AN857 データシートの表示（PDF） - Microchip Technology

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AN857

Brushless DC Motor Control Made Easy

Microchip Technology 

We are using PWM to control start-up current, so why

not use it as a speed control also? We will use the Ana-

log-to-Digital Converter (ADC), of the PIC16F877 to

read a potentiometer and use the voltage reading as

the relative speed control input. Only 8 bits of the ADC

are used, so our speed control will have 256 levels. We

want the relative speed to correspond to the relative

potentiometer position. Motor speed is directly propor-

tional to applied voltage, so varying the PWM duty

cycle linearly from 0% to 100% will result in a linear

speed control from 0% to 100% of maximum RPM.

Pulse width is determined by continuously adding the

ADC result to the free running Timer0 count to deter-

mine when the drivers should be on or off. If the addi-

tion results in an overflow, then the drivers are on,

otherwise they are off. An 8-bit timer is used so that the

ADC to timer additions need no scaling to cover the full

range. To obtain a PWM frequency of 10 kHz Timer0

must be running at 256 times that rate, or 2.56 MHz.

The minimum prescale value for Timer0 is 1:2, so we

need an input frequency of 5.12 MHz. The input to

Timer0 is FOSC/4. This requires an FOSC of 20.48 MHz.

That is an odd frequency, and 20 MHz is close enough,

so we will use 20 MHz resulting in a PWM frequency of

9.77 kHz.

AN857

There are several ways to modulate the motor drivers.

We could switch the high and low side drivers together,

or just the high or low driver while leaving the other

driver on. Some high side MOSFET drivers use a

capacitor charge pump to boost the gate drive above

the drain voltage. The charge pump charges when the

driver is off and discharges into the MOSFET gate

when the driver is on. It makes sense then to switch the

high side driver to keep the charge pump refreshed.

Even though this application does not use the charge

pump type drivers, we will modulate the high side driver

while leaving the low side driver on. There are three

high side drivers, any one of which could be active

depending on the position of the rotor. The motor drive

word is 6-bits wide, so if we logically AND the drive

word with zeros in the high driver bit positions, and 1’s

in the low driver bit positions, we will turn off the active

high driver regardless which one of the three it is.

We have now identified 4 tasks of the control loop:

• Read the sensor inputs

• Commutate the motor drive connections

• Read the speed control ADC

• PWM the motor drivers using the ADC and Timer0

addition results

At 20 MHz clock rate, control latency, caused by the

loop time, is not significant so we will construct a simple

polled task loop. The control loop flowchart is shown in

Figure 6 and code listings are in Appendix B.

 2002-2011 Microchip Technology Inc.

DS00857B-page 7

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