LT1123 Datasheet by Analog Devices Inc.

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1
LT1123
1123fb
Low Dropout
Regulator Driver
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.
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
5V Low Dropout Regulator Dropout Voltage
FEATURES
DESCRIPTIO
U
TYPICAL APPLICATIO
U
620
10µF*
SEALED
LEAD ACID
5.4 TO 7.2V
*REQUIRED IF DEVICE IS
MORE THAN 6" FROM MAIN
FILTER CAPACITOR
OUTPUT = 5V/4A
10µF
LT1123 TA01
20
REQUIRED FOR STABILITY
(LARGER VALUES INCREASE
STABILITY)
MOTOROLA
MJE1123
DRIVE
LT1123
GND
FB
+
+
OUTPUT CURRENT (A)
0
DROPOUT VOLTAGE (V)
0.3
0.4
0.5
4
LT1123 TA02
0.2
0.1
01235
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
U O O O DRWE FE GND L7LJUEAR
2
LT1123
1123fb
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
ORDER PART
NUMBER
LT1123CST
ABSOLUTE AXI U RATI GS
WWWU
Storage Temperature Range .................65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
PACKAGE/ORDER I FOR ATIO
UU
W
(Note 1)
ORDER PART
NUMBER
LT1123CZ
ST PART MARKING
3
2
1
FRONT VIEW
TAB IS
GND
FB
GND
DRIVE
ST PACKAGE
3-LEAD PLASTIC SOT-223
BOTTOM VIEW
DRIVE FB GND
Z PACKAGE
3-LEAD TO-92 PLASTIC
1123
θ
JA
AT TAB 20°C/W T
JMAX
= 125°C, θ
JA
= 220°C/W
PARAMETER CONDITIONS MIN TYP MAX UNITS
Feedback Voltage I
DRIVE
= 10mA, T
J
= 25°C 4.90 5.00 5.10 V
5mA I
DRIVE
100mA
3V V
DRIVE
20V 4.80 5.00 5.20 V
Feedback Pin Bias Current V
FB
= 5.00V, 2V V
DRIVE
15V 300 500 µA
Drive Current V
FB
= 5.20V, 2V V
DRIVE
15V 0.45 1.0 mA
V
FB
= 4.80V, V
DRIVE
= 3V 125 170
V
FB
= 0.5V, V
DRIVE
= 3V, 0°C T
J
100°C 25 100 150
Drive Pin Saturation Voltage I
DRIVE
= 10mA, V
FB
= 4.5V 1.4 V
I
DRIVE
= 125mA, V
FB
= 4.5V 2.0
Line Regulation 5V < V
DRIVE
< 20V 1.0 ±20 mV
Load Regulation I
DRIVE
= 10 to 100mA –5 –50 mV
Temperature Coefficient of V
OUT
0.2 mV/°C
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
ELECTRICAL CHARACTERISTICS
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
Consult LTC Marketing for parts specified with wider operating temperature ranges.
400 ‘ 600 Vpg=5V E E g // g / g 300 g 400 a / 2 <_( e="" 3="" e="" 5="" e="" x="" zoo="" 200="" 3="" e="" s="" 2="" w="" 2="" le="" 2="" mu="" 0="" 0="" 25="" 50="" 75="" h10="" ‘25="" 0="" 25="" 50="" 75="" wu="" temperature="" (“0)="" temperature="" my="" 500="" 25="" w="" w="" 3="" ,="" \="" 5="" 400="" 20="" -="" e="" 5="" “zone="" l:="" w="" 5="" 300="" e="" ‘5="" g:="" g="" \="" ‘="" n42“:="" m="" t="" :i25°c="" z="" 5="" 2m="" j="" e="" m="" \="" g="" m:25°0="" e="" a="" we="" 05="" n="" u="" ‘="" ‘="" n="" w="" 2="" a="" 4="" 5="" n="" 20="" an="" an="" an="" we="" mu="" m="" 750="" 725="" u="" 25="" 50="" 75="" mo="" 12="" feedback="" pwvoltage="" (v)="" drwecurrentwa)="" temperature="" (we)="" l7ljdww="">
3
LT1123
1123fb
TEMPERATURE (°C)
0
100
FEEDBACK PIN BIAS CURRENT (µA)
200
300
400
25 50 75 100
LT1123 G01
125
V
FB
= 5V
Feedback Pin Bias Current
vs Temperature
FEEDBACK PIN VOLTAGE (V)
0
FEEDBACK PIN BIAS CURRENT (µA)
300
400
500
4
LT1123 G04
200
100
01235
T
J
= 125°C
T
J
= 0°C
T
J
= 25°C
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
Output Voltage vs Temperature
TEMPERATURE (°C)
0
0
MINIMUM DRIVE PIN CURRENT (µA)
200
400
600
25 50 75 100
LT1123 G02
125
100
300
500
V
DRIVE
= 3V
Minimum Drive Pin Current
