LT1123 LINER [Linear Technology], LT1123 Datasheet - Page 6

• Manufacturer Part Number: LT1123
• Description: Low Dropout Regulator Driver
• Manufacturer: LINER [Linear Technology]

SELECTING R
In order to select R
of drive current that will give the required value of output
current. For circuits using the MJE1123 as a pass transis-
tor this can be done using the graph of Dropout Voltage vs
Output Current (Figure 2). For example, 20mA of drive
current will guarantee a dropout voltage of 0.3V at 1A of
output current. For circuits using transistors other than
the MJE1123 the user must characterize the transistor to
determine the drive current requirements. In general it is
recommended that the user choose the lowest value of
drive current that will satisfy the output current require-
ments. This will minimize the stress on circuit compo-
nents during overload conditions.
Figure 4 can be used to select the value of R
required drive current and the minimum input voltage.
Curves are shown for 20mA, 50mA, and 120mA drive
current corresponding to the specified base drive currents
for the MJE1123. The data for the curves was generated
using the following formula:
where V
LT1123
A
6
PPLICATI
R
V
PNP pass transistor
V
LT1123
I
The current through R
DRIVE
IN
BE
DRIVE
D
100
1k
10
= (V
= the minimum input voltage to the circuit
= the maximum emitter/base voltage of the
5
D
= the minimum drive current required
= the maximum Drive pin voltage of the
IN
6
O
Figure 4. R
D
– V
7
U
the user should first choose the value
BE
S
8
– V
9
I FOR ATIO
D
U
V
DRIVE
10
Resistor Value
IN
I
I
I
DRIVE
DRIVE
DRIVE
B
11
is assumed to be 1mA
)/(I
12
= 20mA
= 120mA
= 50mA
DRIVE
W
13
LT1123 F04
14
+ 1mA)
D
15
based on the
U
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:
For 50mA drive current assume:
For 120mA drive current assume:
The R
R
current. The chart includes a number for power dissipa-
tion for the LT1123 and R
R
INPUT
VOLTAGE
Note that in some conditions R
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.
5.5V
6.0V
7.0V
8.0V
9.0V
10.0V
D
D
Selection Chart
for the most useful range of input voltage and output
D
Selection Chart lists the recommended values for
V
V
V
V
V
V
OUTPUT CURRENT:
DROPOUT VOLTAGE:
R
Power (LT1123)
Power (R
R
Power (LT1123)
Power (R
R
Power (LT1123)
Power (R
R
Power (LT1123)
Power (R
R
Power (LT1123)
Power (R
R
Power (LT1123)
Power (R
BE
DRIVE
BE
DRIVE
BE
DRIVE
D
D
D
D
D
D
= 0.95V at I
= 1.2V at I
= 1.4V at I
= 1.75V
= 1.9V
= 2.1V
D
D
D
D
D
D
)
)
)
)
)
)
C
C
C
= 2A
= 4A
= 1A
D
.
0 – 1A
0.05W
0.12W
0.05W
0.13W
0.06W
0.16W
0.06W
0.17W
0.20W
0.07W
0.22W
0.07W
D
120
150
180
240
270
330
0.3V
may be replaced with a
0 – 2A
0.14W
0.32W
0.35W
0.36W
0.42W
0.47W
0.52W
0.15W
0.14W
0.15W
0.16W
0.17W
110
130
0.4V
43
51
75
91
0.76W
0.89W
0.97W
1.11W
1.25W
0 – 4A
0.37W
0.38W
0.38W
0.41W
0.43W
0.75V
20
27
36
43
51
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