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AN155

Rev. 1.1 9

swapped during the layout phase for optimum routing to

the MOSFET gates. The stepping pattern has been

changed accordingly. The port output current is

sufficient to drive most small power MOSFETs with sub-

microsecond switching times.

The I/O pins of the MCU are by default configured as

inputs with a weak pull-up transistor. This has the

inadvertent effect of turning on all four transistors

momentarily when the MCU is first powered up. This is

usually not a problem for unipolar stepper motors since

the current will be limited by the winding resistance of

the motor. Bipolar drive will require either pull-down

resistors or inverting gate drivers to accommodate the

default pull-ups.

P0.6 is used to drive an LED indicator that illuminates

while the motor is moving. The C2 data signal is pin

shared with the active-low switch “SW1” on pin 10. The

switch is used to initiate a pre-programmed move so

that the board can be used without the serial interface

when desired.

The C2 reset signal is pin shared with a manual reset

button on pin 8. Momentarily pressing the reset button

will reset the MCU and turn all of the output transistors

off. Note that the MCU will be held in reset for as long as

the reset button is held down. As a result, all four output

transistors will be turned on while the reset button is

depressed.

4.2. Power Electronics

The power MOSFET selected for the stepper motor

reference design is the Fairchild FDS8926A. These are

small low on-resistance power MOSFETs in an SO8

package. These MOSFETs were chosen for their 30 V

rating, 3 V gate, and SO8 package. A maximum drain to

source rating of 30 V is required to drive the 12 V

stepper motor. The MOSFET chosen should be

compatible with a 3 V MCU. The relevant on-resistance

rating of the FDS8926A is 38 mΩ at 2.5 V. These

MOSFETs should easily handle 2-3 A in this application.

As a practical matter, the large dc wall-mounted

transformer used with the reference design has a rating

of 1 A at 9 V. The open load voltage of this supply is

about 12 V. The output voltage decreases to 9 V at 1 A.

This is sufficient to drive a small Nema 17 stepper motor

with a voltage rating of 12 V or 9.6 V and a resistance of

30 Ω or greater. A regulated lab supply can be used to

drive larger stepper motors up to 3 A.

The 330 Ω resistors were chose to provide turn-off time

of just under 1 μs. During the drain to source rise time,

the gate to source voltage is about 1.5 V. This is called

the plateau voltage and will depend on the threshold

and transconductance of the chosen MOSFET. The

plateau voltage may be obtained from the gate charge

curve as shown in Figure 6. Neglecting the V

of the

MCU, there is about 1.5 V across the gate drive resistor.

The 330 Ω resistors provide about 5 mA of gate drive

current during turn off. The gate to drain gate charge

of this MOSFET is about 5 nC. This gives a

switching time of about 1 μs. The measured values are

very close to this value. The measured V

of the MCU

is about 100 mV at this current.

It is not advisable to turn off the MOSFETs too fast. Using

no resistor or too small of a resistor will result in much

faster turn-off. The turn-off time will determine the rate of

the change in current or di/dt. The overshoot voltage at

turn-off depends on the unclamped inductance and the

di/dt. Excessive overshoot voltage could damage the

power MOSFETs. An excessively fast turn-off may also

hinder the ability of the Zener clamp to effectively protect

the Power MOSFET. The resistor also protects the MCU

from excessive currents and voltage transients.

The stepper motor reference design uses a clamp

circuit to protect the power MOSFETS from excessive

overshoot voltage. A zener voltage of 27 V was chosen

to protect the 30 V MOSFETs. This clamp circuit is a

cautionary measure to protect the MOSFETs when used

with different motors and voltage supplies. In many fixed

applications it is possible to just let the MOSFETs

avalanche during turn-off. The MOSFETs must be

capable of handling the peak current and energy stored

in the unclamped inductance. The current and motor

inductance will vary greatly from motor to motor. The

clamp circuit is useful when a single drive board might

be used with various motors.

4.3. Voltage Regulator

The voltage regulator is an LM2973-3.3. This is a 3.3 V

low-dropout regulator in a SOT223 package. The

maximum continuous input voltage for this device is 26 V.

It is important to consider the maximum open circuit

voltage of the power supply when selecting a voltage

, Gate Charge (nC)

, Gate-Source Votage (V)

510152025

Vplateau = 1.5 V

Figure 6. MOSFET Gate Charge

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 35 36

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