vs Temperature
DRIVE CURRENT (mA)
0
0
DRIVE PIN VOLTAGE (V)
1.0
2.5
40 80 100
LT1123 G05
0.5
2.0
1.5
20 60 120 140
TJ = 125°C
TJ = 0°C
TJ = 25°C
VFB = 4.5V
Drive Pin Saturation Voltage
vs Drive Current
CCHARA TERISTICS
UW
ATYPICALPER
FORCE
FEEDBACK PIN VOLTAGE (V)
0
0
DRIVE CURRENT (mA)
50
100
150
1234
LT1123 G03
56
200
T
J
= 125°C
T
J
= –50°C
T
J
= 25°C
V
DRIVE
= 3V
Drive Current
vs Feedback Pin Voltage
Feedback Pin Bias Current
vs Feedback Pin Voltage
UU
U
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.
L7LJUEAR
4
LT1123
1123fb
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.
UU
U
PI FU CTIO S
+
5V
CURRENT
LIMIT
THERMAL
LIMIT
GROUND
DRIVE
FB
LT1123 SBD01
SI PLIFIED
W
BLOCK DIAGRA
W
FU CTIO AL DESCRIPTIO
U
UU
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
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.
ting Tms 1123 The fthe 123 dnve 20mA ages anbe aphs 1123 ame been ested voh- J7. T 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 PNPwrll limit the output current of the regulator which will minimize the stress on the regu- lator circuit under overload conditions. RD is chosen based onthe operating requirements ofthecircuit, prima- rily dropout voltage and output current. Dropout Voltage The dropout voltage of an LT1123-based regulatorcircuit 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 ofthe LT1 1 23. The LT1123 can sink up to 150mA of base current (150mA typ, 125mA min) when output voltage is up near the regulating point (5V). LTLJflfl
5
LT1123
1123fb
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 620. For circuits that are
required to run at junction temperatures in excess of
100°C the recommended value of R
B
is 300.
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.
APPLICATIO S I FOR ATIO
WUUU
R
D
R
B
620
V
IN
V
OUT
= 5V
10µF ALUM
LT1123 F01
DRIVE
LT1123
GND
FB
+
MJE1123
tum: 12mm u I 2 a 4 Figure 2. Maximum Drnpnutantag an ma ATURE (”0) Figure 3. Drnpnut Voltage vs Temperature Selecting RD InordertoselectRDthe usershouldfirstchoosethevalue of drive current that will give the required value of output current. For circuits using the MJE1123 as a pass - DRlVE = 20'“ lnmvg = sum - now = tzumA Vm Figure 4. RD Resistnr Value 6 L7 Lll‘lEN2 memo;
6
LT1123
1123fb
Dropout Voltage
DRIVE CURRENT OUTPUT CURRENT TYP MAX
20mA 1A 0.16V 0.3V
50mA 1A 0.13V 0.25V
2A 0.25V 0.4V
120mA 1A 0.2V 0.35V
4A 0.45V 0.75V
Figure 3. Dropout Voltage vs Temperature
Figure 2. Maximum Dropout Voltage
Selecting R
D
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
0
DROPOUT VOLTAGE (V)
0.75
1.0
4
LT1123 F02
0.50
0.25
123
0
OUTPUT CURRENT (A)
BASED ON
MJE1123 SPECS
I
DRIVE
= 20mA
I
DRIVE
= 120mA
I
DRIVE
= 50mA
CASE TEMPERATURE (°C)
20
DROPOUT VOLTAGE (V)
0.55
0.65
0.75
80
LT1123 F03
0.45
0.35
40 60 100
0.25
0.15
0.05
120
I
C
= 4A, I
B
= 0.12A
I
C
= 2A, I
B
= 0.05A
I
C
= 1A, I
B
= 0.02A
DROPOUT VOLTAGE
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 R
D
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:
R
D
= (V
IN
– V
BE
– V
DRIVE
)/(I
DRIVE
+ 1mA)
where:
V
IN
= the minimum input voltage to the circuit
V
BE
= the maximum emitter/base voltage of the
PNP pass transistor
V
DRIVE
= the maximum drive pin voltage of the
LT1123
I
DRIVE
= the minimum drive current required.
