The thermal characteristics of an IC depend on four
factors:
Output Voltage Sensing
1. Maximum Ambient Temperature T (°C)
A
GM6603 series is three terminal regulator, so they can-
not provide true remote load sensing. Load regulation
is limited by the resistance of the conductors connect-
ing the regulator to the load. For best results, GM6603
should be connected as shown in Figure 2.
2. Power Dissipation P (Watts)
D
3. Maximum Junction Temperature T (°C)
J
4. Thermal Resistance Junction to ambient R
JA
Q
(°C/W)
These relationship of these four factors is expressed
by equation (1):
Conductor
Parasitic
R
V
V
V
C
OUT
T = T + P X R
J A D
IN
IN
Resistance
Q
JA
GM6603-3.3
Maximum ambient temperature and power dissipa-
tion are determined by the design while the maxi-
mum junction temperature and thermal resistance
depend on the manufacturer and the package type.
R
GND
LOAD
(a) Fixed Version
The maximum power dissipation for a regulator is ex-
pressed by equation (2):
Conductor
Parasitic
P
D(max)
={V -V } I
IN(max) OUT(min) OUT(max)
+ V I
IN(max) Q
V
R
V
Resistance
V
IN
C
OUT
IN
GM6603-A
where:
R1
A
DJ
R
V
is the maximum input voltage,
LOAD
IN(max)
V
is the minimum output voltage,
OUT(min)
R2
I
is the maximum output current
OUT(max)
(b) Adjustable Version
I is the maximum quiescent current at I
Q
.
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 ther-
mal resistance. Like series electrical resistances,
(a),(b)
FIGURE 2
Conductor Parasitic Resistance Effects are
Minimized by this Grounding Scheme For Fixed
and Adjustable Output Regulators
Calculating Power Dissipation and Heat
Sink Requirements
these resistances are summed to determine R
,
QJA
the total thermal resistance between the junction
and the air. This is expressed by equation (3):
GM6603 series precision linear regulators include ther-
mal shutdown and current limit circuitry to protect the
devices. However, high power regulators normally oper-
ate at high junction temperatures so it is important to
calculate the power dissipation and junction tempera-
tures accurately to be sure that you use and adequate
R
= R + R + R
QJC QCS
QJA
QSA
Where all of the following are in °C/W:
R
R
R
is thermal resistance of junction to case,
is thermal resistance of case to heat sink,
is thermal resistance of heat sink to ambient
QJC
QCS
QSA
heat sink. The case is connected to V
on GM6603
OUT
air
so electrical isolation may be required for some appli-
cations. Thermal compound should always be used
with high current regulators like the GM6603.
The value for R
is calculated using equation (3)
QJA
and the result can be substituted in equation (1).
The value for R is 3.5°C/W for a given package
QJC
type based on an average die size. For a high cur-
rent regulator such as GM6603, the majority of the
heat is generated in the power transistor section.
7
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