High side current sense
The below image is taken from Analog Devices design note DN1045.
what is the use of M1 and M2?
What is the use of REF and D1?
1 answer
The sensed current may be at a +90V potential (VIN). The LTC2063 op-amp power supply range goes only to +5.25V . It's not a high voltage op-amp. So, the op-amp in this circuit floats just below the high voltage rail. That presents two questions.
How to level-shift the output of the current sensor op-amp and reference it to ground?
M1 and RDRIVE act as a level shifter. M1 is tared for more than 90V between drain and source.
How to create the negative supply voltage for the op-amp?
REF, M2, R2 form a linear regulator that's powered from the high voltage rain. It's regulated output voltage is referenced to VIN, so it's a negative output regulator. The app note says that the negative supply of the op-amp in this circuit is VIN - 3V .
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