參數(shù)資料
型號(hào): MC74LCX2952SD
廠商: ON SEMICONDUCTOR
元件分類: 總線收發(fā)器
英文描述: LVC/LCX/Z SERIES, 8-BIT REGISTERED TRANSCEIVER, TRUE OUTPUT, PDSO24
封裝: PLASTIC, SSOP-24
文件頁數(shù): 32/43頁
文件大?。?/td> 383K
代理商: MC74LCX2952SD
Reliability Information
MOTOROLA
LCX DATA
BR1339 — REV 3
248
Power Dissipation Equation
PD + VCC CPVCC
s
i
+1
FOUTi ) VCC [DICCn]
12
) (VCC * VOH) (VOH * VOL)
s
i
+1
CLiFOUTi )
h
i
+1
VOH
RDi
34
) (VOL) (VOH * VOL)
s
i
+1
CLiFOUTi )
l
i
+1
(VCC * VOL)
RUi
56
The power dissipation equation is made up of five major
factors controlled by the user which contribute to increased
power dissipation:
1 Frequency of operation (output switching frequency)
2 Input voltage levels
3 Output loading (capacitive and resistive)
4VCC level
5 Duty cycle
Each of these five factors are addressed in the estimating
equation except duty cycle. Duty cycle can be addressed by
“weighting” the power dissipation equation terms
appropriately.
The first current term is ICCD, with the device unloaded. It
is caused by the internal switching of the device. Static ICC is
so small for LCX, that when estimating power dissipation, it is
ignored.
CPVCC
s
i
+1
Fouti
This term represents the ICC current with absolutely no
load. This measurement is taken without the output pins
connected to the board. The CP for a device is calculated by:
CP +
ICC(@50MHz) * ICC(@1MHz)
VCC(49MHz)s
“s” is the number of outputs switching. CP may vary slightly
from part to part within a product family.
The next term is from current due to holding the CMOS
inputs at VCC–0.6V rather than at the rail voltages. This term
becomes insignificant as load and frequency increase.
ICCn
ICC is the through current when holding the input High of a
device to VCC–0.6V. This value is typically 300A or less. “n”
is the number of inputs held at this level.
The third term is current through the upper structure of the
device. It is caused by the external capacitive load and the
output frequency. If a capacitive load exists then this term can
become very significant.
(VOH * VOL)
s
i
+1
CLiFOUTi
VOH–VOL is the voltage swing of the output. CL is the output
load (this could vary from output to output). FOUT is the output
frequency which can also vary from output to output.
The fourth term stems from current through the upper
structure due to an external resistive load to ground.
As the output frequency increases, the measured current
approaches that of static High outputs.
h
i
+1
VOH
RDi
RD is an external pull–down resistor. A different value load
could be applied to each output.
The fifth current term is determined by the output
capacitive load and the output frequency on the lower
structure of the device. If this load exists than this term is also
significant.
(VOH * VOL)
s
i
+1
CLiFOUTi
All variables are the same as with the third term with the
exception that this is current flowing through the lower
structure of the IC. This current is not ICC, but rather current
that is “sinked” from an external source.
The final term is due to an external load connected to VCC.
This term includes both switching and static Low outputs.
l
i
+1
(VCC * VOL)
RUi
As with term five, this is current that flows through the lower
structure of the IC. This current too is not ICC.
Example of Thermal Calculations
Junction temperature can be estimated using the following
equation:
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