The current through R
B
is assumed to be 1mA
Figure 4. RD Resistor Value
VIN
6
RD
8
1k
LT1123 F04
10
100
715
14
131211
10
9
IDRIVE = 20mA
IDRIVE = 120mA
IDRIVE = 50mA
5
APPLICATIO S I FOR ATIO
WUUU
Power (RD) 013w o 35w 0 78W 7 DV RD 130$! 75!! 27!} Power (L11 123) D 06W D14W D 33W Power (RD) 016W 0 38W 0 now 3 DV RD 240$! 91!! 38!} Power (L11 123) D 06W D15W D 33W Power (RD) 017W 0 42W 0 97W 9 DV RD 270$! 110$! 43!} Power (L11 123) D 20W D1SW D 41W PoweriRD) 007W 047W 1 11W 10.0V RD 330$! 130$! 51!} Power (L11 123) D 22W 0 17W 0 43W Power (RD) 0 07W 0 52W 1 25W Note that in some conditions RD maybe replaced With a short. This is possible in circuits where an overload is unlikeiy and the input voltage and drive requirements are iow. See the section on Thermai Considerations for more information. L7LJDWW n 400 425 450 475 500 525 550 575 50 ouwur CURRENT (A) Figure 5. Shari-Circuit Currenl inr 3|) Randnm Figure 5. MJE1123 Ic vs II;
7
LT1123
1123fb
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:
V
BE
= 0.95V at I
C
= 1A
V
DRIVE
= 1.75V
For 50mA drive current assume:
V
BE
= 1.2V at I
C
= 2A
V
DRIVE
= 1.9V
For 120mA drive current assume:
V
BE
= 1.4V at I
C
= 4A
V
DRIVE
= 2.1V
The R
D
Selection Chart lists the recommended values for
R
D
for the most useful range of input voltage and output
current. The chart includes a number for power dissipa-
tion for the LT1123 and R
D
.
Current Limit
For regulator circuits using the LT1123, current limiting is
achieved by limiting the base drive to the external PNP
pass transistor. This means that the actual system current
limit will be a function of both the current limit of the
LT1123 and the Beta of the external PNP. Beta-based
current limit schemes are normally not practical because
of uncertainties in the Beta of the pass transistor. Here the
drive characteristics of the LT1123 combined with the
Beta characteristics of the MJE1123 can provide reliable
Beta-based current limiting. This is shown in Figure 5
where the current limit of 30 randomly selected transis-
tors is plotted. The spread of current limit is reasonably
well controlled.
5.5V R
D
12043––
Power (LT1123) 0.05W 0.14W ––
Power (R
D
) 0.12W 0.32W ––
6.0V R
D
1505120
Power (LT1123) 0.05W 0.15W 0.37W
Power (R
D
) 0.13W 0.35W 0.76W
7.0V R
D
1807527
Power (LT1123) 0.06W 0.14W 0.38W
Power (R
D
) 0.16W 0.36W 0.89W
8.0V R
D
2409136
Power (LT1123) 0.06W 0.15W 0.38W
Power (R
D
) 0.17W 0.42W 0.97W
9.0V R
D
27011043
Power (LT1123) 0.20W 0.16W 0.41W
Power (R
D
) 0.07W 0.47W 1.11W
10.0V R
D
33013051
Power (LT1123) 0.22W 0.17W 0.43W
Power (R
D
) 0.07W 0.52W 1.25W
INPUT
VOLTAGE
0A to 4A
0.75V
OUTPUT CURRENT:
DROPOUT VOLTAGE:
0A to 1A
0.3V
0A to 2A
0.4V
RD Selection Chart
Note that in some conditions R
D
may be replaced with a
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.
