LT1123 LINER [Linear Technology], LT1123 Datasheet - Page 5

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

output at 5V. The drive pin of the LT1123 can only sink
current; R
PNP. R
the minimum drive pin current is less than the emitter/
base voltage of the external PNP at light loads. The
recommended value for R
required to run at junction temperatures in excess of
100 C the recommended value of R
R
and to limit the power dissipation in the LT1123. Limiting
the drive current to the PNP will limit the output current of
the regulator which will minimize the stress on the regu-
lator circuit under overload conditions. R
based on the operating requirements of the circuit, prima-
rily dropout voltage and output current.
DROPOUT VOLTAGE
The dropout voltage of an LT1123 based regulator circuit
is determined by the V
PNP when it is driven with a base current equal to the
available drive current of the LT1123. The LT1123 can sink
up to 150mA of base current (150mA typ., 125mA min.)
when output voltage is up near the regulating point (5V).
The available drive current of the LT1123 can be reduced
by adding a resistor (R
the section below on current limit). The MJE1123 is
specified for dropout voltage (V
output current and up to 120mA of base drive current. The
chart below lists the operating points that can be guaran-
A
D
PPLICATI
is used to limit the drive current available to the PNP
B
must be sized so that the voltage drop caused by
B
is required to provide pullup on the base of the
V
Figure 1. Basic Regulator Circuit
IN
O
U
LT1123
DRIVE
CE
GND
S
D
) in series with the drive pin (see
saturation voltage of the discrete
R
R
620
D
B
B
I FOR ATIO
FB
U
is 620 . For circuits that are
CE
sat.) at several values of
B
MJE1123
W
+
is 300 .
10µF ALUM
V
OUT
LT1123 F01
D
= 5V
is chosen
U
Dropout Voltage
DRIVE CURRENT
teed by the combined data sheets of the LT1123 and
MJE1123. Figure 2 illustrates the chart in graphic form.
Although these numbers are only guaranteed by the data
sheet at 25 C, Dropout Voltage vs Temperature (Figure 3)
clearly shows that the dropout voltage is nearly constant
over a wide temperature range.
120mA
20mA
50mA
Figure 3. Dropout Voltage vs Temperature
0.50
0.25
0.75
0.75
0.65
0.55
0.45
0.35
0.25
0.15
0.05
1.0
Figure 2. Maximum Dropout Voltage
0
20
0
I
DRIVE
BASED ON
MJE1123 SPECS
OUTPUT CURRENT
I
I
I
C
C
C
= 20mA
= 4A, I
40
= 2A, I
= 1A, I
CASE TEMPERATURE (°C)
1
OUTPUT CURRENT (A)
B
B
B
1A
1A
2A
1A
4A
= 0.12A
= 0.05A
= 0.02A
60
I
DRIVE
I
2
DRIVE
= 50mA
80
= 120mA
3
100
LT1123 F02
LT1123 F03
DROPOUT VOLTAGE
0.16V
0.13V
0.25V
0.2V
0.45V
TYP
120
4
LT1123
0.3V
0.25V
0.4V
0.35V
0.75V
MAX
5