Noise on Vcc line

[Abe]

Hi,
I am trying to construct a robot with LMD18245 from National
Semiconductor as motor drivers and a DS89C420 as the controller. Since
7 of the LMDs are being used to run 7 different motors, I need to know
the following:
1. Do I need to add capacitors across each LMD18245's Vcc and GND (as
suggested in the datasheet) or do I just add one big cap between the
Vcc and GND of the PCB which houses all the LMD18245s? The
microcontroller PCB is separate but gets
power from the same 12V SLA battery source.
2. I presume that there will be considerable noise on the 12V Vcc line
anyways. Now will this affect the 7805 voltage regulator o/p which
will power the microcontroller?
3. If answer to Q2 is yes, then will a 15V zener (Cathode connected to
Vcc) and a 100uF cap across the Vcc and gnd i/ps to the 7805 voltage
regulator, do the required job of reducing the noise? I am assuming
that the zener eliminates all spikes above 15V and the cap filters the
voltage dips below 12V due to the LMD 18245s.

Thanks in advance
Abhijit Karnik
INDIA

 

[Steve]

abe@eth.net (Abe) wrote in message news:<10a7b3ec.0310160703.78ee36ef@posting.google.com>...

1. Do I need to add capacitors across each LMD18245's Vcc and GND (as
suggested in the datasheet)

This would be best practice. "One big cap" won't do as good a job
smoothing fast transients (high frequency)... also, the closer you can
put the caps to the individual ICs the better. If you really wanted
to drop one of the caps, drop the 100uF and keep the 1uF.

2. I presume that there will be considerable noise on the 12V Vcc line
anyways. Now will this affect the 7805 voltage regulator o/p which
will power the microcontroller?

Not if it's appropriately filtered with a 0.1uF bypass cap on the
input and output pins. (also see below)

3. If answer to Q2 is yes, then will a 15V zener (Cathode connected to
Vcc) and a 100uF cap across the Vcc and gnd i/ps to the 7805 voltage
regulator, do the required job of reducing the noise? I am assuming
that the zener eliminates all spikes above 15V and the cap filters the
voltage dips below 12V due to the LMD 18245s.

Well, for my money, forget the zener and just go with a big cap.
Remember the 0.1uF cap as well and you should be fine.

Sounds like you might be drawing a lot of current from the 7805, have
you checked the power calculations?

nifty

 

[Abe]

Sounds like you might be drawing a lot of current from the 7805, have
you checked the power calculations?

nifty

Not exactly. THe 7805 sources only the uC and some latches.