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LT1123
1123fb
The following assumptions were made in calculating the
data for the curves. Resistors are 5% tolerance and the
values shown on the curve are nominal.
For 20mA drive current assume:
V
BE
 = 0.95V at I
C
 = 1A
V
DRIVE
 = 1.75V
For 50mA drive current assume:
V
BE
 = 1.2V at I
C
 = 2A
V
DRIVE
 = 1.9V
For 120mA drive current assume:
V
BE
 = 1.4V at I
C
 = 4A
V
DRIVE
 = 2.1V
The R
D
 Selection Chart lists the recommended values for
R
D
 for the most useful range of input voltage and output
current. The chart includes a number for power dissipa-
tion for the LT1123 and R
D
.
Current Limit
For regulator circuits using the LT1123, current limiting is
achieved by limiting the base drive to the external PNP
pass transistor. This means that the actual system current
limit will be a function of both the current limit of the
LT1123 and the Beta of the external PNP. Beta-based
current limit schemes are normally not practical because
of uncertainties in the Beta of the pass transistor. Here the
drive characteristics of the LT1123 combined with the
Beta characteristics of the MJE1123 can provide reliable
Beta-based current limiting. This is shown in Figure 5
where the current limit of 30 randomly selected transis-
tors is plotted. The spread of current limit is reasonably
well controlled.
5.5V
R
D
120&
43&

Power (LT1123)
0.05W
0.14W

Power (R
D
)
0.12W
0.32W

6.0V
R
D
150&
51&
20&
Power (LT1123)
0.05W
0.15W
0.37W
Power (R
D
)
0.13W
0.35W
0.76W
7.0V
R
D
180&
75&
27&
Power (LT1123)
0.06W
0.14W
0.38W
Power (R
D
)
0.16W
0.36W
0.89W
8.0V
R
D
240&
91&
36&
Power (LT1123)
0.06W
0.15W
0.38W
Power (R
D
)
0.17W
0.42W
0.97W
9.0V
R
D
270&
110&
43&
Power (LT1123)
0.20W
0.16W
0.41W
Power (R
D
)
0.07W
0.47W
1.11W
10.0V
R
D
330&
130&
51&
Power (LT1123)
0.22W
0.17W
0.43W
Power (R
D
)
0.07W
0.52W
1.25W
INPUT
VOLTAGE
0A to 4A
0.75V
OUTPUT CURRENT:
DROPOUT VOLTAGE:
0A to 1A
0.3V
0A to 2A
0.4V
R
D
 Selection Chart
Note that in some conditions R
D
 may be replaced with a
short. This is possible in circuits where an overload is
unlikely and the input voltage and drive requirements are
low. See the section on Thermal Considerations for more
information.
Figure 6. MJE1123 I
C
 vs I
B
Figure 5. Short-Circuit Current for 30 Random Devices
OUTPUT CURRENT (A)
4.00
3
4
5
4.75
5.25
6.00
LT1123 F05
2
1
0
4.25 4.50
5.00
5.50 5.75
6
7
8
9
10
11
12
13
14
15
I
B
 (A)
0
0
1
3
4
5
0.10
9
2
0.05
0.15
6
7
8
APPLICATIO S I FOR ATIO
U
U
U
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