NCP1086
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8
The case is connected to V
OUT
, and electrical isolation
may be required for some applications. Thermal compound
should always be used with high current regulators such as
these.
The thermal characteristics of an IC depend on the
following four factors:
1. Maximum Ambient Temperature T
A
(°C)
2. Power dissipation P
D
(W)
3. Maximum junction temperature T
J
(°C)
4. Thermal resistance junction to ambient R
q
JA
(°C/W)
These four are related by the equation
T
J
+ T
A
) P
D
R
qJA
(eq. 1)
The maximum ambient temperature and the power
dissipation are determined by the design while the
maximum junction temperature and the thermal resistance
depend on the manufacturer and the package type.
The maximum power dissipation for a regulator is:
P
D(max)
+
{
V
IN(max)
* V
OUT(min)
}
I
OUT(max)
) V
IN(max)
I
Q
(eq. 2)
where:
V
IN(max)
is the maximum input voltage,
V
OUT(min)
is the minimum output voltage,
I
OUT(max)
is the maximum output current, for the application
I
Q
is the maximum quiescent current at I
OUT(max)
.
A heatsink effectively increases the surface area of the
package to improve the flow of heat away from the IC and
into the surrounding air.
Each material in the heat flow path between the IC and the
outside environment has a thermal resistance. Like series
electrical resistances, these resistances are summed to
determine R
q
JA
, the total thermal resistance between the
junction and the surrounding air.
1. Thermal Resistance of the junction to case, R
q
JC
(°C/W)
2. Thermal Resistance of the case to Heatsink, R
q
CS
(°C/W)
3. Thermal Resistance of the Heatsink to the ambient
air, R
q
SA
(°C/W)
These are connected by the equation:
R
qJA
+ R
qJC
) R
qCS
) R
qSA
(eq. 3)
The value for R
q
JA
is calculated using Equation 3 and the
result can be substituted in Equation 1.
The value for R
q
JC
is 3.5°C/W. For a high current
regulator such as the NCP1086 the majority of the heat is
generated in the power transistor section. The value for
R
q
SA
depends on the heatsink type, while R
q
CS
depends on
factors such as package type, heatsink interface (is an
insulator and thermal grease used?), and the contact area
between the heatsink and the package. Once these
calculations are complete, the maximum permissible value
of R
q
JA
can be calculated and the proper heatsink selected.
For further discussion on heatsink selection, see application
note “Thermal Management,” document number
AND8036/D via our website at www.onsemi.com.
