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Design Considerations
•
To minimize temperature variation across the surface, the thermal resistance is measured from the junction
to the outside surface of the package (case) closest to the chip mounting area when that surface has a
proper heat sink.
•
To define a value approximately equal to a junction-to-board thermal resistance, the thermal resistance is
measured from the junction to the point at which the leads attach to the case.
•
If the temperature of the package case (T
computed from the value obtained by the equation (T
As noted earlier, the junction-to-case thermal resistances quoted in this data sheet are determined using the first
definition. From a practical standpoint, that value is also suitable to determine the junction temperature from a case
thermocouple reading in forced convection environments. In natural convection, the use of the junction-to-case
thermal resistance to estimate junction temperature from a thermocouple reading on the case of the package will
yield an estimate of a junction temperature slightly higher than actual temperature. Hence, the new thermal metric,
thermal characterization parameter or Ψ
of the junction temperature in natural convection when the surface temperature of the package is used. Remember
that surface temperature readings of packages are subject to significant errors caused by inadequate attachment of
the sensor to the surface and to errors caused by heat loss to the sensor. The recommended technique is to attach a
40-gauge thermocouple wire and bead to the top center of the package with thermally conductive epoxy.
4.2 Electrical Design Considerations
Use the following list of recommendations to ensure correct DSP operation.
•
Provide a low-impedance path from the board power supply to each
board ground to each
Use at least four 0.01–0.1 µF bypass capacitors for the core and PLL power and six 0.01–0.1 µF bypass
•
capacitors for I/O power positioned as closely as possible to the four sides of the package to connect the
power source to
V
CC
•
Ensure that capacitor leads and associated printed circuit traces that connect to the chip
are less than 0.5 inch per capacitor lead.
•
Use at least a four-layer PCB with two inner layers for
4-2
) is determined by a thermocouple, thermal resistance is
T
, has been defined to be (T
JT
CAUTION
This device contains protective circuitry to
guard against damage due to high static
voltage or electrical fields. However, normal
precautions are advised to avoid application
of any voltages higher than maximum rated
voltages to
this high-impedance circuit.
Reliability of operation is enhanced if unused
inputs are tied to an appropriate logic voltage
level (for example, either GND or V
pin.
GND
.
GND
DSP56311 Technical Data, Rev. 8
– T
)/P
.
J
T
D
– T
)/P
. This value gives a better estimate
J
T
D
).
CC
pin on the DSP and from the
V
CC
and
.
V
GND
CC
and
pins
V
GND
CC
Freescale Semiconductor
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