Figure 6. MJE1123 IC vs IB
Figure 5. Short-Circuit Current for 30 Random Devices
OUTPUT CURRENT (A)
4.00
NUMBER OF UNITS
3
4
5
4.75 5.25 6.00
LT1123 F05
2
1
04.25 4.50 5.00 5.50 5.75
6
7
8
9
10
11
12
13
14
15
I
B
(A)
0
0
I
C
(A)
1
3
4
5
0.10
9
LT1123 F06
2
0.05 0.15
6
7
8
APPLICATIO S I FOR ATIO
WUUU
Vw (V) Figure 7. iner in LT1123 L7LJUEAR
8
LT1123
1123fb
The curve in Figure 6 can be used to determine the range
of current limit of an LT1123 regulator circuit using an
MJE1123 as a pass transistor. The curve was generated
using the Beta versus I
C
curve of the MJE1123. The
minimum and maximum value curves are extrapolated
from the minimum and maximum Beta specifications.
Thermal Conditions
The thermal characteristics of three components need to
be considered; the LT1123, the pass transistor and R
D
.
Power dissipation should be calculated based on the
worst-case conditions seen by each component during
normal operation.
The worst-case power dissipation in the LT1123 is a
function of drive current, supply voltage and the value of
R
D
. Worst-case dissipation for the LT1123 occurs when
the drive current is equal to approximately one half of its
maximum value. Figure 7 plots the worst-case power
dissipation in the LT1123 versus R
D
and V
IN
. The graph
was generated using the following formula:
PV–V
4R ;R 10
DIN BE 2
DD
=
()
>
where:
V
BE
= the emitter/base voltage of the PNP pass
transistor (assumed to be 0.6V)
For some operating conditions R
D
may be replaced with a
short. This is possible in applications where the operating
requirements (input voltage and drive current) are at the
low end and the output will not be shorted. For R
D
= 0 the
following formula may be used to calculate the maximum
power dissipation in the LT1123.
P
D
= (V
IN
– V
BE
)(I
DRIVE
)
where:
V
IN
= maximum input voltage
V
BE
= emitter/base voltage of PNP
I
DRIVE
= required maximum drive current
The maximum junction temperature rise above ambient
for the LT1123 will be equal to the worst-case power
dissipation multiplied by the thermal resistance of the
device. The thermal resistance of the device will depend
upon how the device is mounted, and whether a heat sink
is used. Measurements show that one of the most effective
ways of heat sinking the TO-92 package is by utilizing the
PC board traces attached to the leads of the package. The
table below lists several methods of mounting and the
measured value of thermal resistance for each method. All
measurements were done in still air.
Package alone ............................................................................. 220°C/W
Package soldered into PC board with plated through
holes only ................................................................................ 175°C/W
Package soldered into PC board with 1/4 sq. in. of copper trace
per lead .................................................................................... 145°C/W
Package soldered into PC board with plated through holes in
board, no extra copper trace, and a clip-on type heat sink:
Thermalloy type 2224B .................................................... 160°C/W
Aavid type 5754 ................................................................ 135°C/W
The maximum operating junction temperature of the
LT1123 is 125°C. The maximum operating ambient tem-
perature will be equal to 125°C minus the maximum
junction temperature rise above ambient.
