Features

MC3PHAC Monolithic Intelligent Motor Controller, Rev. 2

Freescale Semiconductor

11

Selectable PWM Frequency — The MC3PHAC accommodates four discrete PWM frequencies and can

be changed dynamically while the motor is running. This resistor can be a potentiometer or a fixed resistor

in the range shown in Table 4. In standalone mode, the PWM frequency is specified by applying a voltage

to the MUX_IN pin while the PWMFREQ_RxD pin is being driven low. Table 4 shows the required voltage

levels on the MUX_IN pin and the associated PWM frequency for each voltage range.

NOTE

The PWM frequencies are based on a 4.00 MHz frequency applied to the

oscillator input.

Selectable PWM Dead Time — Besides being able to specify the PWM frequency, the blanking time

interval between the on states of the complementary PWM pairs can also be specified. Refer to the graph

in Figure 9 for the resistance value versus dead time. Figure 9 assumes a 6.8 k

standalone mode, this is done by

supplying a voltage to the MUX_IN pin while the DT_FAULTOUT pin is being driven low. In this way, dead

time can be specified with a scaling factor of 2.075

µs per volt, with a minimum value of 0.5 µs. In PC

master software mode, this value can be selected to be anywhere between 0 and 32

µs.

In both standalone and PC master software modes, the dead time value can be written only once. Further

updates of this parameter are locked out until a reset condition occurs.

Speed Control — The synchronous motor frequency can be specified in real time to be any value from

1 Hz to 128 Hz by the voltage applied to the SPEED pin. The scaling factor is 25.6 Hz per volt. This

parameter can also be controlled directly from PC master software in real time.

The SPEED pin is processed by a 24-bit digital filter to enhance the speed stability in noisy environments.

This filter is only activated in standalone mode.

Acceleration Control — Motor acceleration can be specified in real time to be in the range from 0.5

Hz/second, ranging to 128 Hz/second, by the voltage applied to the ACCEL pin. The scaling factor is 25.6

Hz/second per volt. This parameter can also be controlled directly from PC master software in real time.

Voltage Profile Generation — The MC3PHAC controls the motor voltage in proportion to the specified

frequency, as indicated in Figure 5.

An ac motor is designed to draw a specified amount of magnetizing current when supplied with rated

voltage at the base frequency. As the frequency decreases, assuming no stator losses, the voltage must

decrease in exact proportion to maintain the required magnetizing current. In reality, as the frequency

decreases, the voltage drop in the series stator resistance increases in proportion to the voltage across

the magnetizing inductance. This has the effect of further reducing the voltage across the magnetizing

inductor, and consequently, the magnetizing current. A schematic representation of this effect is

Table 4. MUX_IN Resistance Ranges and Corresponding PWM Frequencies

Voltage Input

PWM Frequency

0 to 1 V

5.291 kHz

1.5 to 2.25 V

10.582 kHz

2.75 to 3.5 V

15.873 kHz

4 to 5 V

21.164 kHz