. I will get back to you as soon as I hear anything.
the datasheet only gives the values for 0mA: 6.0V and for 10mA: 7.5V.
The series pass transistor is usually a MOSFET with some on-resistance which will be more or less constant.
The 10mA range equals to 1.5V overhead voltage. In my opinion the rule of three is a way to approximately calculate the necessary input voltage, which would be 6.6V and with some safety: 6.8V.
According to the product owner, it is recommended to use the 7.5 V supply voltage since it is already specified for 10 mA current drive.
Let me know if you need anything else.
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