© 2001 Microchip Technology Inc. DS00807A-page 1

AN42Low-Cost DC Motor Speed Control with CMOS ICs

FIGURE 1: 12VDC speed control and current limit.

Author: Scott Sangster,Microchip Technology, Inc.

INTRODUCTIONTwo low-cost CMOS ICs manage a 12 VDC, current-limited speedcontrol circuit for DC brush motors. The circuit design (seeFigure 1) uses PWM (pulse width modulation) to chop the effectiveinput voltage to the motor. Use of CMOS devices gives the benefitsof low power, minimal heat and improved longevity. The overalldesign is simple, inexpensive and reliable, and is useful in applica-tions such as embedded DC motor control where efficiency,economy and performance are essential.

CIRCUIT CONCEPTA CMOS update of the popular bipolar-555 timer device is usedbecause it draws much lower operating currents and thus runs coolerand lasts longer in typical operating environments. By adjusting thewiper of the speed-control potentiometer, output-signal duty cycle, orpulse width, can be varied from 2% to 98%. Operating frequency isfixed at 20kHz, to remain in the inaudible range.

Two signals are generated: the PWM signal, and a direction signal(high = forward, low = reverse). These signals become inputs to theTC4469, a CMOS Quad MOSFET driver. Its logic inputs allowproper output chopping and commutation.

OUTPUTSFour outputs drive a MOSFET H-bridge, to provide rapid motor-speed changes and motor-direction reversal. The lower rail powerMOSFETs are N-channel devices, the upper MOSFETs are P-channel. All four are driven directly by the TC4469.

In the lower rail devices, a small series resistor helps prevent gateoscillation and slows transition time, helping the upper device tostay "OFF." For motor voltages over 12VDC, a resistor divider andlow-cost level shift transistor can be added easily and economicallyto maintain a 15VDC gate drive for the upper rail MOSFETs. Sincethe ICM7555 and TC4469 need negligible current, a simple linearregulator can power them from the positive motor supply when it isabove 15VDC. Zener diodes can be used to help protect the gatesfrom supply transients. Gate-to-source capacitors help keep theupper MOSFETs "OFF" when the lower MOSFET in the same legturns "ON," causing a high dV/dT. Keeping the upper MOSFETgate drive impedance low in the "OFF" state will also help.

A sense resistor in the ground leg of the H-bridge provides an easyway to sense motor current, pulse by pulse, regardless of forwardor reverse motor rotation. This signal is filtered and applied to theICM7555 to inhibit PWM generation if motor current exceeds theallowed value.

CONCLUSIONSThis flexible, inexpensive circuit eliminates costly PWM devicesand complex floating upper rail drives, while still delivering efficientmotor control and protection.

94

13126

31

58

1011

TC4469

0.01µF7 2

Direction

100kΩ Speed

1kΩ

+12V

7

36

52

1 4

ICM7555

820pF

1N4148 47Ω

47Ω

2200pF2N3904

0.047µF

220µF 0.1µF 1kΩ 0.01µF 1kΩ0.01µF

Servo Motor

1kΩ

0.18Ω

10kΩ

+12V

1kΩ

1kΩ

814

+

1N4148

DS00807A-page 2 2001 Microchip Technology Inc

AN42

Information contained in this publication regarding deviceapplications and the like is intended through suggestion onlyand may be superseded by updates. It is your responsibility toensure that your application meets with your specifications.No representation or warranty is given and no liability isassumed by Microchip Technology Incorporated with respectto the accuracy or use of such information, or infringement ofpatents or other intellectual property rights arising from suchuse or otherwise. Use of Microchip’s products as critical com-ponents in life support systems is not authorized except withexpress written approval by Microchip. No licenses are con-veyed, implicitly or otherwise, under any intellectual propertyrights.

Trademarks

The Microchip name and logo, the Microchip logo, PIC, PICmicro,PICMASTER, PICSTART, PRO MATE, KEELOQ, SEEVAL,MPLAB and The Embedded Control Solutions Company are reg-istered trademarks of Microchip Technology Incorporated in theU.S.A. and other countries.

Total Endurance, ICSP, In-Circuit Serial Programming, Filter-Lab, MXDEV, microID, FlexROM, fuzzyLAB, MPASM,MPLINK, MPLIB, PICC, PICDEM, PICDEM.net, ICEPIC,Migratable Memory, FanSense, ECONOMONITOR, SelectMode and microPort are trademarks of Microchip TechnologyIncorporated in the U.S.A.

Serialized Quick Term Programming (SQTP) is a service markof Microchip Technology Incorporated in the U.S.A.

All other trademarks mentioned herein are property of theirrespective companies.

© 2001, Microchip Technology Incorporated, Printed in theU.S.A., All Rights Reserved.

Printed on recycled paper.

Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July 1999. The Company’s quality system processes and procedures are QS-9000 compliant for its PICmicro® 8-bit MCUs, KEELOQ® code hopping devices, Serial EEPROMs and microperipheral products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001 certified.

Microchip Technology Inc. DS00807A - page 3

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