Features

MC3PHAC Monolithic Intelligent Motor Controller, Rev. 2

Freescale Semiconductor

9

High Precision Calculations — Up to 32-bit variable resolution is employed for precision control and

smooth performance. For example, the motor speed can be controlled with a resolution of 4 mHz.

Smooth Voltage Transitions — When the commanded speed of the motor passes through

±1 Hz, the

voltage is gently applied or removed depending on the direction of the speed change. This eliminates any

pops or surges that may occur, especially under conditions of high-voltage boost at low frequencies.

High-Side Bootstrapping — Many motor drive topologies (especially high-voltage drives) use

optocouplers to supply the PWM signal to the high-side transistors. Often, the high-side transistor drive

circuitry contains a charge pump circuit to create a floating power supply for each high-side transistor that

is dependent on low-side PWMs to develop power. When the motor has been off for a period of time, the

charge on the high-side power supply capacitor is depleted and must be replenished before proper PWM

operation can resume.

To accommodate such topologies, the MC3PHAC will always provide 100 ms of 50 percent PWM drive

to only the low-side transistors each time the motor is turned on. Since the top transistors remain off

during this time, it has the effect of applying zero volts to the motor, and no motion occurs. After this

period, motor waveform modulation begins, with PWM drive also being applied to the high-side

transistors.

Fast Velocity Updating — During periods when the motor speed is changing, the rate at which the

velocity is updated is critical to smooth operation. If these updates occur too infrequently, a ratcheting

effect will be exhibited on the motor, which inhibits smooth torque performance. However, velocity

profiling is a very calculation intensive operation to perform, which runs contrary to the previous

requirement.

In the MC3PHAC, a velocity pipelining technique is employed which allows linear interpolation of the

velocity values, resulting in a new velocity value every 189

µs (252 µs for 15.9 kHz PWMs). The net result

is ultra smooth velocity transitions, where each velocity step is not perceivable by the motor.

Dynamic Bus Ripple Cancellation — The dc bus voltage is sensed by the MC3PHAC, and any

deviations from a predetermined norm (3.5 V on the dc bus input pin) result in corrections to the PWM

values to counteract the effect of the bus voltage changes on the motor current. The frequency of this

calculation is sufficiently high to permit compensation for line frequency ripple, as well as slower bus

voltage changes resulting from regeneration or brown out conditions. See Figure 4.

Selectable Base Frequency — Alternating current (ac) motors are designed to accept rated voltage at

either 50 or 60 Hz, depending on what region of the world they were designed to be used. The MC3PHAC

can accommodate both types of motors by allowing the voltage profile to reach maximum value at either

50 or 60 Hz. This parameter can be specified at initialization in standalone mode, or it can be changed at

any time in PC master software mode.

Selectable PWM Polarity — The polarity of the PWM outputs may be specified such that a logic high on

a PWM output can either be the asserted or negated state of the signal. In standalone mode, this

parameter is specified at initialization and applies to all six PWM outputs. In PC master software mode,

the polarity of the top PWM signals can be specified separately from the polarity of the bottom PWM

signals.

This specification can be done at any time, but once it is done, the polarities are locked and cannot be

changed until a reset occurs. Also, any commands from PC master software that would have the effect

of enabling PWMs are prevented by the MC3PHAC until the polarity has been specified.