6
GM6605
Output Voltage Sensing
The GM6605 series are three terminal regulators, so
they cannot provide true remote load sensing. Load
regulation is limited by the resistance of the conductors
connecting the regulator to the load. For best results
the GM6605 should be connected as shown in Figure
2.
Calculating Power Dissipation and Heat
Sink Requirements
The GM6605 series precision linear regulators include
thermal shutdown and current limit circuitry to protect
the devices. However, high power regulators normally
operate at high junction temperatures so it is important
to calculate the power dissipation and junction temper-
atures accurately to be sure that you use and adequate
heat sink. The case is connected to V
on the
OUT
GM6605, so electrical isolation may be required for
some applications. Thermal compound should always
be used with high current regulators like the GM6605.
GM6605-3.3
V
IN
V
OUT
R
LOAD
R
C
GND
V
IN
Conductor
Parasitic
Resistance
(a) Fixed Version
(b) Adjustable Version
GM6605-A
ADJ
V
OUT
V
IN
V
IN
R
C
R
LOAD
R1
R2
(a),(b)
Figure 2
Conductor Parasitic Resistance Effects are
Minimized by this Grounding Scheme For Fixed
and Adjustable Output Regulators
The thermal characteristics of an IC depend four fac-
tors:
1. Maximum Ambient Temperature T (°C)
A
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):
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):
where:
V
is the maximum input voltage,
IN(max)
V
is the minimum output voltage,
OUT(min)
I
is the maximum output current
OUT(max)
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 thermal re-
sistance. Like series electrical resistances, these re-
sistance are summed to determine R
, the total
Q
JA
thermal resistance between the junction and the air.
This is expressed by equation (3):
Where all of the following are in °C/W:
R
is thermal resistance of junction to case,
Q
JC
R
is thermal resistance of case to heat sink,
Q
CS
R
is thermal resistance of heat sink to ambient air
Q
SA
The value for R
is calculated using equation (3)
Q
JA
and the result can be substituted in equation (1). The
value for R
is 3.5°C/W for a given package type
Q
JC
based on an average die size. For a high current reg-
ulator such as the GM6605 the majority of the heat is
generated in the power transistor section.
T = T + P X R
........
JAD
JA
(1)
Q
P
= { V
- V
} I
+ V
I
)
D(max)
IN(max)
OUT(min)
OUT(max
IN(max) Q ........(2)
R
= R
+ R
+ R
JA
JC
CS
SA ........(3)
QQQQ
Conductor
Parasitic
Resistance