MAX4372 High-side Current Amp Gotcha

MAX4372 output and boost transistor base drive
MAX4372 output and boost transistor base drive

Maxim’s MAX4372 is a high-side current sense amp that simplifies DC-DC converters, battery chargers / monitors, and stuff like that. It’s a nice gadget because it has a 28-V common-mode voltage rating that’s independent of the supply voltage. That means you can monitor the current from a relatively high voltage source with a low-resistance in series with the supply’s positive lead.

I’m using it as part of a solar panel characterization circuit for a Circuit Cellar column, where it’s a key part of a simple DC-DC boost converter that will illustrate how maximum power point tracking works.

The current sense resistor is a 0.5 Ω (that’s a capital Omega, if your browser just burped) resistor that produces 150 mV from the maximum 300 mA I’ll be drawing from the small panels in my collection. That’s the maximum rated full-scale sense voltage in the datasheet.

The scope shot shows that it works like a champ, with the DC-DC converter’s transistor base drive toggling at the 100 and 200 mA setpoints. There’s a touch of deliberate hysteresis in the comparators, hence the overshoot.

The datasheet has one Absolute Maximum Rating value that I’d managed to overlook:

Differential Input Voltage (VRS+ - VRS-) ... ±0.3V

What that means is that you must not, under any circumstances, apply more than 300 mV across the sense resistor. That translates into 600 mA for my circuit, which seems unlikely.

I was bringing the board up with a bench power supply set to 4 V and connected through an 8-Ω resistor to simulate a solar panel’s declining voltage-vs-current characteristic, with the supply set to a 300 mA current limit. Worked fine until I managed to connect the power supply without the resistor, at which point the MAX4372 stopped working.

What most likely happened was that the booster drive transistor stayed on a mite too long while the microcontroller was rebooting, which drove the inductor into saturation and put the supply directly across the sense resistor: 4 V / 0.5 Ω = 8 A, if the supply was up to it (which it isn’t). Not for very long, mind you, just long enough to kill the MAX4372 stone cold dead.

The best solution, which doesn’t appear anywhere in the datasheet or the app notes, seems to be a Schottky diode across the sense resistor. When the sense voltage exceeds the more-or-less 300 mV diode forward threshold, it’s clamped to a (presumably) safe value.

I haven’t tested this yet, but I don’t see any other way to prevent toasting the MAX4372 during the inevitable brief circuit glitches and faults. Even in an operational circuit, you might well see a brief short-to-ground that would apply the full supply voltage across the sense resistor: poof!

Oh, yeah, if you’ve just blown your two samples, Maxim nails you with a $15 shipping charge (!) to order more. I picked up ten through DigiKey for 60% more per piece, but just $4 in USPS postage.

I wanted to get some MAX4322 high-current op amps, but they’re non-stock items. Fooey!

Memo to self: Use the diode, Ed!

Update: see the comments for a better idea. More details later.

4 thoughts on “MAX4372 High-side Current Amp Gotcha

  1. If you use the schottky diode alone as a protective device, you’ll need a big one. The diode works by diverting the full current around the shunt, so it has to handle all that current.

    The input bias current for the chip is only 1uA (as long as the shunt is at least 2V above ground). That means you could put 100 ohms in series with each sense lead with negligable effect on accuracy. Then put a small schottky on the chip side of the 100 ohm resistors to clamp the differential voltage that the chip sees. That would protect the chip up to current levels that either blow the shunt, or overheat the 100 ohm resistors. I don’t know how sharp the turn-on knee of a schottky is – if it draws too much current at 150mV, the drop across the 100 ohm resistors could contribute unacceptable error.

  2. I like it… much better than what I was thinking, that’s for sure.
    Quick measurement: a 1N5819 has the usual exponential I-V curve (no surprise there) and conducts about 130 uA at 150 mV forward bias.
    So: measure the voltage across the 0.5 ohm sampling resistor through two 24 ohm resistors, with back-to-back Schottky diodes snuggled up against the MAX4372. The resistors drop maybe 3 mV each, for a total error under 5%; for this application, that’s pretty much spot on.
    I’ll do a formal post of all this in a few days, with a nice graph and all, but I think you’ve got it right.
    Update: sent a note to a friend with major juice @ Maxim and they’ll put a suggestion in the datasheet to protect fools such as I.

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