Logic Design for Array-Based Circuits
Copyright © 1996, 2001, 2002 Donnamaie E. White
Last Edit July 22, 2001
Power dissipation is a measure of the amount of heat that must be handled by the array, its package, and the board on which they are mounted and cooled by whatever means is used. Handling this heat, called thermal management, is a fundamental design requirement. Estimating the heat to be dissipated requires a worst-case maximum power dissipation computation.
The computation of the worst-case maximum power dissipation for a circuit has two major components, DC (quiescent) power and AC (dynamic) power dissipation. Each in turn is composed of several elements. While DC power dissipation can be reasonably bounded by worst-case equations based on P = I*V, AC power dissipation without hardware emulation-assist and complex software is at best a rough guess.
Cell Types Define the Power Computation
Array and cell types can be broken down into four basic categories: CMOS, bipolar, BiCMOS and super-speed bipolar (see Table 7-1):
Table 7-1 Cell Type Vs. Power Component
DC Power in a Bipolar Array
DC power dissipation computation for a bipolar array is composed of interface macro DC power, internal macro DC power, overhead current DC power and ECL static output power dissipation. DC power is a function of the current dissipated, adjusted for state dependencies, junction temperature, voltage levels, ECL termination values, and any conditional geo-metry power down of IOEF. Typical DC power can be computed to a given level of detail from the netlist of the circuit using:
where I is the adjusted typical macro current from all current sources (IOEF and internal current) and V is the power supply. A worst-case multiplier can be used to convert typical DC power into worst-case power for a specific set of conditions. Some components of DC power are shown in Table 7-2.
Table 7-2 DC Power Computation - Bipolar
The array vendor may specify a typical macro current (summing internal and IOEF values) and overhead current, or a typical macro PDC term, where a power supply has been assumed and conversion factors exist to adjust for differences. In this case, additional information as to ECL I/O bias power usage will be supplied, as well as some means of handling ECL
DC power due to termination current. Regardless of the variations in "specsmenship", the methods are algebraically the same, as they must be.
DC Power in a BiCMOS Array
The DC power component will be the significant factor in computing power for the interface macros in a BiCMOS array. The power computation is the same as was applied to the bipolar array, except that the computation is made for the interface macros and not all macros in the circuit. Both the power due to the current sources in the interface macros and the power due to overhead current must be computed.
Overhead current (ICC and IEE) is that current dissipated by the voltage threshold generators and bias circuitry. It will be dissipated even if no macros are placed on any of the cells, i.e., it exists as soon as the chip is powered on.
For some arrays, overhead current and therefore overhead power may be specified as variable. It may depend on whether a specific type of I/O macro was used anywhere on the circuit, or on the number of such macros and their placement.
Placement-dependent computations cannot occur until after place and route. Pre-placement computations must assume the worst possible conditions.
Overhead power is the overhead current times the power supply, computed as for the macro power dissipation.
DC Power in a CMOS Array
DC power in a CMOS array is due to a static DC current (transistors are ON) and a leakage DC current (transistors are OFF). There may also be an overlap current, a result of both paired transistors being ON as they change state.
The total value for leakage current will be low and needs to be measured when the circuit can be configured with all devices off. Static DC current should be approximately zero, or low enough not to consider. Power dissipation due to overlap current is no more than 10% of the total power dissipation. These figures are guidelines only. The designer should review the design manual for the specific array to be used to determine the power characteristics for that array.
TTL Outputs - CMOS Arrays
Power dissipation due to TTL outputs on a CMOS array needs to be included in the total power computation. It is a function of the duty cycle of the outputs and the sink current.
where n is the number of TTL loads, V is the output low voltage and I is the sink current for the macro.
The specification may show a Pdc value instead of current for the individual macro.
ECL Static Output Power - All Arrays
ECL output macros on bipolar or BiCMOS arrays dissipate a static power based on the termination resistor value (off-chip termination). The com-mon estimate for this current is:
where I is the termination current value (14 mA for a 50 ohm termination) and V = 1.3V, the voltage swing for -5.2V termination.