LT1123
Low Dropout
Regulator Driver
FEATURES
■
■
■
■
■
■
■
■
■
DESCRIPTIO
Extremely Low Dropout
Low Cost
Fixed 5V Output, Trimmed to
±1%
700µA Quiescent Current
1mV Line Regulation
5mV Load Regulation
Thermal Limit
4A Output Current Guaranteed
Available in a 3-Pin TO-92 Package
The LT
®
1123 is a 3-pin bipolar device designed to be used
in conjunction with a discrete PNP power transistor to
form an inexpensive low dropout regulator. The LT1123
consists of a trimmed bandgap reference, error amplifier,
and a driver circuit capable of sinking up to 125mA from
the base of the external PNP pass transistor. The LT1123
is designed to provide a fixed output voltage of 5V.
The drive pin of the device can pull down to 2V at 125mA
(1.4V at 10mA). This allows a resistor to be used to reduce
the base drive available to the PNP and minimize the
power dissipation in the LT1123. The drive current of the
LT1123 is folded back as the feedback pin approaches
ground to further limit the available drive current under
short-circuit conditions.
Total quiescent current for the LT1123 is only 700µA. The
device is available in a low cost TO-92 package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATIO
5V Low Dropout Regulator
0.5
SEALED
LEAD ACID
5.4 TO 7.2V
+
10µF*
620Ω
20Ω
DRIVE
*REQUIRED IF DEVICE IS
MORE THAN 6" FROM MAIN
FILTER CAPACITOR
†
DROPOUT VOLTAGE (V)
MOTOROLA
MJE1123
0.4
0.3
LT1123 FB
OUTPUT = 5V/4A
0.2
+
GND
REQUIRED FOR STABILITY
(LARGER VALUES INCREASE
STABILITY)
10µF
†
LT1123 TA01
0.1
0
0
1
U
Dropout Voltage
3
4
2
OUTPUT CURRENT (A)
5
LT1123 TA02
U
1123fb
1
LT1123
ABSOLUTE
AXI U
RATI GS
U
W W
U
W
(Note 1)
Drive Pin Voltage (V
DRIVE
to Ground) ..................... 30V
Feedback Pin Voltage (V
FB
to Ground) .................... 30V
Operating Junction Temperature Range ... 0°C to 125°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
PACKAGE/ORDER I FOR ATIO
FRONT VIEW
3
TAB IS
GND
2
1
FB
GND
DRIVE
ORDER PART
NUMBER
LT1123CST
ST PART MARKING
1123
ST PACKAGE
3-LEAD PLASTIC SOT-223
θ
JA
AT TAB
≈
20°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The
●
denotes the specifications which apply over the full operating temperature range, otherwise specifications are at T
A
= 25°C.
PARAMETER
Feedback Voltage
CONDITIONS
I
DRIVE
= 10mA, T
J
= 25°C
5mA
≤
I
DRIVE
≤
100mA
3V
≤
V
DRIVE
≤
20V
Feedback Pin Bias Current
Drive Current
V
FB
= 5.00V, 2V
≤
V
DRIVE
≤
15V
V
FB
= 5.20V, 2V
≤
V
DRIVE
≤
15V
V
FB
= 4.80V, V
DRIVE
= 3V
V
FB
= 0.5V, V
DRIVE
= 3V, 0°C
≤
T
J
≤
100°C
I
DRIVE
= 10mA, V
FB
= 4.5V
I
DRIVE
= 125mA, V
FB
= 4.5V
5V < V
DRIVE
< 20V
∆I
DRIVE
= 10 to 100mA
●
●
●
●
●
●
Drive Pin Saturation Voltage
Line Regulation
Load Regulation
Temperature Coefficient of V
OUT
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
2
U
BOTTOM VIEW
W
ORDER PART
NUMBER
LT1123CZ
DRIVE
FB
GND
Z PACKAGE
3-LEAD TO-92 PLASTIC
T
JMAX
= 125°C,
θ
JA
= 220°C/W
MIN
4.90
4.80
TYP
5.00
5.00
300
0.45
170
100
1.4
2.0
1.0
–5
0.2
MAX
5.10
5.20
500
1.0
150
UNITS
V
V
µA
mA
