MICROELECTRONICS
A
A
Power Management
GM1117
1.0A LOW DROPOUT PRECISION LINEAR REGULATORS
The thermal characteristics of an IC depend four fac-
tors:
1. Maximum Ambient Temperature T
A
(°C)
2. Power Dissipation P
D
(Watts)
3. Maximum Junction Temperature T
J
(°C)
4. Thermal Resistance Junction to ambient R
Q
JA
(°C/W)
The relationship of these four factors is expressed by
equation (1):
T
J
= T
A
+ P
D
X R
Q
JA
........(1)
Maximum ambient temperature and power dissipa-
tion are determined by the design while the maximum
junction temperature and thermal resistance depend
on the manufacturer and the package type.
The maximum power dissipation for a regulator is ex-
pressed by equation (2):
P
D(max)
= { V
IN(max)
- V
OUT(min)
} I
OUT(max
)
+ V
IN(max)
I
Q
........(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
I
Q
is the maximum quiescent current at I
OUT(max)
.
A heat sink effectively increases the surface area of
the package to improve the flow of heat away from
the IC into the air. Each material in the heat flow path
between the IC and the environment has a thermal re-
sistance. Like series electrical resistances, these re-
sistance are summed to determine R
QJA
,
the total
thermal resistance between the junction and the air.
This is expressed by equation (3):
R
Q
JA
= R
Q
JC
+ R
Q
CS
+ R
Q
SA
........(3)
Where all of the following are in °C/W:
R
QJC
is thermal resistance of junction to case,
R
QCS
is thermal resistance of case to heat sink,
R
QSA
is thermal resistance of heat sink to ambient air
GM1117
8
The value for R
QJA
is calculated using equation (3)
and the result can be substituted in equation (1). The
value for R
QJC
is 3.5°C/W for a given package type
based on an average die size. For a high current reg-
ulator such as the GM1117 the majority of the heat is
generated in the power transistor section.
GAMMA [ GAMMA MICROELECTRONICS INC. ]
