LT1123CST データシートの表示（PDF） - Linear Technology
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LT1123CST Datasheet PDF : 16 Pages
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LT1123
APPLICATIO S I FOR ATIO
Dropout Voltage
DRIVE CURRENT
20mA
50mA
120mA
OUTPUT CURRENT
1A
1A
2A
1A
4A
DROPOUT VOLTAGE
TYP
MAX
0.16V
0.3V
0.13V
0.25V
0.25V
0.4V
0.2V
0.45V
0.35V
0.75V
1.0
BASED ON
MJE1123 SPECS
0.75
IDRIVE = 120mA
0.50
IDRIVE = 20mA
0.25
IDRIVE = 50mA
0
0
1
2
3
4
OUTPUT CURRENT (A)
LT1123 F02
Figure 2. Maximum Dropout Voltage
0.75
0.65
0.55
IC = 4A, IB = 0.12A
0.45
0.35
IC = 2A, IB = 0.05A
0.25
IC = 1A, IB = 0.02A
0.15
0.05
20
40
60
80 100 120
CASE TEMPERATURE (°C)
LT1123 F03
Figure 3. Dropout Voltage vs Temperature
transistor 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 requirements. This will minimize the stress on
circuit components during overload conditions.
Figure 4 can be used to select the value of RD based on the
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:
RD = (VIN – VBE – VDRIVE)/(IDRIVE + 1mA)
where:
VIN = the minimum input voltage to the circuit
VBE = the maximum emitter/base voltage of the
PNP pass transistor
VDRIVE = the maximum drive pin voltage of the
LT1123
IDRIVE = the minimum drive current required.
The current through RB is assumed to be 1mA
1k
IDRIVE = 20mA
IDRIVE = 50mA
100
IDRIVE = 120mA
Selecting RD
In order to select RD the user should first choose the value
of drive current that will give the required value of output
current. For circuits using the MJE1123 as a pass
6
10
56
78
9 10 11 12 13 14 15
VIN
LT1123 F04
Figure 4. RD Resistor Value
1123fb
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