125
25
V
±20
–50
mV
mV
mV/°C
1123fb
LT1123
TYPICAL PERFOR A CE CHARACTERISTICS
Feedback Pin Bias Current
vs Temperature
400
600
MINIMUM DRIVE PIN CURRENT (µA)
V
FB
= 5V
FEEDBACK PIN BIAS CURRENT (µA)
DRIVE CURRENT (mA)
300
200
100
0
25
50
75
100
TEMPERATURE (°C)
125
Feedback Pin Bias Current
vs Feedback Pin Voltage
500
2.5
FEEDBACK PIN BIAS CURRENT (µA)
400
DRIVE PIN VOLTAGE (V)
OUTPUT VOLTAGE (V)
300
T
J
= 125°C
T
J
= 25°C
100
T
J
= 0°C
0
200
0
1
3
4
2
FEEDBACK PIN VOLTAGE (V)
PI FU CTIO S
Drive Pin:
The drive pin serves two functions. It provides
current to the LT1123 for its internal circuitry including
start-up, bias, current limit, thermal limit and a portion of
the base drive current for the output Darlington. The sum
total of these currents (450µA typical) is equal to the
minimum drive current. This current is listed in the speci-
fications as Drive Current with V
FB
= 5.2V. This is the
minimum current required by the drive pin of the LT1123.
The second function of the drive pin is to sink the base
drive current of the external PNP pass transistor. The
available drive current is specified for two conditions.
Drive current with V
FB
= 4.80V gives the range of current
available under nominal operating conditions, when the
device is regulating. Drive current with V
FB
= 0.5V gives the
range of drive current available with the feedback pin
pulled low as it would be during start-up or during a short-
circuit fault. The drive current available when the feedback
pin is pulled low is less than the drive current available
when the device is regulating (V
FB
= 5V). This can be seen
in the curve of Drive Current vs V
FB
Voltage in the Typical
Performance Characteristics curves. This can provide
some foldback in the current limit of the regulator circuit.
1123fb
U W
LT1123 G01
Minimum Drive Pin Current
vs Temperature
200
V
DRIVE
= 3V
500
150
400
300
200
100
0
0
25
50
75
100
TEMPERATURE (°C)
125
0
Drive Current
vs Feedback Pin Voltage
V
DRIVE
= 3V
T
J
= 125°C
T
J
= 25°C
100
T
J
= –50°C
50
0
1
5
2
3
4
FEEDBACK PIN VOLTAGE (V)
6
LT1123 G02
LT1123 G03
Drive Pin Saturation Voltage
vs Drive Current
5.03
Output Voltage vs Temperature
V
FB
= 4.5V
2.0
T
J
= 0°C
1.5
T
J
= 125°C
1.0
T
J
= 25°C
5.02
5.01
5.00
4.99
4.98
4.97
–50 –25
0.5
0
5
LT1123 G04
0
20
80
60
40
100
DRIVE CURRENT (mA)
120
140
50
25
75
0
TEMPERATURE (°C)
100
125
LT1123 G05
LT1123 G06
U
U
U
3
LT1123
PI FU CTIO S
All internal circuitry connected to the drive pin is designed
to operate at the saturation voltage of the Darlington
output driver (1.4 to 2V). This allows a resistor to be
inserted between the base of the external PNP device and
the drive pin. This resistor is used to limit the base drive to
the external PNP below the value set internally by the
LT1123, and also to help limit power dissipation in the
LT1123. The operating voltage range of this pin is from
0V to 30V. Pulling this pin below ground by more than one
V
BE
will forward bias the substrate diode of the device.
