Page 1 of 1

PWM frequency ?

Posted: Sat Oct 12, 2013 6:09 pm
by kert
Hi,

Whats a reasonable peak PWM frequency that A495x can be driven at ? Hopefully i should be able to switch it above 20Khz without too many problems ?

Obviously the switching losses will be higher with higher frequency. Unfortunately the datasheet doesnt even mention typical values at say 500hz, 1khz, 10khz.

Re: PWM frequency ?

Posted: Sun Oct 13, 2013 1:03 am
by andrew
kert wrote:Hi,

Whats a reasonable peak PWM frequency that A495x can be driven at ? Hopefully i should be able to switch it above 20Khz without too many problems ?

Obviously the switching losses will be higher with higher frequency. Unfortunately the datasheet doesnt even mention typical values at say 500hz, 1khz, 10khz.
The data sheet is a bit thin on that topic, the only hint it gives is that the it's <30 kHz when referring to load supply current.

Re: PWM frequency ?

Posted: Mon Oct 14, 2013 7:07 am
by kert
So can the shield be safely driven at 20khz ?

Re: PWM frequency ?

Posted: Mon Oct 14, 2013 9:41 pm
by andrew
kert wrote:So can the shield be safely driven at 20khz ?
We don't see any reason why not. If you want to supply your sketch we can test it for you on our gear first.

Re: PWM frequency ?

Posted: Sun Nov 03, 2013 5:20 pm
by kert
I'll be trying it out today, will update on results here.

Couple other small EE questions on the shield
- the A4954 inputs are driven by 7408 gate, any particular reason why it couldnt be driven MCU outputs directly ?
- it bugs me not to know - i have seen a few circuits with a nF cap (C4) across the LEDs, what does it really do ?

EDIT: Tested with 62khz, works just fine. As i was doing this on Atmega ADK, just do this in setup()
int divider = 0x01;
TCCR3B = TCCR3B & 0b11111000 | divider;
TCCR4B = TCCR4B & 0b11111000 | divider;

As per http://playground.arduino.cc/Main/TimerPWMCheatsheet

Re: PWM frequency ?

Posted: Mon Nov 04, 2013 10:02 pm
by andrew
kert wrote:I'll be trying it out today, will update on results here.

Couple other small EE questions on the shield
- the A4954 inputs are driven by 7408 gate, any particular reason why it couldnt be driven MCU outputs directly ?
- it bugs me not to know - i have seen a few circuits with a nF cap (C4) across the LEDs, what does it really do ?

EDIT: Tested with 62khz, works just fine. As i was doing this on Atmega ADK, just do this in setup()
int divider = 0x01;
TCCR3B = TCCR3B & 0b11111000 | divider;
TCCR4B = TCCR4B & 0b11111000 | divider;

As per http://playground.arduino.cc/Main/TimerPWMCheatsheet
Nice one. The capacitor is for decoupling.
Using the AND gates (74HC08) allows speed control via one PWM output per DC motor. So when the PWM signal is HIGH, the logic input to the A4954 also goes HIGH. Without the AND gates there wouldn't be any speed control unless both logic inputs were from a PWM output on the MCU. This method saves PWM outputs, only need one per DC motor instead of two.