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
R77 & R83 also determine the 49 mV ADC operational bias for current sensing, which corresponds to the motor
regeneration operation range.
For air conditioner applications and other applications that only work in motoring mode, the bias can be as low
as possible to reserve enough ADC range for the motoring mode, since negative current sensing would not
occur.
Figure 17
Current feedback configuration in MCEWizard
Please note the following two design guidelines for direct AD sample:
The external RC network’s input gain is less than 1. Normally ADC internal gain needs to be configured to
X3, X6 or X12 to lower the required input signal range, and reduce shunt resistor power dissipation. RS2
must be much higher than 10 mOhm to achieve enough current sensing signal-to-noise ratio (SNR), and
improve control performance: For example, to achieve sufficiently high SNR, motor current sensing counts
need be over 30~50% of the AD range (traced Iu peak over 600 cts~1000 cts) at the rate motor current, and
PCB ground noise lower than 5% of total sampled signal (check in MCEDesigner trace result). Very low
current sensing SNR might cause motor start to fail, or not running smoothly at low power.
There is NO common-mode noise-rejection ability, compared to external op-amp sample mode. Direct AD
current-sensing mode needs to deal with the PCB GND network very carefully to reduce grounding noise as
much as possible, to make shunt resisters as close as possible to IMC302A IC and DC bus capacitor. Please
note that any voltage difference between IMC302A’s ADC ground and shunt resistor ground will be
considered as an “input signal” and will deteriorate control performance or increase motor audible noise.
