Digital Power Supply Bypassing

MC3PHAC Monolithic Intelligent Motor Controller, Rev. 2

Freescale Semiconductor

15

Digital Power Supply Bypassing

Fast signal transitions connected internally on these pins place high, short-duration current demands on

the power supply. To prevent noise problems, take special care to provide power supply bypassing at the

frequency-response ceramic capacitor, such as a 0.1

µF, paralleled with a bulk capacitor in the range of

1

µF to 10 µF for bypassing the digital power supply.

Analog Power Supply Bypassing

digital converter (ADC). On the schematics in this document, analog ground is labeled with an A and other

grounds are digital grounds. Analog power is labeled as +5 A. It is good practice to isolate the analog and

digital +5 volt power supplies by using a small inductor or a low value resistor less than 5 ohms in series

the ADC’s voltage reference.

Decoupling of these pins should be per the digital power supply bypassing, described previously. ADC

Grounding Considerations

Printed circuit board layout is an important design consideration. In particular, ground planes and how

grounds are tied together influence noise immunity. To maximize noise immunity, it is important to get a

good ground plane under the MC3PHAC. It is also important to separate analog and digital grounds. That

is why, shown on the schematics, there are two ground designations, analog ground is marked with an A

and other grounds are digital grounds. GND is the digital ground plane and power supply return. GNDA

is the analog circuit ground. They are both the same reference voltage, but are routed separately, and tie

together at only one point.

Power-Up/Power-Down

When power is applied or removed, it is important that the inverter’s top and bottom output transistors in

the same phase are not turned on simultaneously. Since logic states are not always defined during power-

up, it is important to ensure that all power transistors remain off when the controller’s supply voltage is

below its normal operating level. The MC3PHAC’s PWM module outputs make this easy by switching to

a high impedance configuration whenever the 5-volt supply is below its specified minimum.

The user should use pullup or pulldown resistors on the output of the MC3PHAC’s PWM outputs to ensure

during power-up and power-down, that the inverter’s drive inputs are at a known, turned off, state.