Fighting LMD18200 - I'm going insane!

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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. <http://www.botsandbytes.com/Schematics/18200.htm 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.

Re: Fighting LMD18200 - I'm going insane!
Quoted text here. Click to load it

... 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.

Re: Fighting LMD18200 - I'm going insane!
snipped-for-privacy@larwe.com says...
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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


Re: Fighting LMD18200 - I'm going insane!
Hi Mark,

Quoted text here. Click to load it

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 <
http://www.larwe.com/1.jpgthough it's not a great
perspective (the clipleads run off to my STK500 and to the motor). Oh,
and <
http://www.larwe.com/2.jpgand <
http://www.larwe.com/3.jpgare
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.

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