CA3282 Intersil Corporation, CA3282 Datasheet - Page 7

• Manufacturer Part Number: CA3282
• Description: Octal Low Side Power Driver with Serial Bus Control
• Manufacturer: Intersil Corporation

For the CA3282, the maximum positive output current rating
is 1A when one output is ON. When ALL outputs are ON, the
rating is reduced to 0.625A because the total maximum cur-
rent is limited to 5A. For any given application, all output
drivers on a chip may or may not have a different level of
loading. The discussion here is intended to provide relatively
simple methods to determine the maximum dissipation and
current ratings as a general solution and, as a special solu-
tion, when all switched ON outputs have the same current
loading.
General Solution
A general equation for dissipation should specify that the
total power dissipation in a package is the sum of all signifi-
cant elements of dissipation on the chip. However, in Power
BiMOS Circuits very little dissipation is needed to control the
logic and predriver circuits on the chip. The over-all chip dis-
sipation is primarily the sum of the I
each channel where the current, I is the output current and
the resistance, R is the NMOS channel resistance, r
of each output driver. As such, the total dissipation, P
output drivers is:
This expression sums the dissipation, P
driver without regard to uniformity of dissipation in each
MOS channel. The dissipation loss in an NMOS channel is:
where the current, I is determined by the output load when
the channel is turned ON. The channel resistance, r
is a function of the circuit design, level of gate voltage and
the chip temperature. Refer to the Electrical Specifications
values for worse case channel resistance.
The temperature rise in the package due to the dissipation is
the product of the on-chip dissipation, P
Junction-to-Case thermal resistance,
junction temperature, T
temperature, T
or
Since this solution relates only to the package, further
consideration must be given to a practical heat sink. The
equation of linear heat flow assumes that the Junction-to-
Ambient thermal resistance,
resistance from Junction-to-Case and the thermal resistance
from Case (heat sink)-to-Ambient,
Ambient thermal resistance,
paths from the chip junction to the ambient temperature (T
environment and can be expressed as:
P k
T J
T C
P D
J
A
=
=
=
=
=
T C
I
T J P D
2
k
∑
JC
n
=
+
–
r DS ON
1
P D
+
P k
CA
C
, the linear heat flow solution is:
JC
JC
J
, given the case (heat sink tab)
JA
JA
, is the sum of the thermal
is the sum of all thermal
2
R dissipation losses in
CA
JC
D
. The Junction-to-
. To determine the
K
and the package
of each output
(EQ. 3A)
DS(ON)
DS(ON)
D
(EQ. 1)
(EQ. 2)
(EQ. 3)
(EQ. 4)
for n
CA3282
A
)
7
Equation 3 and Equation 3A may be expressed as:
Not all Integrated Circuit packages have a directly definable
case temperature because the heat is spread thru the lead
frame to a PC Board which is the effective heat sink.
Calculation Example 1
For the CA3282,
temperature, as an application design solution, should not
exceed 150
mines the dissipation, P
Assume the package is mounted to a heat sink having a
thermal resistance of 6
dissipation, P
perature, T
thermal resistance is:
The solution for junction temperature by Equation 5 is :
T
Calculation Example 2
Using the CA3282 maximum Junction-to-Ambient Thermal
Resistance,
the worst case Junction Temperature, T
an application design solution for the maximum ambient
temperature or dissipation. For example; Using Equation 1
and assuming a device dissipation, P
allowable Ambient Temperature, T
calculated as follows:
T
Equal Current Loading Solution
Where a given application has equal current loading in the
output drivers, equal r
may be assumed. As such, a convenient method to show
rating boundaries is to substitute the dissipation Equation 2
into the junction temperature Equation 3. For m outputs that
are ON with equal currents, where I = I
have the following solution for dissipation:
T
T
or
P D
I
JA
A
J
J
A
=
= 100
= 150
=
=
=
=
T
---------------------------------------------------- -
m
T
JC
m
A
J
o
o
C + 3W x 9
C - 1.0W x 45
+
–
+
P
P
P k
JC
A
T J T C
o
D
CA
D
C. For any given application, Equation 1 deter-
= 100
JA
D
=
–
= 9
r
= 3W. Assume the operating ambient tem-
m
value of 45
DS ON
JA
JA
o
o
C/W
JC
o
C/W = 127
I
2
C. The calculated Junction-to-Ambient
o
= 3
C/W = 105
o
DS(ON)
D
C/W and, for a given application, the
r DS ON
.
o
C/W and the worst case junction
o
C/W (no external heat sink) and
o
C
o
and temperature conditions
C
A
D
from Equation 5A is
of 1W, the maximum
C
1
of 150
= I
2
..... = I
o
C we have
(EQ. 5A)
(EQ. 5)
(EQ. 6)
(EQ. 7)
m
, we