1-bit digital-to-analog converter

[julien....@gmail.com]

Hi,

I like the idea of the Gator and was reading your "circuit ideas" page
when I saw your examples of driving stepper motors. As I use several
steppers for robots (www.pobot.org) using L297/L298 or L6208, I am
very interested by the A3977.

I don't understand the "1-bit digital-to-analog converter" trick :
what signal should be outputted on the PD6 to change the speed of the
motor ? not a PWM but a pulse ?

Best regards,
Julien.

[ruggedc...@gmail.com]

On May 10, 4:26 pm, julien.holt...@gmail.com wrote:
> Hi,
>
> I like the idea of the Gator and was reading your "circuit ideas" page
> when I saw your examples of driving stepper motors. As I use several
> steppers for robots (www.pobot.org) using L297/L298 or L6208, I am
> very interested by the A3977.

Hello Julien,

The A3977 is just an example -- the L6208 can also be used, as can the
L297/298.

> I don't understand the "1-bit digital-to-analog converter" trick :
> what signal should be outputted on the PD6 to change the speed of the
> motor ? not a PWM but a pulse ?

This is actually fairly well explained by Figure 9 (and associated
text) of the L6208 data sheet.

PD6 does not change the speed of the motor; it changes the amount of
current that the motor receives. The speed is controlled (for a
stepper) by how quickly the motor is stepped at its control inputs
(i.e., through PD4/PD5). The current can affect the speed indirectly,
as higher currents allow higher torques, which means greater speeds
can be maintained under load.

To understand the "1-bit digital-to-analog converter" trick it helps
to think of the PWM waveform as an LED dimmer. For a constant
frequency, the DUTY CYCLE of the PWM waveform will control the
brightness of an LED, because the more often the LED is on (relative
to when it is off), the more current it carries (on average) so the
brighter it will look. At a high enough frequency, you don't see the
fast on-off-on-off action of the PWM because your eyes can't respond
that fast; they "integrate" the LED switching and "compute" an average
brightness.

The resistor and capacitor do the same thing in that circuit: they
integrate the PWM waveform and produce a constant, average value that
is proportional to the PWM waveform's duty cycle.

Hope that helps,

Rugged Circuits Support