The worst-case power dissipation in R
D
needs to be
calculated so that the power rating of the resistor can be
determined. The worst-case power in the resistor will
occur when the drive current is at a maximum. Figure 8
plots the required power rating of R
D
versus supplyFigure 7. Power in LT1123
VIN (V)
6
RD ()
8
1k
LT1123 F07
10
100
715
14
131211
10
9
5
0.4W
0.7W
0.1W
0.2W
0.3W
0.5W
THERMAL
RESISTANCE
APPLICATIO S I FOR ATIO
WUUU
Vw (V) Figure 8. iner in BB L7LJDWW
9
LT1123
1123fb
voltage and resistor value. Power dissipation can be
calculated using the following formula:
PV–V –V
R
RD IN BE DRIVE 2
=
()
where:
V
BE
= emitter/base voltage of the PNP pass transistor
V
DRIVE
= voltage at the drive pin of the LT1123
= V
SAT
of the drive pin in the worst case
The worst-case power dissipation in the PNP pass transis-
tor is simply equal to:
P
MAX
= (V
IN
– V
OUT
)(I
OUT
)
where
V
IN
= Maximum V
IN
I
OUT
= Maximum I
OUT
The thermal resistance of the MJE1123 is equal to:
70°C/W Junction to Ambient (no heat sink)
1.67°C/W Junction to Case
The PNP will normally be attached to either a chassis or a
heat sink so the actual thermal resistance from junction to
ambient will be the sum of the PNP’s junction to case
thermal resistance and the thermal resistance of the heat
sink or chassis. For nonstandard heat sinks the user will
need to determine the thermal resistance by experiment.
The maximum junction temperature rise above ambient
for the PNP pass transistor will be equal to the maximum
power dissipation times the thermal resistance, junction
to ambient, of the PNP. The maximum operating junction
temperature of the MJE1123 is 150°C. The maximum
operating ambient temperature for the MJE1123 will be
equal to 150°C minus the maximum junction temperature
rise.
The SOT-223 package is designed to be surface mounted.
Heat sinking is accomplished by using the heat spreading
capabilities of the PC board and its copper traces. The
thermal resistance from junction to ambient can be as low
as 50°C/W. This requires a reasonably sized PC board with
at least one layer of copper to spread the heat across the
board and couple it into the surrounding air.
The table below can be used as a guideline in estimating
thermal resistance. Data for the table was generated using
1/16" FR-4 board with 1oz copper foil.
Table 1.
Copper Area Thermal Resistance
Topside* Backside Board Area (Junction to Ambient)
2500 sq. mm 2500 sq. mm 2500 sq. mm 50°C/W
1000 sq. mm 2500 sq. mm 2500 sq. mm 50°C/W
225 sq. mm 2500 sq. mm 2500 sq. mm 58°C/W
100 sq. mm 2500 sq. mm 2500 sq. mm 64°C/W
1000 sq. mm 1000 sq. mm 1000 sq. mm 57°C/W
1000 sq. mm 0 1000 sq. mm 60°C/W
* Tab of device attached to topside copper
For the LT1123 the tab is ground so that plated through
holes can be used to couple the tab both electrically and
thermally to the ground plane layer of the board. This will
help to lower the thermal resistance.
Thermal Limiting
The thermal limit of the LT1123 can be used to protect both
the LT1123 and the PNP pass transistor. This is accom-
plished by thermally coupling the LT1123 to the power
transistor. There are clip type heat sinks available for the
TO-92 package that will allow the LT1123 to be mounted
to the same heat sink as the PNP pass transistor. One
example is manufactured by IERC (part #RUR67B1CB).
The LT1123 should be mounted as close as possible to the
Figure 8. Power in RD
VIN (V)
6
RD ()
8
1k
LT1123 F08
10
100
715
14
131211
10
9
5
1W
0.5W
0.25W
2W
APPLICATIO S I FOR ATIO
WUUU
L7LJUEAR
10
LT1123
1123fb
PNP. If the output of the regulator circuit can be shorted,
heat sinking must be adequate to limit the rate of tempera-
ture rise of the power device to approximately 50°C/
minute. This can be accomplished with a fairly small heat
sink, on the order of 3 to 4 square inches of surface area.
Design Example
Given the following operating requirements:
5.5V < V
IN
< 7V
I
OUTMAX
= 1.5A
Max ambient temperature = 70°C
V
OUT
= 5V
1.
The first step is to determine the required drive current.
This can be found from the Maximum Dropout Voltage
curve. 50mA of drive current will guarantee 0.4V drop-
out at an output current of 2A. This satisfies our
requirements.
I
DRIVE
= 50mA
2. The next step is to determine the value of R
D
. Based on
50mA of drive current and a minimum input voltage of
5.5V, we can select R
D
from the graph of Figure 4. From
the graph the value of R
D
is equal to 50, so we should
use the next lowest 5% value which is 47.
R
D
= 47
3. We can now look at the thermal requirements of the
circuit.
