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Fighting LMD18200 - I'm going insane!

I'm trying to breadboard something to show a colleague, and my LMD18200 is being recalcitrant. The circuit is so simple it's practically a reference ckt:

Pin 1 - 22n ceramic to pin 2 Pin 2 - Goes to motor (output 1) Pin 3 - Direction input Pin 4 - Brake input, tied low (not braking) Pin 5 - PWM input, tied to +5V Pin 6 - +24V input Pin 7 - GND Pin 8 - Not used (current sense output). Pin 9 - Not used (thermal flag output) Pin 10 - Output 2 Pin 11 - 22n ceramic to pin 10.

Additionally, I have a 220uF 63V bypass cap between pins 6 and 7.

When I set pin 3 @ +5V, output 1 shows a slightly asymmetric 24V square wave with a period of about 15.2ms, but the motor is at least running in the right direction. Very suspicious that this noise is so close to AC line frequency.

When I have pin 3 LOW, I see an odd noise waveform on both outputs, and the chip starts to draw about 450mA on the 24V line. (The motor only draws 60mA).

The 22n caps should nominally be 10nF, but I didn't have any lying around so I subbed in the closest value I have. But these don't seem to be critical anyway, because I still see exactly the same symptom when I remove those caps.

What am I doing wrong? I googled and found a few sample circuits on the web, e.g. and except for some sugar in those circuits that isn't in mine, I'm doing exactly the same things in exactly the same ways.

Very frustrating, because I've used this chip many times before and it was always very simple.

Reply to
Lewin A.R.W. Edwards
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... and the circuit was good, but the chip was bad. I foolishly assumed that since the Digi-Key seal was still intact, the chip was good.

Reply to
Lewin A.R.W. Edwards

Whew! Had me going there. I use the same circuit--except that I do away with the bootstrap caps, since the motor is always full on forward or full on reverse. I couldn't see how the bootstrap caps could give the symptoms you saw.

How were you breadboarding that circuit? The odd pinout of the LMD18200T convinced me to go straight to a PC board.

Mark Borgerson

Reply to
Mark Borgerson

Hi Mark,

I bolted the chip to a piece of protoboard. I then took a pair of pliers and straightened the odd pins at the "knee" so they stick up at

90 degrees to the chip face. I then soldered wires to them. There's a picture at though it's not a great perspective (the clipleads run off to my STK500 and to the motor). Oh, and and are what it's going into. The wooden pieces are just wedging that nose piece in place while I decide what angle to put on the leading edge of the pipe. Painstaking work, I'm afraid. It will look much better once it's all stuck together and painted.

This is going to be a pool toy of sorts for the aforementioned colleague. Flashing lights in the nose so it looks more hi-tech. Basically it contains a couple of the modules from my sub that I already have debugged, but it's not intelligent - it just swims about randomly.

Reply to
Lewin A.R.W. Edwards

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