LINEAR TECHNOLOGY LT1123 User Manual

FEATURES

■

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

LT1123

Low Dropout

Regulator Driver

U

DESCRIPTIO

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.

TYPICAL APPLICATIO

5V Low Dropout Regulator Dropout Voltage

SEALED

LEAD ACID

5.4 TO 7.2V

*REQUIRED IF DEVICE IS
MORE THAN 6" FROM MAIN
FILTER CAPACITOR

†

REQUIRED FOR STABILITY
(LARGER VALUES INCREASE
STABILITY)

+

10µF*

DRIVE

LT1123

GND

620Ω

20Ω

FB

U

MOTOROLA
MJE1123

+

LT1123 TA01

OUTPUT = 5V/4A

†

10µF

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.

0.5

0.4

0.3

0.2

DROPOUT VOLTAGE (V)

0.1

0

1

0

OUTPUT CURRENT (A)

3

4

2

5

LT1123 TA02

1123fb

1

LT1123

WWWU

ABSOLUTE AXI U RATI GS

(Note 1)

Drive Pin Voltage (V
Feedback Pin Voltage (V

to Ground) ..................... 30V

DRIVE

to Ground) .................... 30V

FB

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec)................. 300°C

Operating Junction Temperature Range ... 0°C to 125°C

UU

W

PACKAGE/ORDER I FOR ATIO

FRONT VIEW

3

FB

TAB IS

GND

ST PACKAGE

3-LEAD PLASTIC SOT-223

θJA AT TAB ≈ 20°C/W

Consult LTC Marketing for parts specified with wider operating temperature ranges.

2

GND

1

DRIVE

ORDER PART

NUMBER

LT1123CST

ST PART MARKING

1123

BOTTOM VIEW

DRIVE FB GND

Z PACKAGE

3-LEAD TO-92 PLASTIC

= 125°C, θJA = 220°C/W

T

JMAX

ORDER PART

NUMBER

LT1123CZ

ELECTRICAL CHARACTERISTICS

The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.

PARAMETER CONDITIONS MIN TYP MAX UNITS

Feedback Voltage I

Feedback Pin Bias Current VFB = 5.00V, 2V ≤ V

Drive Current VFB = 5.20V, 2V ≤ V

Drive Pin Saturation Voltage I

Line Regulation 5V < V

Load Regulation ∆I

Temperature Coefficient of V

OUT

= 10mA, TJ = 25°C 4.90 5.00 5.10 V

DRIVE

5mA ≤ I
3V ≤ V

= 4.80V, V

V

FB

V

= 0.5V, V

FB

DRIVE

I

DRIVE

DRIVE

≤ 100mA

DRIVE

≤ 20V ● 4.80 5.00 5.20 V

DRIVE

≤ 15V ● 300 500 µA

DRIVE

≤ 15V ● 0.45 1.0 mA

DRIVE

= 3V ● 125 170

DRIVE

= 3V, 0°C ≤ TJ ≤ 100°C 25 100 150

DRIVE

= 10mA, VFB = 4.5V 1.4 V
= 125mA, VFB = 4.5V 2.0

< 20V ● 1.0 ±20 mV

DRIVE

= 10 to 100mA ● –5 –50 mV

0.2 mV/°C

Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.

2

1123fb

TEMPERATURE (°C)

–50

OUTPUT VOLTAGE (V)

5.01

5.02

5.03

25 75

LT1123 G06

5.00

4.99

–25 0

50 100 125

4.98

4.97

LPER

FEEDBACK PIN VOLTAGE (V)

0

0

DRIVE CURRENT (mA)

50

100

150

1234

LT1123 G03

56

200

TJ = 125°C

TJ = –50°C

TJ = 25°C

V

DRIVE

= 3V

LT1123

UW

R

F

O

ATYPICA

CCHARA TERIST

E

C

ICS

Feedback Pin Bias Current
vs Temperature

400

VFB = 5V

300

200

FEEDBACK PIN BIAS CURRENT (µA)

100

0

25 50 75 100

TEMPERATURE (°C)

Feedback Pin Bias Current
vs Feedback Pin Voltage

500

400

300

200

TJ = 25°C

100

FEEDBACK PIN BIAS CURRENT (µA)

TJ = 125°C

LT1123 G01

TJ = 0°C

125

Minimum Drive Pin Current
vs Temperature

600

V

= 3V

DRIVE

500

400

300

200

100

MINIMUM DRIVE PIN CURRENT (µA)

0

0

25 50 75 100

TEMPERATURE (°C)

Drive Pin Saturation Voltage
vs Drive Current

2.5
VFB = 4.5V

2.0

1.5

1.0

DRIVE PIN VOLTAGE (V)

0.5

TJ = 0°C

TJ = 125°C

TJ = 25°C

Drive Current
vs Feedback Pin Voltage

125

LT1123 G02

Output Voltage vs Temperature

0

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 speci­fications as Drive Current with VFB = 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.

1

0

FEEDBACK PIN VOLTAGE (V)

U

3

2

4

UU

LT1123 G04

0

5

0

40

20

DRIVE CURRENT (mA)

80

60

100

120

LT1123 G05

140

Drive current with VFB = 4.80V gives the range of current
available under nominal operating conditions, when the
device is regulating. Drive current with VFB = 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 short­circuit fault. The drive current available when the feedback
pin is pulled low is less than the drive current available
when the device is regulating (VFB = 5V). This can be seen
in the curve of Drive Current vs VFB Voltage in the Typical
Performance Characteristics curves. This can provide
some foldback in the current limit of the regulator circuit.

1123fb

3

LT1123

U

UU

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

will forward bias the substrate diode of the device.

BE

This condition can only occur if the power supply leads are

W

SI PLIFIEDWBLOCK DIAGRA

DRIVE

reversed and will not damage the device if the current is
limited to less than 200mA.

Feedback Pin (V

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.

): The feedback pin also serves two

FB

CURRENT

LIMIT

THERMAL

LIMIT

GROUND

U

U

U

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 incor­porates 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

–

+

5V

LT1123 SBD01

series with the drive pin. This resistor is used to limit the
base drive to the PNP and also to limit the power dissipa­tion 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.

FB

4

1123fb

WUUU

APPLICATIO S I FOR ATIO

LT1123

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 specifica­tions 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 volt­ages 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; RB is required to provide pull up on the base of the
PNP. RB 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 RB is 620Ω. For circuits that are
required to run at junction temperatures in excess of
100°C the recommended value of RB is 300Ω.

R

DRIVE

LT1123

GND

B

620Ω

R

D

MJE1123

V

FB

+

LT1123 F01

OUT

10µF ALUM

= 5V

V

IN

Figure 1. Basic Regulator Circuit

RD 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 regu­lator circuit under overload conditions. RD is chosen
based on the operating requirements of the circuit, prima­rily dropout voltage and output current.

Dropout Voltage

The dropout voltage of an LT1123-based regulator circuit
is determined by the VCE 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 (RD) in series with the drive pin (see
the section below on current limit). The MJE1123 is
specified for dropout voltage (VCE 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 guaran­teed 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