Category: Electronics Workbench

Contrary to what I’d thought, the MAX4372 circuitry has a simple gain error: it’s about 10% low over the full-scale 300 mA current range.

A bench supply produces 5 V through an 8 Ω resistor, although the slope of the purple line is more like 7.3 Ω. Close enough.

The blue line is the current sense voltage, which is exactly the same as the setpoint voltage plus a little PWM noise contributing to the waviness. Unlike the previous solar-powered chart, the bench supply voltage doesn’t drop enough to saturate the current sink, so the result is a nice straight line.

The red line is the MAX4372 output, which is consistently 10% low right up to the end; I can fix that with simple software scaling. The curve doesn’t flatten out, either, because the common-mode voltage across the sense resistor stays well above the it-stops-working-well limit around 2 V.

Conspicuous by its absence is any sign of nonlinearity due to the Schottky protection diode across the sense terminal inputs. The full-scale sense voltage is 300 mA x 0.5 Ω = 150 mV, which is sufficiently below the 1N5819 threshold of about 300 mV.

The picture shows the hack-job mod I applied to the circuit board; basically a cut-and-solder job with 10 Ω SMD resistors and a through-hole 1N5819. Yes, I stacked those two chips to get 5 Ω on the -Sense input; it’s a nice way to get good fixed ratios.

Despite what the stripes look like, both of those through-hole resistors are 1.0 Ω: brown-black-gold-gold.

The MAX4372T, the heart of this discussion, is the nearly invisible black rectangle just in front of the diode’s right-hand lead.

Although I should take a look at the high-value resistor / no diode protection circuitry, this one will suffice for now. It’s worth mentioning that I haven’t managed to burn this MAX4372 out, despite perpetrating much the same indignities on it as I did to the others, so the diode protection really is working.

Whew!

As noted here, there’s a difference between the current setpoint (controlled by the PWM analog outputs) and the measured values. As it turns out, there’s a better way to look at those datapoints.

This is a graph of measured current against the setpoints. Looks pretty good to me, apart from a teensy offset error. There really isn’t much in the way of a gain error over the entire range.

Having had a bit of time to think this over, the measured current-sink current should generally be numerically equal to the setpoint value, simply because there’s an external op-amp forcing that to be true. The twiddlepot adjusting the op-amp gain doesn’t enter into this, because the loop forces that voltage to match the PWM output. So, duh, the purple line should be spot on, at least up to the point where the sink transistor saturates.

What’s more interesting is that, over this range, the MAX4372 output is also spot on, which is not obvious from the previous chart. It flattens out when the common-mode voltage at the sense resistor drops below a volt, more or less, which is what the datasheet leads you to believe.

The datapoints comes from the same panel on a different day, so the points don’t quite line up if you’re comparing them. The brown solar panel voltage curve flattens out when the current sink transistor saturates, but the panel can continue to supply increasing current into a dead short, so the current continues to rise for a bit.

After I get the Circuit Cellar column laid to rest, I gotta figure all this out from first principles, then run the current up to 300 mA from the dreaded bench supply.

But the short answer seems to be that the Schottky protection circuitry doesn’t have much effect up through 75 mV. Which seems reasonable, come to think of it.

Here’s the result of using the Schottky diode input protection circuit I proposed there. I used 10 and 5 ohm resistors, twice the values shown in that schematic.

The circuit runs a load test and determines the maximum power point (MPP) of a solar photovoltaic panel every minute. The points represent the results of about two hours of winter-afternoon sunlight.

The test program applies an increasing load in steps of 10 mA (it’s a small panel, OK?) and records the corresponding panel voltage. One combination of current and voltage extracts the most power from the panel; that’s the MPP.

Obviously, the MPP varies with the amount of sunlight falling on the panel, so the result of the test is a cloud of points. That’s what you see in the graph: the highest points represent the most intense sunshine, the lower points come from shadows and changing sun angle.

The load is applied through a current sink that draws 100 mA per volt, as generated from a microcontroller PWM output. That’s pretty well calibrated by twiddling a gain pot, so I think it’s quite close.

The graph shows that the 10 mA steps recorded by the now-well-protected MAX4372 high-side current amp are low by about 10%, regardless of the absolute current level. That’s more error than predicted by the SPICE model (even with the larger resistors) and may represent contributions from something other than the protection network.

However, it does look as though a simple calibration routine could compensate for the error. That’s a simple matter of software, right?

Most important of all, the MAX4372 has survived the usual mistreatment. The load test is essentially DC, where the inductor counts as a piece of wire. Under those conditions, the combination of a stiff voltage source and an imposed load exceeding 600 mA produces a lethal differential voltage across the current-sense resistor. The diode clamps that voltage to 300 mV or so, which is enough to protect the MAX4372.

Rumor from my source at Maxim says the protection circuitry inside the MAX4372 can withstand maybe 50 mA, so the high-value external resistor approach (without the diode) may be the better way to go. Getting rid of the nonlinear diode should be a win…

Update: A different plot shows a different result. I think the offset comes from something other than the protection circuitry.