PSU - High side current sense

Projects / Smart Bench Supply

I have decided to move this topic outside of my main page as I am still not happy with what I have.  Some of you might question my sanity of not using a proper part.   They all look fine at first glance until you go and read the datasheet.  That's why I still have some INA201 unused.

7.4.2.4 Low V SENSE Case 2: V SENSE < 20 mV, 0 V ≤ V CM ≤ V S

For my application, it can fall within this condition.  One way to get around this is to add 20mV offset to the input.

For the INA201, this is a 50V/V * 20mV = 1V offset at the output that I have to remove.
It is ~1/3 of the input range of my ADC.

LT6100:

The common mode range is Vcc + 1.4V to 48V.  This means that the part has to be sitting on a negative rail and the output needs to be level shifted.  Unfortunately I only have one of these in questionable condition.

Old details page archive as follows:

High Side Current Sense

The high side current sense circuit is a lot more complicated because of requirement 2.

1. The load current can be sensed at the output of the LDO as it draws different amount of currents based on supply voltage, temperature and load requirements. The feedback resistor needs to be connected after the sense resistor to compensate for the drop.
2. VCC rail can reach 0V.

R3 is the current sampling resistor.  This adds up to 0.05R x 3A = 0.15V drop at full load.

The I*R voltage drop is level shifted to a negative rail via a voltage controlled current source (R5, R2, U3 and M2).  The negative rail provides a DC offset for the circuits to work as V2 can sometime drops to 0V.  The voltage gain is set by resistor ratio of R2/R1. A gain of 20 gives 1V/A.

Voltage across R2 drive a voltage controlled current sink (U2, R1 and M1). This current is converted back to a ground reference voltage using a transimpedance amplifier (U1 and R4). The ratio of R4/R1 determines the voltage gain.

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This is a revised version.  The constant current circuit has been replaced. The 20K resistor is on a DC offset as the opamp buffer U1 cannot swing to 0V. The offset is subtracted off with the different amplifier.

There is an issue with the opamp Vos interfering with low current reading. LTSpice thinks it is about 20mA, but it'll be better or worse depending on luck. 

Note: the constant current sink, level translating circuits are left out.  Revise circuit here.

This is not easily solvable with common parts as the common mode input of opamp needs to include the positive supply rail.  A precision opamp with low dc offset does not usually reach the positive rail.  A rail to rail opamp compromises by using two complementary input stages to cover the entire input range resulting in the following type of performance - good in the middle but worse off on either ends of the rail.  If we are talking about trimmed parts, but those are unicorns.

Fixes:

The value of sampling resistor can be increased so that this nonlinearity will only show up for a lower current. The minimum LDO load can be used to move the operating point above this.

2. TI INA201
As mentioned before, this requires a 20mV offset to be inserted into input and 1V removed from the output.  A precision voltage reference, a voltage divider, a floating supply and opamp for subtracting the output are needed.  I do have some samples of this.

3. Analog/LinearTech LT6100
This requires operating from a negative supply to meet the common mode requirement and an opamp to convert the output back to ground reference. This approach has the least complexity with less gotcha.  I only have one sample.

There are some non-linearity below 8.6mA.  We can operate above that by increasing minimum LDO load slightly.

The -5V rail can be provided by a switched cap converter.  A RC filter is needed to clean up the output.

As the negative rail is initially ramped up, there is a glitch at the output.  It is not something that would cause any issues.

While looking through my own project directory under Current sensing, I rediscovered a part: MAX44284  So far I have not found any flaws yet. It is worth looking at if you require accurate current sensing.

• Very Low 2μV Input Offset Voltage (MAX44284F/H)
• Extremely Low 50nV/°C Input Offset Tempco
• Low 0.05% Gain Error
• -0.1V to +36V Wide Input Common-Mode Range - covers full range without needing a work around.

The only reason why I might not be using this is that I don't own any of these parts. It is much hard to get free samples these days.