Application Notes: continued
5V to mP and
System
Power
VIN
VOUT
C1*
0.1mF
C **
2
10mF
CS-8120
R
RST
to mP
RESET
Port
RESET
ENABLE
C
RST
*C1 is required if regulator is far from power source filter.
**C2 is required for stability.
Figure 6. Circuit showing output compensation capacitor.
will keep the die temperature below 150¡C.
Calculating Power Dissipation
in a Single Output Linear Regulator
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.
The maximum power dissipation for a single output regu-
lator (Figure 7) is:
Heat Sinks
PD(max)= VIN(max) Ð VOUT(min)}
I
OUT(max) + VIN(max) Q
I
(1)
{
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.
where:
VIN(max) is the maximum input voltage,
VOUT(min) is the minimum output voltage,
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
IOUT(max) is the maximum output current for the applica-
tion, and
IQ is the quiescent current the regulator consumes at
to determine the value of RQJA
:
IOUT(max)
.
R
QJA = RQJC + RQCS + RQSA
(3)
where:
Once the value of PD(max) is known, the maximum permis-
sible value of RQJA can be calculated:
R
R
R
QJC = the junctionÐtoÐcase thermal resistance,
QCS = the caseÐtoÐheatsink thermal resistance, and
QSA = the heatsinkÐtoÐambient thermal resistance.
150¡C - TA
(2)
RQJA
=
PD
The value of RQJA can then be compared with those in
the package section of the data sheet. Those packages
with RQJA's less than the calculated value in equation 2
R
QJC appears in the package section of the data sheet. Like
QJA, it too is a function of package type. RQCS and RQSA
R
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.
I
IN
I
OUT
V
IN
Smart
Regulator
V
OUT
Control
Features
}
I
Q
Figure 7. Single output regulator with key performance parameters
labeled.
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
