The power dissipation of a general-purpose logic IC can be calculated by obtaining the static current and dynamic current, and then multiplying that current by the voltage applied to the IC.
Power dissipation can be obtained by multiplying the above current by the voltage applied to an IC.
Static power dissipation: PS
While general-purpose logic ICs are in a static state (i.e., while its input signal remains unchanged), little current flows in it except tiny leakage current that flows across the internal reverse-biased pn junction (known as static supply current, ICC). Static power dissipation is ICC multiplied by the supply voltage.
PS = VCC x ICC
VCC: Voltage applied to a logic IC
ICC: Static supply current shown in the datasheet
Dynamic power dissipation
Dynamic supply current is the current that flows in general-purpose logic while its input transitions between High and Low. This current flows during the charging and discharging of capacitance. It is necessary to consider both parasitic capacitance (internal equivalent capacitance) and load capacitance. Dynamic power dissipation is obtained by multiplying this current by the voltage applied to the P-channel or N-channel MOSFET. Here, for the sake of simplicity, the VCC value at which the maximum current flows is used for power calculation.
Dynamic power dissipation due to load capacitance (CL): PL
PL is dissipated when an external load is charged and discharged as shown by the Fig. 1.
The amount of charge (Q) stored on the load capacitance is calculated as follows:
QL = CL x VCC
CL: Load capacitance
Let the output signal frequency be fOUT (= 1/TOUT). Then, the average current (IL) is expressed as follows:
IL = QL / T = CL * VCC * fOUT
Hence, dynamic power dissipation (PL) is:
PL = VCC * IL = CL * VCC^2 * fOUT
If an IC has multiple outputs, its dynamic power dissipation can be calculated as follows:
PL = VCC^2 * Σ (CLn* fOUTn)
Dynamically Consumed Power with Equivalent Internal Capacitive CPD: PPD
General-purpose logic ICs have various parasitic capacitances as shown by Fig. 2. These capacitances are equivalently expressed as CPD. (Actually, CPD is calculated from power dissipation at relatively high frequency (1 MHz) under a zero-load condition.) PPD is the power dissipated by the equivalent capacitance of an IC and can be considered in the same manner as PL. Note, however, that PPD is calculated at input frequency (fIN):
PPD = VCC * IL = CPD * VCC^2 * fIN
总功耗:PTTL
可通过静态功耗(PS)和动态功耗(PL+PPD)的总和,得出总功耗(PTTL):
PTTL=PS+PL+PPD
The following documents also contain related information.
以下文档也包含相关信息:
应用说明
Glossary of CMOS Logic Ics
CMOS Logic ICs Selection Guide
Basics of CMOS Logic ICs
CMOS Logic ICs Usage Considerations
CMOS Logic Ics Standard series
CMOS Logic Ics TC74HC/HCT Series
CMOS Logic Ics TC74AC/ACT Series
CMOS Logic Ics TC74VHC/VHCT/VHC9/VHCV Series
CMOS Logic Ics TC74VCX/LCX Series
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