Design Considerations
SDA
tSU; DAT
tf
tr
tBUF
tf
tr
tHD; STA
tSP
tLOW
SCL
tSU; STA
tHD; STA
tSU; STO
S
SR
P
S
tHD; DAT
tHIGH
Figure 22. Timing Definition for Fast and Standard Mode Devices on the I2C Bus
9 Design Considerations
9.1 Thermal Design Considerations
An estimation of the chip junction temperature, TJ, can be obtained from the equation:
TJ = TA + (RΘJA x PD)
Where,
TA = Ambient temperature for the package (°C)
RΘJA = Junction-to-ambient thermal resistance (°C/W)
PD = Power dissipation in the package (W)
The junction-to-ambient thermal resistance is an industry-standard value that provides a
quick and easy estimation of thermal performance. Unfortunately, there are two values in
common usage: the value determined on a single-layer board and the value obtained on a
board with two planes. For packages such as the PBGA, these values can be different by
a factor of two. Which value is closer to the application depends on the power dissipated
by other components on the board. The value obtained on a single layer board is
appropriate for the tightly packed printed circuit board. The value obtained on the board
with the internal planes is usually appropriate if the board has low-power dissipation and
the components are well separated.
When a heat sink is used, the thermal resistance is expressed as the sum of a junction-to-
case thermal resistance and a case-to-ambient thermal resistance:
RΘJA = RΘJC + RΘCA
Where,
RΘJA = Package junction-to-ambient thermal resistance (°C/W)
MC56F8458x Advance Information Data Sheet, Rev. 2, 06/2012.
58
Freescale Semiconductor, Inc.
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