Worst-case power in the LT1123 will be equal to:
V–V
4R
IN(MAX) BE
2
D
()
Given: V
IN(MAX)
= 7V, V
BE
= 0.6V, R
D
= 47
Then: P
MAX
(LT1123) = 0.22W.
APPLICATIO S I FOR ATIO
WUUU
Assuming a thermal resistance of 150°C/W, the maximum
junction temperature rise above ambient will be equal to
(P
MAX
)(150°C/W) = 33°C. The maximum operating junc-
tion temperature will be equal to the maximum ambient
temperature plus the junction temperature rise above
ambient. In this case we have (maximum ambient = 70°C)
plus (junction temperature rise = 33°C) is equal to 103°C.
This is well below the maximum operating junction tem-
perature of 125°C for the LT1123.
The power rating for R
D
can be found from the plot of
Figure 8 using V
IN
= 7V and R
D
= 47. From the plot, R
D
should be sized to dissipate a minimum of 1/2W.
The worst-case power dissipation, for normal operation,
in the MJE1123 will be equal to:
(V
INMAX
– V
OUT
)(I
OUTMAX
) = (7V – 5V)(1.5A) = 3W
The maximum operating junction temperature of the
MJE1123 is 150°C. The difference between the maximum
operating junction temperature of 150°C and the maxi-
mum ambient temperature of 70°C is 80°C. The device
must be mounted to a heat sink which is sized such that the
thermal resistance from the junction of the MJE1123 to
ambient is less than 80°C/3W = 26.7°C/W.
It is recommended that the LT1123 be thermally coupled
to the MJE1123 so that the thermal limit circuit of the
LT1123 can protect both devices. In this case the ambient
temperature for the LT1123 will be equal to the tempera-
ture of the heat sink. The heat sink temperature, under
normal operating conditions, will have to be limited such
that the maximum operating junction temperature of the
LT1123 is not exceeded.
Refer to Linear Technology’s list of Suggested Manufac-
turers of Specialized Components for information on
where to find the required heat sinks, resistors and capaci-
tors. This listing is available through Linear Technology’s
marketing department.
Lia \I ll L7LJDWW
11
LT1123
1123fb
Isolated Feedback for Switching Regulators 5V/2A Regulator with Remote Sensing
5V Regulator with Antisat Miminizes
Ground Pin Current in Dropout
TYPICAL APPLICATIO S
U
SWITCHING
REGULATOR
VIN
1k
5V
OUTPUT
LT1123 TA03
DRIVE
LT1123
GND
FB
LT1123 TA08
600
100µF
OR LARGER
REMOTE
LOAD
75
7V
100
100
DRIVE
LT1123
GND
FB
+
MJE1123
5V
OUTPUT
LT1123 TA04
10µF
ALUM
1N4148
1N4148
1k
V
IN
620
2N2907
MJE1123
DRIVE
LT1123
GND
FB
+
M > _:_% 5V Shunl Regulalnr ur Va 3%? 22L; % r4 12 L7LJUEAR
12
LT1123
1123fb
5V/1A Regulator with Shutdown
TYPICAL APPLICATIO S
U
5V/1A
OUTPUT
LT1123 TA09
620
50k
6V
GEL CELL 10µF
ALUM
MPSA12
10µF
ALUM
HI = ON
LO = OFF
1/6
MM74C906
(OPEN COLLECTOR
OUTPUT) DRIVE
LT1123
GND
FB
+
+
MJE1123
5V Shunt Regulator or Voltage Clamp
Undervoltage Indicator On for VIN < (VZ +5V)
