Servo - April 2009

considerably cheaper, which was another requirement as I intended to buy them by the dozens. My hypothesis is that by parallelizing these boards J1:1 together, a higher amperage rating could be obtained.

As you can see in Photo 1, the controller is not only tiny but the component count is considerably small. In reality, there is no need for much more as the motor driver itself is packaged on the tiny

J1: 4

16-pin SOIC package.

The controller has all the capacitors needed for the charge pump (the H-bridge uses four N-channel power MOSFET transistors and a higher internal voltage is generated to enable the high side drivers), the bulk capacitance for high current transient management — especially when the motor changes direction — and the internal regulator voltage bypass capacitor.

A sense resistor allows for a current max to be set. A zero ohm resistor signals the device to limit current to at least 2.8A. That’s a lot of juice! If you want to limit the current to less than 2.8A, place a resistor such that the voltage at the SENSE pin is 500 mV. The equation is:

Connector Name

Motor Fault

J1:2 MODE

J1: 3 PHASE

SLEEP

J1: 5

J1: 6

J2

J3

J4

J5

ENABLE

GND

V+

GND

A

B

Description

Connected to the NFault pin. Needs to be pulled up to rail ( 3.3V or 5V, depending on system).

Controls current decay modes (fast vs. slow)

Controls direction

Allows for very little current consumption when on sleep mode

Enable/Disable and/or speed control if PWM’d

Microcontroller Ground

Solder Motor Battery Positive Terminal (8V to 36V)

Solder Motor Battery Ground Terminal

Solder Motor Positive Terminal

Solder Motor Negative Terminal

TABLE 1

500 mV

RSENSE =

DesiredCurrent(mA)

Four pads (J2 through J5) allow for battery power and motor leads to be soldered. A six-pin connector (J1) offers the inputs and outputs necessary for the microcontroller application to interface (FAULT, MODE, ENABLE, SLEEP, PHASE, and GROUND).

Table 1 shows the connections to power supply, motor and microcontroller signals.

Figure 1 Shows the DC motor controller Block Diagram and the connections needed to be made. Any microcontroller should be able to tackle on the task of sending these signals and reading the NFAULT output. I decided to leave the NFAULT disconnected for early tests, but will definitely use this feature as a trouble shooting tool on later projects.

Do be warned that I have found the NFAULT to get asserted a few times when the motor starts as the large inrush current is bound to be seen as a short. It is a matter of learning how much time the particular motor needs to start and ignore these NFAULT triggers for that time. Depending on the motor, you may see some of these NFAULT pulses when the motor switches direction, in which case the firmware must take this fact into account, as well.

Artwork used to generate the board can be downloaded from my website at www.avayanelectronics.com or from the SERVO website at www.servomagazine.com. I usually post firmware and other details, as well.

Figure 2 shows the schematic for the entire controller design. Six passive components, a chip, and a bare board offer a low count.

controllers in parallel and increase current capability? I found the A3950 to allow for this enhanced power potential by plugging two controllers in parallel.

There are various ways of attaching the two controllers in parallel. I tried three different venues and the cleanest is shown in Photo 2. The item to look for is cable length. I am not certain what would happen if cables are grossly different in length, other than possibly inducing some fault on one of the controllers if the signals do get out of phase. However, with cables the same size, I was able to control the motor with two controllers in the same fashion as I had done with one with an added plus, that is a twofold increase in current!