LT1123CST Просмотр технического описания (PDF) - Linear Technology
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LT1123CST Datasheet PDF : 16 Pages
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LT1123
APPLICATIO S I FOR ATIO
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:
VBE = 0.95V at IC = 1A
VDRIVE = 1.75V
For 50mA drive current assume:
VBE = 1.2V at IC = 2A
VDRIVE = 1.9V
For 120mA drive current assume:
VBE = 1.4V at IC = 4A
VDRIVE = 2.1V
The RD Selection Chart lists the recommended values for
RD for the most useful range of input voltage and output
current. The chart includes a number for power dissipa-
tion for the LT1123 and RD.
RD Selection Chart
INPUT OUTPUT CURRENT:
VOLTAGE DROPOUT VOLTAGE:
5.5V
6.0V
7.0V
8.0V
9.0V
10.0V
RD
Power (LT1123)
Power (RD)
RD
Power (LT1123)
Power (RD)
RD
Power (LT1123)
Power (RD)
RD
Power (LT1123)
Power (RD)
RD
Power (LT1123)
Power (RD)
RD
Power (LT1123)
Power (RD)
0A to 1A
0.3V
120Ω
0.05W
0.12W
150Ω
0.05W
0.13W
180Ω
0.06W
0.16W
240Ω
0.06W
0.17W
270Ω
0.20W
0.07W
330Ω
0.22W
0.07W
0A to 2A
0.4V
43Ω
0.14W
0.32W
51Ω
0.15W
0.35W
75Ω
0.14W
0.36W
91Ω
0.15W
0.42W
110Ω
0.16W
0.47W
130Ω
0.17W
0.52W
0A to 4A
0.75V
––
––
––
20Ω
0.37W
0.76W
27Ω
0.38W
0.89W
36Ω
0.38W
0.97W
43Ω
0.41W
1.11W
51Ω
0.43W
1.25W
Note that in some conditions RD 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.
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.
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
4.00 4.25 4.50 4.75 5.00 5.25 5.50 5.75 6.00
OUTPUT CURRENT (A)
LT1123 F05
Figure 5. Short-Circuit Current for 30 Random Devices
9
8
7
6
5
4
3
2
1
0
0
0.05
0.10
0.15
IB (A)
LT1123 F06
Figure 6. MJE1123 IC vs IB
1123fb
7
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