AN2433 Freescale Semiconductor / Motorola, AN2433 Datasheet - Page 18

• Manufacturer Part Number: AN2433
• Description: 5V to 3V Design Considerations
• Manufacturer: Freescale Semiconductor / Motorola

AN2433/D
Digital Signals
18
V
V
GND
GND
I_sense -
I_sense -
I_sense +
I_sense +
(Vref/2)
(Vref/2)
Current sensing is used for motor over-current protection as well as
determination of accurate back emf voltage. A current sensing resistor is often
used in low-cost applications (
return path of the DC-Bus lines for DC-Bus current measurement, or in the
individual legs of the inverter bridge for phase current measurements. The
voltage drop created on the current sense resistor is amplified using an
operational amplifier. The gain of the operational amplifier is set to get the
output signal in the MCU voltage range reference, i.e. 0V – 5V, 0V – 3.3V.
The output voltage for zero input current is set to the middle of the ADC range
by the Voltage GND level, which is usually derived from a reference voltage
generator.
Compared to 5V systems there is no change except that the chosen
operational amplifiers must operate on 3.3V single supply with rail-to-rail
output. Among suitable amplifiers are the On Semiconductor MC3320x and the
MC3350x rail-to-rail types.
In motor control applications, the MCU input signals serve as a user interface
(i.e. switches, push buttons) and as a means of communication and detection
of the state of control/feedback signals (i.e. zero crossing detection, enable
signal, etc.).
Compared to 5V systems, the input signals need to be adjusted to the 3.3V
level and the implementation of this adjustment depends on the actual signal
circuitry. Some examples are shown in
No other modifications are needed.
Freescale Semiconductor, Inc.
For More Information On This Product,
R1
R1
R2
R2
5V to 3V Design Considerations
Figure 16. Current Sensor
Go to: www.freescale.com
R4
R4
-
-
+
+
R3
R3
Figure 16
Interfacing to Microcontroller Inputs
). This resistor is placed in the ground
Input
Input
A/D
A/D
MCU
MCU
MOTOROLA
.