DSP56004ROM datasheet(78/82 Pages) FREESCALE

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4-4

DSP56004/D, Rev. 3

MOTOROLA

Design Considerations

Power Consumption Considerations

POWER CONSUMPTION CONSIDERATIONS

Power dissipation is a key issue in portable DSP applications. Some of the factors

which affect current consumption are described in this section. Most of the current

consumed by CMOS devices is Alternating Current (AC), which is charging and

discharging the capacitances of the pins and internal nodes.

Current consumption is described by the formula:

Equation 3:

where:

C = node/pin capacitance

V = voltage swing

f = frequency of node/pin toggle

The Maximum Internal Current (ICCImax) value reflects the typical possible

switching of the internal buses on best-case operation conditions, which is not

necessarily a real application case. The Typical Internal Current (ICCItyp) value

reflects the average switching of the internal buses on typical operating conditions.

For applications that require very low current consumption:

•

Minimize the number of pins that are switching.

•

Minimize the capacitive load on the pins.

•

Connect the unused inputs to pull-up or pull-down resistors.

•

Disable unused peripherals.

•

Disable unused pin activity.

Example 4-1

Current Consumption

For an I/O pin loaded with 50 pF capacitance, operating at 5.5 V, and with a 81 MHz clock, toggling

at its maximum possible rate (20 MHz), the current consumption is:

Equation 4:

CVf

I50

–

5.5

5.5mA

Freescale Semiconductor, Inc.

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