Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: szhang on October 18, 2008, 01:48:15 AM

Title: lmd18200 h-bridge question
Post by: szhang on October 18, 2008, 01:48:15 AM

I am somewhat puzzled by the LMD18200's design decisions.  If you look at the truth table for PWM, you'll notice that if you hold brake low, dir to whatever, and pulse PWM, the driver'll pulse the motor when the PWM is high and short the motor when PWM is low (effectively braking the motor).  Can anyone enlighten me on why this is case instead of letting the motor coast while the control signal is low.

Here is the truth table for the LMD 18200

PWM   DIR     Brake     Active Drivers
H    |    H    |    L    |    Source 1, Sink 2
H    |    L    |    L    |    Sink 1, Source 2
L    |    X    |    L    |    Source 1, Source 2
H    |    H    |    H    |    Source 1, Source 2
H    |    L    |    H    |    Sink 1, Sink 2
L    |    X    |    H    |    NONE

Title: Re: lmd18200 h-bridge question
Post by: Soeren on October 21, 2008, 04:11:51 PM

Quote from: szhang on October 18, 2008, 01:48:15 AM

Can anyone enlighten me on why this is case instead of letting the motor coast while the control signal is low.

It is for tighter control (but it is a more power hungry method).

If you want it to coast instead, just invert the PWM signal and feed it to the brake input - the inversion needs to have a propagation delay of at least 1µs (could be done by eg. the PWM signal feeding an R/C circuit of 1kOhm/1nF respectively, before going to an inverter with a schmitt trigger input).
If you want to retain the brake function, it can be done with a bit of logic.
If you drive the LMD18200 from a µcontroller, this can all be done in software.