User guide
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V 1.0.1
2021-01-27
REF-AIRCON-C302A-IM564 user guide
Air conditioner application reference design kits
System and functional description
2.2.5
Design tips for direct motor current sensing
From the point view of the AD converter, the current input scaling value is the product of the voltage over shunt
resistor and the gain of the external current-sense amplifier, as shown in Figure 15.
Figure 15
Current feedback scaling outside IMC302A
Direct AD sample is used in the motor section of REF-AIRCON-C302A-IM564 reference design, i.e., motor current
sensing is done without an external OPA (Only C51, R83 & R77 used for operational gain, bias and low pass
filter), as shown in Figure 16.
The real AD input voltage is decided by the divider of R83 & R77, with bias of 49 mV and gain of 0.99:
𝐴𝐷 𝑖𝑛𝑝𝑢𝑡 =
𝑅
83
𝑅
77
+ 𝑅
83
𝑅𝑆
1
∗ 𝐼
𝑏𝑢𝑠
=
1𝐾
0.01𝐾 + 1𝐾
𝑅𝑆
1
∗ 𝐼
𝑏𝑢𝑠
=
0.99
∗ 𝑅𝑆
1
∗ 𝐼
𝑏𝑢𝑠
R77 & C51 is a low pass filter (LPF) for switching noise suppression, whose time constant is about 0.1~0.5 µs
normally.
A 20 mOhm, 5 W single shunt resistor (50110 package), and 1 k & 10 Ohm pull-up net are the default
configuration for the motor section of the REF-AIRCON-C302A-IM564 reference design. In other words, a 1 A
motor current produces 19.8 mV voltage to the ADC input, so the MCEWizard’s “Motor1 Current Input Scaling”
needs input at 19.8 mV/A (20*0.99=19.8).
Please note that “Internal Current Feedback Amplifier Gain” (Question 84 for MCEWizard V2.3.0.0) is ADC’s
internal gain, which defines the MAX ADC input range at V
CC
/Gain. For a 5 V power supply, the ADC sensing range
is 0 ~ 833 mV if 6 times the internal gain is selected, and 0 ~ 416.6 mV if 12 times the internal gain is selected.
Figure 16
Direct AD sample (current feedback without external op-amp)
