AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 18

• Manufacturer Part Number: AN1857
• Description: A 3-Phase ac Induction Motor Control System Based on the MC68HC908MR32
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
Analog
Optoisolation
Circuit
18
ANALOG INPUT
(V
In
)
100 k
R1
Next, NAND gate, U1A, inverts the input signal. Assuming a logic low at
the input (across R1), U1A’s output is high. With no forward bias on the
input diode of the optocoupler, the optocoupler’s output transistor is off,
producing a logic high. This logic high is inverted by U2A to produce a
logic low at the output. Conversely, when the input (across R1) is high,
the output of U1A is low. Forward bias at the input of the optocoupler
causes light to shine on the optocoupler’s photodiode which produces a
leakage current that flows into the optocoupler’s base. With base current
supplied, the optocoupler’s transistor is on and the optocoupler’s output
is low. That low input into U2A causes the output of U2A is go high. The
block as a whole, therefore, is non-inverting.
Figure 10
analog optoisolation circuit used on the optoisolation board. The circuits
are used to couple several feedback signals from the power board to the
controller board. Those signals are the phases A, B, and C current
feedback, system bus current, and bus voltage, heatsink temperature
and phase A, B, and C back-EMF signals.
GNDA_PS
Figure 10. Analog Optoisolation Circuit
Freescale Semiconductor, Inc.
6
5
+
–
For More Information On This Product,
A1
MC34072D
7
is a simplified schematic diagram of the basic design of the
Go to: www.freescale.com
100
R2
1
2
3
4
PD1
HCNR201#300
LED
ISO1
PD2
6
5
GNDA_CB
5
6
+
–
100 k
4
R3
A2
MC33502/D
7
ANALOG OUTPUT
(V
MOTOROLA
Out
)
AN1857