Power Considerations
Last Edit July 22, 2001
State Dependent Current - Bipolar
For some bipolar interface macros, as defined in the macro library documentation,
the current is state-dependent. The ICC and IEE values for many of the
interface macros are specified for HIGH and LOW input. Bidirectionals
are specified for enabled and disabled states
To compute actual power for any given state of the circuit, the state
and operating duty cycle of each I/O would have to be known, requiring
a detailed vector analysis
Estimating State-Dependent Current
Without specific and unique operating duty cycles, the following procedure
is recommended:
- For 2-state outputs (HIGH, LOW), calculate IEE and ICC as the average
of the two values (50% HIGH, 50% LOW)
- For 3-state outputs (HIGH, LOW, Z or INPUT) calculate as 50% disabled
high impedance Z state or input mode, 25% enabled HIGH and 25% enabled
LOW
Usage-Dependent Overhead (Bias) Power
Several arrays specify overhead power dissipation as a function of the
number of ECL interface macros used, without reference to placement. A
placement restriction is implied in such calculations
Placement-Dependent Overhead Current
For those arrays with placement-dependent overhead current, the knowledge
that a designer might influence power by careful placement is dangerous.
Placement is driven by speed performance requirements and array placement
restrictions. Only after these criteria are satisfied can any approach
toward reducing overhead current be attempted, and then it will be heavily
restricted
As a rule, the differences between the maximum worst-case overhead current
and the minimum do not warrant the violation of the performance criteria
Example
The AMCC Q24000B BiCMOS array has placement-dependent overhead current.
In the 100% ECL mode of operation, with a supply of -5.2V, a military
circuit would have 1.14W due to overhead. Of this, 40% is due to a fixed
component and 60% is due to the variable component of overhead. If the
array has no ECL interface macros, the minimum value can be applied
The breakdown for this one array is shown in Table 7-10
Table 7-10 Components Of Overhead Current
Description |
Value |
Generator Name |
fixed component |
51.0 mA |
- |
ECL inputs |
9.6 mA |
VTA10K or VTA100K |
ECL inputs + outputs |
72.0 mA |
VRB |
The numbers are the same for an ECL 100K array, whether or not both ECL
10K and ECL 100K outputs are used. Either ECL 10KH or ECL 10K compatibility
is assumed.
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