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Q. What is meant by “hottest die” power dissipation?
A. "Hottest die power dissipation" refers to the active transceiver chip. That is, at any given time, the hottest die will be for the transceiver for the "active" (transmitting) bus channel; i.e., either Channel A or Channel B. Q. What equation should I use for performing a thermal calculation for the Mini-ACE™ Plus? What is the ?JC for the hybrid? A. The junction-to-case thermal resistance (?JC)for Mini-ACE™ (Plus) is 6.8 degrees C per watt.
That is, TJ = TC + (6.8•PDISS), where TJ = hottest die junction temperature, TC = hybrid case temperature, and PDISS = hottest die dissipation. Q.. The Mini-ACE™ power ratings are given for transmitter duty cycles of 25%, 50%, 100%, and Idle. What does this mean? A. This refers to the percentage of time that the bus controller is actually transmitting on the 1553 bus; that is, not transmitting or receiving, just transmitting. For example, if the BC sends a message with a command word + 32 data words to each of 10 RTs once per second, the duty cycle would be:
10*((1 + 33)*(20 µs))/1 sec = 0.66% Q. If the BC is polling each of 10 RTs once per second, can I use the 25% duty cycle? A. Referring to the calculation above, you can almost use the "idle" duty cycle. Whatever your actual duty cycle is, you can calculate the power usage and dissipation; it's a linear interpolation. Q. Do the power ratings given apply to a dual bus? A. Yes. MIL-STD-1553 operates as a "standby redundant" bus. That is, the BC usually sends most of its messages on the "primary" bus (typically Bus A). The BC will only switch over to the secondary bus (typically Bus B) if it detects a failure on the primary bus, or if there is a failure of one (or more) RTs on the primary bus.
Clause 4.6.3.1 of MIL-STD-1553B states that “Only one data bus shall be active at a given time …”. On most 1553 terminals, it is not possible to transmit on both 1553 buses simultaneously. Q. I have a BUS-63125 transceiver on a board that doesn't get -15 V in production board test. If I ground the -15 V input to the board during test instead of -15 V, will this damage the BUS-63125? A. Grounding the -15V input to the BUS-63125 transceiver will not damage the BUS-63125. Of course, the BUS-63125 won't be able to transmit or receive in this condition.
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