This condition can only occur if the power supply leads are
reversed and will not damage the device if the current is
limited to less than 200mA.
Feedback Pin (V
FB
):
The feedback pin also serves two
functions. It provides a path for the bias current of the
reference and error amplifier and contributes a portion of
the drive current for the Darlington output driver. The sum
total of these currents is the Feedback Pin Bias Current
(300µA typical). The second function of this pin is to
provide the voltage feedback to the error amplifier.
SI PLIFIED BLOCK DIAGRA
DRIVE
FU CTIO AL DESCRIPTIO
The LT1123 is a 3-pin device designed to be used in
conjunction with a discrete PNP transistor to form an
inexpensive ultralow dropout regulator. The device incor-
porates a trimmed 5V bandgap reference, error amplifier,
a current-limited Darlington driver and an internal thermal
limit circuit. The internal circuitry connected to the drive
pin is designed to function at the saturation voltage of the
Darlington driver. This allows a resistor to be inserted in
4
W
U
U
U
U
W
U
U
–
CURRENT
LIMIT
THERMAL
LIMIT
FB
+
5V
LT1123 SBD01
GROUND
series with the drive pin. This resistor is used to limit the
base drive to the PNP and also to limit the power dissipa-
tion in the LT1123. The value of this resistor will be defined
by the operating requirements of the regulator circuit. The
LT1123 is designed to sink a minimum of 125mA of base
current. This is sufficient base drive to form a regulator
circuit which can supply output currents up to 4A at a
dropout voltage of less than 0.75V.
1123fb
LT1123
APPLICATIO S I FOR ATIO
U
V
IN
R
B
620Ω
MJE1123
R
D
DRIVE
FB
LT1123
V
OUT
= 5V
The LT1123 is designed to be used in conjunction with an
external PNP transistor. The overall specifications of a
regulator circuit using the LT1123 and an external PNP will
be heavily dependent on the specifications of the external
PNP. While there are a wide variety of PNP transistors
available that can be used with the LT1123, the specifica-
tions given in typical transistor data sheets are of little use
in determining overall circuit performance.
Linear Technology has solved this problem by cooperating
with Motorola to design and specify the MJE1123. This
transistor is specifically designed to work with the LT1123
as the pass element in a low dropout regulator. The
specifications of the MJE1123 reflect the capability of the
LT1123. For example, the dropout voltage of the MJE1123
is specified up to 4A collector current with base drive
currents that the LT1123 is capable of generating (20mA
to 120mA). Output currents up to 4A with dropout voltages
less than 0.75V can be guaranteed.
The following sections describe how specifications can be
determined for the basic regulator. The charts and graphs
are based on the combined characteristics of the LT1123
and the MJE1123. Formulas are included that will enable
the user to substitute other transistors that have been
characterized. A chart is supplied that lists suggested
resistor values for the most popular range of input volt-
ages and output current.
Basic Regulator Circuit
The basic regulator circuit is shown in Figure 1. The
LT1123 senses the voltage at its feedback pin and drives
the base of the PNP (MJE1123) in order to maintain the
output at 5V. The drive pin of the LT1123 can only sink
current; R
B
is required to provide pull up on the base of the
PNP. R
B
must be sized so that the voltage drop caused by
the minimum drive pin current is less than the emitter/
base voltage of the external PNP at light loads. The
recommended value for R
B
is 620Ω. For circuits that are
required to run at junction temperatures in excess of
100°C the recommended value of R
B
is 300Ω.
W
U U
+
GND
10µF ALUM
LT1123 F01
Figure 1. Basic Regulator Circuit
R
D
is used to limit the drive current available to the PNP
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
D
is chosen
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
CE
saturation voltage of the discrete
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
D
) in series with the drive pin (see
the section below on current limit). The MJE1123 is
specified for dropout voltage (V
CE
sat.) at several values of
output current and up to 120mA of base drive current. The
chart below lists the operating points that can be guaran-
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.
1123fb
5
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