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2020-5-22
UG2020-11 REF-Vacuum-C101-2ED User Guide
6.1.2
External op-amp configuration and calculation for current feedback
The current input scaling value is the product of the shunt resistance in milliohms and gain of external current
sense amplifier for the REF-Vacuum-C101-2ED reference design, as shown in Figure 20.
Figure 20
Current feedback scaling outside IMC101T
A 2 mOhm 3 W 2512 package shunt resistor and 10X external-gain operational amplifier (op-amp) is the default
configuration for the REF-Vacuum-C101-2ED reference design, which means 1 A DC bus current produces 20 mV
voltage to ADC input, MCEWizard’ s “Motor1 Current Input Scaling” needs input of 20 mV/A.
Please note that the Internal Current Feedback Amplifier Gain (Question 84 for MCEWizard V2.3.0.0) is ADC’s
internal gain, which defines the maximum ADC input range to V
CC
/Gain. For 3.3 V power supply, the ADC sensing
range is 0~1.1 V if 3X internal gain is selected, and 0~550 mV if 6X internal gain is selected, 0~275 mV if 12X
internal gain is selected.
Figure 21
Current feedback with external op-amp
Figure 21 shows the external amplifier gain circuit in REF-Vacuum-C101-2ED reference design, which is a typical
differential amplifier, with input low pass filter and operational bias. C4, R30, R31 R10, R18 build the input
differential mode LPF, which can damping parts of PWM switching noise. The default LPF time constants is
2*RC = 2*(R31//R10)*C4 = 100 ns, which corresponds to DC bus current ring time. The typical setup time is
0.1~0.5 us, which depends on PCB layout, DC bus decouple capacitor and MOS gate driver configuration.
For better common-mode, noise-rejection ability, a balanced differential amplifier is highly recommended,
which means R19 is equal to the value of R6 & R8 in parallel.
The divider of R6 & R8 provide operational bias for the amplifier, which affects the maximum motoring current
and regeneration current range. With the default configuration of REF-Vacuum-C101-2ED, R6=22 k & R8=220 k,
the op-amp operational bias is 3300 mV * 22 / (22+220) = 300 mV, which means:
-
Op-amp outputs 300 mV at zero motor current (motor stopped)
