negative rail for low side current sense?

I thought of just outputting a square wave from the MCU into a charge pump inverter, but 4.6v minus two diode drops is only a few volts, and not a lot of milliamps (you're limited by the pin's max output). It might be enough, but if a solid 1A buck/boost negative rail is cheap and easy enough...

That sounds like multiple op amps per leg... either both signals need an opamp to offset them, plus a third to difference them, or if you try to reduce the opamps, you can't offset them enough *and* amplify them at the same time (which is what I'm seeing now).

The IKCM15L60GAXKMA1 power module has a design without the separate short sense (just three resistors) but it uses diodes to "or" the signals into the sense pin. The side effect of that is you need larger sense resistors (36mR instead of 5mR, and 3W instead of 0.5W) which creates a larger voltage swing, which means the negative swings are much larger. This kind of design assumes the motor will always be "running" which is not true in my case (a hard stop caused the fault in the last design)

You kinda want differential anyway to avoid ground drops, with that much current.

Yup, but that doesn't stop the sensed value from going negative...

lørdag den 19. marts 2022 kl. 02.13.16 UTC+1 skrev DJ Delorie:

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FLAG 512 544 I_V FLAG 512 912 I_W FLAG 1424 624 I_SC SYMBOL OpAmps\\AD824 208 112 R0 SYMATTR InstName U1 SYMBOL voltage 304 -64 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL voltage -320 -32 R0 WINDOW 3 -334 50 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value SINE(0 {Ipeak} 100 10m) SYMATTR InstName V2 SYMBOL res 320 80 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 45k SYMBOL res 144 80 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL res -16 176 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL res 0 128 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R4 SYMATTR Value 91k SYMBOL res -224 176 R0 SYMATTR InstName R5 SYMATTR Value 0.005 SYMBOL res -224 16 R0 SYMATTR InstName R6 SYMATTR Value 1 SYMBOL OpAmps\\AD824 208 480 R0 SYMATTR InstName U2 SYMBOL voltage -320 336 R0 WINDOW 3 -334 50 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value SINE(0 {Ipeak} 100 10m 0 120) SYMATTR InstName V4 SYMBOL res 320 448 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 45k SYMBOL res 144 448 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R8 SYMATTR Value 1k SYMBOL res -16 544 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R9 SYMATTR Value 1k SYMBOL res 0 496 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R10 SYMATTR Value 91k SYMBOL res -224 544 R0 SYMATTR InstName R11 SYMATTR Value 0.005 SYMBOL res -224 384 R0 SYMATTR InstName R12 SYMATTR Value 1 SYMBOL OpAmps\\AD824 208 848 R0 SYMATTR InstName U3 SYMBOL voltage -320 704 R0 WINDOW 3 -334 50 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value SINE(0 {Ipeak} 100 10m 0 240) SYMATTR InstName V6 SYMBOL res 320 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R13 SYMATTR Value 45k SYMBOL res 144 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R14 SYMATTR Value 1k SYMBOL res -16 912 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R15 SYMATTR Value 1k SYMBOL res 0 864 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R16 SYMATTR Value 91k SYMBOL res -224 912 R0 SYMATTR InstName R17 SYMATTR Value 0.005 SYMBOL res -224 752 R0 SYMATTR InstName R18 SYMATTR Value 1 SYMBOL schottky 768 112 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D1 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 768 560 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D2 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 768 928 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D3 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 1072 528 R0 SYMATTR InstName R20 SYMATTR Value 1k SYMBOL OpAmps\\AD824 1312 560 R0 SYMATTR InstName U4 SYMBOL res 1072 336 R0 SYMATTR InstName R21 SYMATTR Value 2.4k SYMBOL res 1072 224 R0 SYMATTR InstName R22 SYMATTR Value 3k SYMBOL res 1072 624 R0 SYMATTR InstName R19 SYMATTR Value 1k TEXT -552 -200 Left 2 !.tran .1 TEXT -672 224 Left 2 !.param Ipeak=10

Interesting! Hadn't thought of that... Thanks!

Yeah, the choice of a gain-of-one current sense amp isn't great. There's gain-of-20 and gain-of-60 available, ST.com shows TSC2010, TSC2011, and for gain-of-100 TSC2012. They don't claim accuracy for big sense voltages, though.

Things I've used for similar setup depending on accuracy need. BLDC driver does not sound like metrology application ;-)

- Large negative rail

- LM7705 to generate -0.23V rail

- Resistor divider (one or two sided) from sense resistor to 3.3V (say, 40 ohms and 3.3V cause the -40mV to show up at 0mV, increase 40ohms to 'lift' the signal).

- Instrumentation amplifier

- Hall current sensor

- Opamp with negative input CM (TL27L4 and successors, TS921...)

-- mikko

Something like this should work.

The caps are optional. Some tricks can be played to make this 4-wire differential, or 3-wire sorta diff which preserves the shared divider.

Autozeroing is still a good idea.

One other thought: set the processor adc input to have a slightly positive offset and then counter that offset in the "ad conversion complete" interupt handler.

Hul

DJ Delorie snipped-for-privacy@delorie.com wrote:

-2V5 seems like a rather large source signal.

Is the sensed current that produces the 'below ground' situation normal or overload? Is a linear response required under those conditions?

If not, why not just ad sensor protection/conditioning?

RL

The lm324's inputs

Interesting idea, but I'd lose some precision in the adc. I saw some solutions that biased the sense the *other* way too - more adc range on the useful side.

The source signal is around 40mV in the old design, closer to 0.5v in the new design (which I'm not happy about)

The opamp changes that to a 0-5V swing centered around 2.5v

It's a servo. When it's speeding up, it's a motor, and everything's positive. When it's slowing down, it's a generator, and the currents are negative.

Classic LM324s can go insane on all four amps if one input pin is pulled a bit below ground.

When you're trying to hold a 0.09 degree step, it might as well be.

One poster had a clever idea for generating this with a charge pump off the main switcher.

That's a new one on me!

I'm not sure I want all the motor power potentially travelling through the MCU's vcc bus.

One poster hinted at these, and I've been looking at the TMCS1101 which would let me move the current sense to the three BLDC wires instead of the negative rail, so I could sense the positively-driven phases as well. It causes some layout issues, but I think it will be the most useful.

easy to try,

I couldn't find any offsets in your schematic and didn't see the gain.

There are a number of op amps that don't have issues with inputs going tens of millivolts below ground. Are you sure its an issue?

Same question - are you actually trying to control the current during the overload, or is it simply an overloaded monitor?

RL

A typo from me - I meant 40 (or 400) ohms to shunt non-GND-end and 3k3 (or 33k) to 3.3V so you're running roughly constant 1mA through. This causes a small gain/offset error, but that's easy to compensate.

I understood. I don't want the high voltage side connected to the 5v (not 3v3) bus at all, even through a 33k resistor, for both safety and noise reasons.