Design Considerations
•
If the temperature of the package case (T ) is determined by a thermocouple, thermal resistance is
T
computed from the value obtained by the equation (T – T )/P .
J
T
D
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 , has been defined to be (T – T )/P . This value gives a better estimate
JT
J
T
D
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
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 ).
CC
Use the following list of recommendations to ensure correct DSP operation.
•
•
•
Provide a low-impedance path from the board power supply to each VCC pin on the DSP and from the
board ground to each GND pin.
Use at least six 0.01–0.1 μF bypass capacitors positioned as close as possible to the four sides of the
package to connect the VCC power source to GND.
Ensure that capacitor leads and associated printed circuit traces that connect to the chip VCC and GND pins
are less than 0.5 inch per capacitor lead.
•
•
Use at least a four-layer PCB with two inner layers for VCC and GND.
Because the DSP output signals have fast rise and fall times, PCB trace lengths should be minimal. This
recommendation particularly applies to the address and data buses as well as the IRQA, IRQB, IRQC, IRQD,
TA, and BG pins. Maximum PCB trace lengths on the order of 6 inches are recommended.
Consider all device loads as well as parasitic capacitance due to PCB traces when you calculate
capacitance. This is especially critical in systems with higher capacitive loads that could create higher
transient currents in the VCC and GND circuits.
•
•
All inputs must be terminated (that is, not allowed to float) by CMOS levels except for the three pins with
internal pull-up resistors (TRST, TMS, DE).
•
•
Take special care to minimize noise levels on the VCCP, GNDP, and GNDP1 pins.
The following pins must be asserted after power-up: RESET and TRST.
DSP56301 Technical Data, Rev. 10
4-2
Freescale Semiconductor
FREESCALE [ Freescale ]
