Application Notes — MC74113 & MC75113
106
Juno Step Motor Control IC User Guide
15
15.7.3 Step Motor Drive with 5A Current Rating
This example shows a step motor drive with leg current sensing. The power train has four half-bridges for the step
motor’s four winding terminals. The input voltage in this example can be up to 56V. It is capable of driving 5A
continuous current with peak current of more than 10A.
The design considerations for the four half-bridges and their current sensing circuits are the same. Using as
example, the half bridge uses N-channel MOSFETs for both the higher and the lower switches to achieve high
efficiency. The half bridge is driven by PWMHighA and PWMLowA through MOSFET driver U1, which is powered
by 12V.
During normal operation, PWMHighA and PWMLowA are active high. For PWMLowA, a logic “1” turns on the
MOSFET Q3. R5, R6 and D3 provide an unsymmetrical turn-on and turn-off capability.
A logic “1” PWMHighA will turn on Q1. C3 is the bootstrapping capacitor, and it is charged through D1 when Q3 is
turned on. C3 provides the power to turn on Q1, and C3 needs to be a low-ESR capacitor such as a ceramic capacitor.
D1 should be a fast switching diode with low leakage current, and its voltage rating should be chosen based on +HV
and the +12V. R3 is optional; it can limit the charging current, especially during power up when C3 is zero voltage.
R1, R4 and D2 provide an unsymmetrical turn-on and turn-off capability.
Upon power up or during reset, PWMHighA and PWMLowA output are high impedance. Therefore, pull-down
resistors R13/R14 ensure that the upper and lower switches are all off so that the half bridge output is high impedance.
Also, when a fault is triggered, PWMHighA and PWMLowA can go into high impedance if configured, and the pull-
down resistors will turn off the MOSFETs and put the output of the half bridge into high impedance. Usually the
MOSFET driver has internal pull-up or pull-down resistors, and the user needs to check the driver’s datasheet and
decide if the external resistors are necessary.
R9 is the current sensing resistor, and U3 is the differential amplifier for signal conditioning. Please see
for more design considerations on leg current sensing.
This design uses dedicated MOSFET drivers and N-channel MOSFETs to achieve high efficiency and high
performance. This example provides a balanced design reference between performance and cost for applications up
to 56V and current up to 10A. With different MOSFETs, higher voltage and current capabilities can be achieved.
Please refer to the low cost DC drive example if cost is critical and to BLDC drive example for high voltage and/or
high current applications in noisy environments.
