CS8251ETVA7 View Datasheet(PDF) - Cherry semiconductor
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CS8251ETVA7 Datasheet PDF : 8 Pages
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Application Notes: continued
Calculating Power Dissipation
in a Dual Output Linear Regulator
The maximum power dissipation for a dual output regu-
lator (Figure 1) is
PD(max) = {VIN(max)–VOUT1(min)}IOUT1(max)+
{VIN(max)–VOUT2(min)}IOUT2(max)+VIN(max)IQ (1)
Where
VIN(max) is the maximum input voltage,
VOUT1(min) is the minimum output voltage from VOUT1,
VOUT2(min) is the minimum output voltage from VOUT2,
IOUT1(max) is the maximum output current, for the appli-
cation
IOUT2(max) is the maximum output current, for the appli-
cation
IQ is the quiescent current the regulator consumes at
IOUT(max).
Once the value of PD(max) is known, the maximum permis-
sible value of RΘJA can be calculated:
RΘJA =
150°C - TA
PD
(2)
The value of RΘJA can then be compared with those in
the package section of the data sheet. Those packages
with RΘJA's less than the calculated value in equation 2
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.
IIN
VIN
Smart
Regulator
}Control
Features
IQ
IOUT1
IOUT2
VOUT1
VOUT2
Figure 1: Dual output regulator with key performance parameters
labeled.
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 RΘJA.
RΘJA = RΘJC + RΘCS + RΘSA
(3)
where
RΘJC = the junction–to–case thermal resistance,
RΘCS = the case–to–heatsink thermal resistance, and
RΘSA = the heatsink–to–ambient thermal resistance.
RΘJC appears in the package section of the data sheet. Like
RΘJA, it too is a function of package type. RΘCS and RΘSA
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.
5
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