Logic Design for Array-Based Circuits

by Donnamaie E. White

Copyright © 1996, 2001, 2002 Donnamaie E. White

 

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.

 

Copyright @ 2001, 2002 Donnamaie E. White, White Enterprises
For problems or questions on these pages, contact dew@Donnamaie.com

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