8
LT1123
1123fb
The curve in Figure 6 can be used to determine the range
of current limit of an LT1123 regulator circuit using an
MJE1123 as a pass transistor. The curve was generated
using the Beta versus I
C
curve of the MJE1123. The
minimum and maximum value curves are extrapolated
from the minimum and maximum Beta specifications.
Thermal Conditions
The thermal characteristics of three components need to
be considered; the LT1123, the pass transistor and R
D
.
Power dissipation should be calculated based on the
worst-case conditions seen by each component during
normal operation.
The worst-case power dissipation in the LT1123 is a
function of drive current, supply voltage and the value of
R
D
. Worst-case dissipation for the LT1123 occurs when
the drive current is equal to approximately one half of its
maximum value. Figure 7 plots the worst-case power
dissipation in the LT1123 versus R
D
and V
IN
. The graph
was generated using the following formula:
P
V–V
4R
;R 10
D
IN BE
2
D
D
=
()
> Ω
where:
V
BE
= the emitter/base voltage of the PNP pass
transistor (assumed to be 0.6V)
For some operating conditions R
D
may be replaced with a
short. This is possible in applications where the operating
requirements (input voltage and drive current) are at the
low end and the output will not be shorted. For R
D
= 0 the
following formula may be used to calculate the maximum
power dissipation in the LT1123.
P
D
= (V
IN
– V
BE
)(I
DRIVE
)
where:
V
IN
= maximum input voltage
V
BE
= emitter/base voltage of PNP
I
DRIVE
= required maximum drive current
The maximum junction temperature rise above ambient
for the LT1123 will be equal to the worst-case power
dissipation multiplied by the thermal resistance of the
device. The thermal resistance of the device will depend
upon how the device is mounted, and whether a heat sink
is used. Measurements show that one of the most effective
ways of heat sinking the TO-92 package is by utilizing the
PC board traces attached to the leads of the package. The
table below lists several methods of mounting and the
measured value of thermal resistance for each method. All
measurements were done in still air.
Package alone ............................................................................. 220°C/W
Package soldered into PC board with plated through
holes only ................................................................................ 175°C/W
Package soldered into PC board with 1/4 sq. in. of copper trace
per lead .................................................................................... 145°C/W
Package soldered into PC board with plated through holes in
board, no extra copper trace, and a clip-on type heat sink:
Thermalloy type 2224B .................................................... 160°C/W
Aavid type 5754 ................................................................ 135°C/W
The maximum operating junction temperature of the
LT1123 is 125°C. The maximum operating ambient tem-
perature will be equal to 125°C minus the maximum
junction temperature rise above ambient.
The worst-case power dissipation in R
D
needs to be
calculated so that the power rating of the resistor can be
determined. The worst-case power in the resistor will
occur when the drive current is at a maximum. Figure 8
plots the required power rating of R
D
versus supplyFigure 7. Power in LT1123
V
IN
(V)
6
R
D
(Ω)
8
1k
LT1123 F07
10
100
715
14
131211
10
9
5
0.4W
0.7W
0.1W
0.2W
0.3W
0.5W
THERMAL
RESISTANCE
APPLICATIO S I FOR ATIO
WUUU
