I know enough of electronics to be dangerous with a screwdriver. My brother Bruce is experimenting with motor control using one of the New Micros TPIC0108B-based NMIH-0030 H-bridge boards, and this past weekend I was attempting to help him work through the datasheet. Along the way I noticed an oddity in the '0030's input -- it appeared that they were simply re-labelling the TPIC's IN1/IN2 inputs as DIR/PWM.
Is this a "reasonable" / "good" / "acceptable" thing to do?
Here's why it doesn't feel right (I'm hoping someone can point out a big hole in my reasoning so I can stop worrying about it):
The TI TPIC0108B data sheet has the following Function Table for IN1+IN2:
-----------------------+--------------------------------- IN1 IN2 OUT1 OUT2 | MODE --- --- ---- ---- | ---- 0 0 Hi-Z Hi-Z | Quiescent supply current mode 0 1 LS HS | Motor turns clockwise 1 0 HS LS | Motor turns counter-clockwise 1 1 HS HS | Brake, both HSDs turned on hard -----------------------+--------------------------------- HSD = high side drivers LSD = low side drivers Hi-Z = high impedance
So... when you want to drive FORWARD (CW), you hold IN1 LOW and put a PWM pulse in IN2. This alternates the motor connections between full-on-CW and Hi-Z (disconnected). So far, so good.
But when you try to do the same thing with REVERSE (CCW), and you hold IN2 LOW and toggle IN1 with your PWM pulse, you're switching the motor connections between full-on-CCW and... BRAKE ???
I didn't hit this previously because my current project uses an L298N -- I set the inputs to control direction and then toggle the Enable line with my PWM.
I feel like I _have_ to be overlooking something somewhere. After all, there are lots of H-bridges out there, and at least _some_ of them seem to be using this approach without letting their magic smoke escape.
(It was a bad weekend. I'm even more mystified how a chip rated with Rds(on) of 0.300ohm per driver -- so 0.600ohm for the bridge -- can handle 3A without exceeding its maximum Continuous power dissipation of 1.29W. When I do the obvious I*I*R on my calculator it gives me 3*3*0.6 = 5.4W. Aaaagh!)
No rush... if it works, it works, and if it doesn't I'll hear about it soon enough (;-).
Frank McKenney
-- "We are not creatures of circumstance; we are creators of circumstance." -- Benjamin Disraeli
-- Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)