Application Note AN4835, High-side current sensing for applications using high common-mode voltage

[TK1]

In your application note, AN4835, you are using resistors R2 and R3. Please can you explain their purpose?

My circuit uses the TSZ121 precision op-amp in the classical method without R2 and R3 and it works equally well. 

Thank you very much for any insight you can provide.

Best regards, TK1

[LCE]

Your picture does not show the important part that makes this work:

The zener diode and the resistor which create the supply reference voltage Vcc_L, which is Vcc_L = Vcc_H - Vzener.

So in this example the first opamp is actually supplied with 150 V and ~145 V.
Basically, R2 sets the gain, R3 allows and controls the output current through the MOSFET.

[TK1]

Thank you very much for your explanation. The Zener concept in HV supplies is an old one and well known.

The important part to me was to understand, the use of additional resistors that reduce accuracy, yet have no obvious benefit. Again, thanks for your fast help. Kind regards, Tom

[LCE]

Okay, then you probably know more about that stuff than I do... :D

[AScha.3 · Accepted solution]

..just guessing:

- R2 sets some current limit (about 1mA) through bsp220, in case of a short/high current, to save the adc input

- R2 + R3 giving some added current gain, to get 3,3 V output with standard resistor values (otherwise 100A -> 3,03 V out)  -- not important, i think :)

[TK1]

This makes total sense. Why didn't I think of that?

Thank you very much for the quick and easy explanation. TK