CS8120
Application Notes: continued
5V to
mP
and
System
Power
R
RST
RESET
ENABLE
C
RST
to
mP
RESET
Port
C
2
**
10mF
V
IN
C
1
*
0.1mF
V
OUT
CS-8120
*C
1
is required if regulator is far from power source filter.
**C
2
is required for stability.
Figure 6. Circuit showing output compensation capacitor.
Calculating Power Dissipation
in a Single Output Linear Regulator
The maximum power dissipation for a single output regu-
lator (Figure 7) is:
P
D(max)
={V
IN(max)
Ð V
OUT(min)
}I
OUT(max)
+ V
IN(max)
I
Q
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 applica-
tion, and
I
Q
is the quiescent current the regulator consumes at
I
OUT(max)
.
Once the value of P
D(max)
is known, the maximum permis-
sible value of R
QJA
can be calculated:
150¡C - T
A
R
QJA
=
P
D
The value of R
QJA
can then be compared with those in
the package section of the data sheet. Those packages
with R
QJA
's less than the calculated value in equation 2
(2)
(1)
will keep the die temperature below 150¡C.
In some cases, none of the packages will be sufficient to
dissipate the heat generated by the IC, and an external
heatsink will be required.
Heat Sinks
A heat sink 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 will have a thermal resistance. Like
series electrical resistances, these resistances are summed
to determine the value of R
QJA
:
R
QJA
= R
QJC
+ R
QCS
+ R
QSA
where:
R
QJC
= the junctionÐtoÐcase thermal resistance,
R
QCS
= the caseÐtoÐheatsink thermal resistance, and
R
QSA
= the heatsinkÐtoÐambient thermal resistance.
R
QJC
appears in the package section of the data sheet. Like
R
QJA
, it too is a function of package type. R
QCS
and R
QSA
are functions of the package type, heatsink and the inter-
face between them. These values appear in heat sink data
sheets of heat sink manufacturers.
(3)
I
IN
V
IN
Smart
Regulator
I
OUT
V
OUT
}
Control
Features
I
Q
Figure 7. Single output regulator with key performance parameters
labeled.
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
