UTMC Application Note __________________________________________________________________ Calculation of Sµ µMMITTM E Stand-Alone Current Utilization The following describes the typical supply current consumed by the SµMMIT E protocol device (UT69151E) during 1553 message processing. Within the SµMMIT E package drawing current is the JA01 protocol die (figure 1). Considering the SµMMIT’s power saving architecture, it is appropriate to calculate supply current with respect to some reference of message rate or duty cycle. Therefore, characterization data and device specifications exist for 0%, 25%, 50%, 87.5%, and 100% duty cycles. These percentages comprehend actual intermessage gap and RT response times defined by the 1553 bus protocol (i.e. 12µs and 4µs, respectively). In rare cases where a maximum number of data words are transferred, the protocol circuitry operates at just above 90% duty cycle, well below the maximum rating. Note: Characterization and specification data do not usually include 100% duty cycle by definition of 1553 not that the devices can not operate at this maximum. Protocol Handler MMU (unused for stand-alone device) SRAM (unused) JA01 die Figure 1. Sµ µMMIT E Device With regard to the current consumption during 1553 message processing, the JA01 circuitry facilitates 1553 operations. This situation is comprehended by data specified in the SµMMIT handbook for the stand-alone device. Table 1 relates typical test data collected, at worst case temperature and voltage, for the stand-alone SµMMIT part in comparison to 40mA standby current specified within the SµMMIT handbook (Chapter 16). Using specification data and typical characterization data at various duty cycles simplifies power and supply current calculations required by the designer. Using information provided by the table on the next page, it is possible to generate total current and subsequent power consumption by the SµMMIT E during 1553 processing. Latest Revision Date: Original Publication Date: 9/20/1999 Page 1 of 3 UTMC Application Note __________________________________________________________________ Table 1. SuMMIT Current vs. Duty Cycle 50 40 35 35.1 35.8 Supply Current 30 (mA) 20 27.6 22.8 typical test values spec. values 10 3.3 0 0 25 50 75 100 Duty Cycle At 0% duty cycle the observed current consumption for the stand-alone SµMMIT E is 3.3mA. Using values for each duty cycle provide the following results: 0% ISµMMIT E = 3.3mA 25% ISµMMIT E = 22.8mA 50% ISµMMIT E = 27.6mA 87.5% ISµMMIT E = 35.0mA 100% ISµMMIT E = 35.8mA Multiplying these total currents by the nominal voltage supply value of 5 Volts provides the following power consumption equations per respective duty cycle: 0% PSµMMIT E = 5V(3.3mA) = 0.017 Watts 25% PSµMMIT E = 5V(22.8mA) = 0.114 Watts 50% PSµMMIT E = 5V(27.6mA) = 0.138 Watts 87.5% PSµMMIT E = 5V(35.0mA) = 0.175 Watts 100% PSµMMIT E = 5V(35.8mA) = 0.179 Watts Latest Revision Date: Original Publication Date: 9/20/1999 Page 2 of 3 UTMC Application Note __________________________________________________________________ From the above it can be seen that under the most intensive 1553 message processing the SµMMIT E consumes considerably less than a fifth of a Watt of power. For the sake of comparison it might be worth noting and calculating absolute worst case power consumption by considering the very guard-banded specification value at 100%. Using data from the table above provides the following: 100%rated ISµMMIT E = 40.0mA Then calculate power using maximum voltage rating: 100%rated PSµMMIT E = 5.5V(40.0mA) = 0.22 Watts Latest Revision Date: Original Publication Date: 9/20/1999 Page 3 of 3