# How can the power dissipation of a general-purpose logic IC be calculated?

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

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

Total Power Dissipation: PTTL

Total power dissipation (PTTL) can be obtained as the sum of static power dissipation (PS) and dynamic power dissipation (PL + PPD):

PTTL=P+ P+ PPD