Freescale Semiconductor MC68881 - Electrical Specifications; M a X I M U M Ratings; Thermal Characteristics - - Pga Package; Power Considerations

SECTION 12
ELECTRICAL SPECIFICATIONS
This section contains electrical specifications and associated timing information for the
MC68881/MC68882 (FPCP).
12.1 MAXIMUM RATINGS
Rating Symbol Value
Supply Voltage VCC -0.3 to +7,0
Input Voltage Vin -0.5 to +7.0
Operating Temperature T A 0 to 70
Storage Temperature Tstfl -55 to + 150
12.2 THERMAL CHARACTERISTICS -- PGA PACKAGE
Characteristic Symbol Value Rating
Thermal Resistance- Ceramic °CA/V
Junction to Ambient 0jA 30*
Junction to Case ejC 15"
*Estimated
Unit This device contains circuitry to pro-
tect the inputs against damage due to
V .... high static voltages or electric fields;
V however, it is advised that normal pre-
cautions be taken to avoid application
°C of any voltage higher than maximum-
°C rated voltages to this high-impedance
circuit. Reliability of operation is en-
hanced if unused inputs are tied to an
appropriate logic voltage level (e.g,,
either GND or VCC).
12.3 POWER CONSIDERATIONS
The average chip-junction temperature, T j, in °C can be obtained from:
Tj =TA+ (PD ° 0JA)
where:
TA
0JA
PD
PINT
PI/O
(1)
= Ambient Temperature, °C
= Package Thermal Resistance, Junction-to-Ambient, °C/W
= PINT+ PI/O
= ICC x VCC, Watts -- Chip Internal Power
= Power Dissipation on Input and Output Pins -- User Determined
For most applications PI/O<PINT and can be neglected.
The following is an approximate relationship between PD and Tj (if PI/o is neglected):
PD = K + (TJ + 273°C) (2)
Solving equations (1) and (2) for K gives:
K=PD" (TA+ 273°C) + 0JA • PD 2 (3)
where K is a constant pertaining to the particular part. K can be determined from equation
(3) by measuring PD (at equilibrium) for a known TA. Using this value of K, the values of
PD and Tj can be obtained by solving equations (1) and (2) iteratively for any value of TA.
The total thermal resistance of a package (eJA) can be separated into two components,
eJC and 0CA, representing the barrier to heat flow from the semiconductor junction to the
package (case) surface (eJC) and from the case to the outside ambient (eCA). These terms
are related by the equation:
0JA = eJC + eCA (4)
MC68881/MC68882 USER'S MANUAL FREESCALE
12-1