DG-IRMCK203 Datasheet, PDF (64/67 Pages) International Rectifier – Application Developer’s Guide

IRMCK203 Application Developer’s Guide
Appendix B IR2175 Current Sensing
Two channels of current feedback interface logic are provided in the IRMCx20x system. Each module measures the
incoming varying duty period of the 130 kHz carrier frequency signal at the IR2175output. Measurement is performed
for both carrier frequency period and on duty period at the same time using fast counters. Counting frequency is 133
MHz with a 33.3 MHz system clock.
The IR2175 are the unique high voltage ICs capable of measuring the motor phase current through an associated shunt
resistor, which can generate ±260mV voltage range. The output of the IR2175 is an open drain with a 130 kHz fixed
carrier frequency where the duty variance is linearly proportional to ±260 mV input voltage. The counting frequency
is 133.3 MHz when the system clock crystal frequency is 33.3 MHz, which yields 10-bit resolution of the current
measurement data from the IR2175.
The offset measurement is automatically added after the 10-bit current measurement has been calculated. The offset
value must be calculated and supplied by external hardware or software.
The period measurement of both the carrier frequency period and the duty period of the IR2175 output signal are
performed. For carrier frequency period measurement, there is a 16-stage averaging filter to smooth out the 130 kHz
carrier period of the IR2175. The multiply/divide computation follows after completing both period measurements.
Divide computation between the carrier frequency period and the duty period alleviates temperature drift of the
incoming data off the IR2175, since variation of these periods uniformly moves in same direction as temperature
changes.
The measured and adjusted data is coherently updated to the host digital system such as a microcontroller, DSP, or
FPGA. A block diagram of the current measurement block is shown in Figure 30.
Phase V
Period Cohesive
Latch
AXK ^A
O
B
B
K
Phase V Period
Counter
Phase V Carrier
Frequency Period
Cohesive Latch
4096
Filter
Phase V Carrier
Frequency Period
Counter
Internal
PWM Sync
signal
IR2175
output falling
edge
Figure 30. Current Feedback Measurement Block
Digital
FIlter
The current feedback module requires a faster clock to count the duty period of the incoming pulse width modulated
signal from the IR2175. This clock rate is designed to work with a frequency between 120 MHz and 133.3 MHz.
Figure 31 depicts a simple time chart of counting.
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