LT1123 TA12
1k
VIN
2.4k
VZ
470k
DRIVE
LT1123
GND
FB
LT1123 TA11
10µF
ALUM
1k
IRL510
DRIVE
LT1123
GND
FB
+
WTERNAL EATTERV ”HI I— we»— _L % MUM 52m GEL CELL : WINE V 2m .u. -'I-): .u. Adjuslin Vw > Vnur .u. L7LJDWW
13
LT1123
1123fb
Battery Backup Regulator
5V OUTPUT
LT1123 TA07
10µF
ALUM
620620
10µF
ALUM
EXTERNAL
POWER
INTERNAL
BATTERY
6V
GEL CELL
1N4148 1N4148
10µF
ALUM
20
DRIVE
LT1123
GND
FB
+
+
MJE1123 MJE1123
+
TYPICAL APPLICATIO S
U
Adjusting VOUT
Adjusting VOUT
V
OUT
*
LT1123 TA13
10µF
ALUM
V
IN
> V
OUT
620
R
D
V
Z
*V
OUT
= (5V + V
Z
)
DRIVE
LT1123
GND
FB
+
MJE1123
V
OUT
*
LT1123 TA14
10µF
ALUM
V
IN
> V
OUT
620
R
D
R
X
I
FB
*V
OUT
= (5V + (I
FB
• R
X
))
I
FB
300µA
DRIVE
LT1123
GND
FB
+
MJE1123
14 L7LJUEAR
14
LT1123
1123fb
ST Package
3-Lead Plastic SOT-223
(Reference LTC DWG # 05-08-1630)
U
PACKAGE DESCRIPTIO
.114 – .124
(2.90 – 3.15)
.248 – .264
(6.30 – 6.71)
.130 – .146
(3.30 – 3.71)
.264 – .287
(6.70 – 7.30)
.0905
(2.30)
BSC
.033 – .041
(0.84 – 1.04)
.181
(4.60)
BSC
.024 – .033
(0.60 – 0.84)
.071
(1.80)
MAX
10°
MAX
.012
(0.31)
MIN
.0008 – .0040
(0.0203 – 0.1016)
10° – 16°
.010 – .014
(0.25 – 0.36)
10° – 16°
RECOMMENDED SOLDER PAD LAYOUT
ST3 (SOT-233) 0502
.129 MAX
.059 MAX
.059 MAX
.181 MAX
.039 MAX
.248 BSC
.090
BSC
050 u 27) 350 L7LJDWW H H H fluwcomoum H H H {1270)LEADD‘MENS‘0N MAX e m 0405:0375) J L m magnum} m 115MHz“) 14a : mm {W} \e ma NDM mnmmannn mmsm by Lmaav Tecunomgy ampammn \s behaved m he mums and vehame Hnwevev nu vesponsmmy \5 assumed my K5 use Lmeaflenhna‘nqy Cammanon makes nu vepvesanr mmn mama mtemannemnn m mums asaesumen nevem \wll nm mmnga on exusunq patent mm:
15
LT1123
1123fb
U
PACKAGE DESCRIPTIO
Z Package
3-Lead Plastic TO-92 (Similar to TO-226)
(Reference LTC DWG # 05-08-1410)
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of circuits as described herein will not infringe on existing patent rights.
.050
(1.27)
BSC
.060 ± .005
(1.524± 0.127)
DIA
.90
(2.286)
NOM
.180 ± .005
(4.572 ± 0.127)
.180 ± .005
(4.572 ± 0.127)
.500
(12.70)
MIN
.050
(1.270)
MAX
UNCONTROLLED
LEAD DIMENSION
.016 ± .003
(0.406 ± 0.076)
5°
NOM
.015 ± .002
(0.381 ± 0.051)
.060 ± .010
(1.524 ± 0.254)
10° NOM
.140 ± .010
(3.556 ± 0.127)
Z3 (TO-92) 0801
321
.098 +.016/–.04
(2.5 +0.4/–0.1)
2 PLCS
TO-92 TAPE AND REEL
REFER TO TAPE AND REEL SECTION OF
LTC DATA BOOK FOR ADDITIONAL INFORMATION
-IH| II- «9E Loam LEVEL J_ T \l ‘mv ‘PACKS WILL SHARE CURRENT LinearTechnology 0 McCarthy BLvd ‘ WW 1408) 43271900 - FAX (403) 43470 poration 035-7417 www.mear cum LT LWL/LT 0505 REV a - PRLNY L7LJHE/AR 7UNEAR TEcHNmosv conpoa
16
LT1123
1123fb
© LINEAR TECHNOLOGY CORPORATION 1992
LT/LWI/LT 0505 REV B • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
FAX: (408) 434-0507
www.linear.com
TYPICAL APPLICATIO S
U
5V/1A Regulator with Shutdown 5V Regulator Powered by Multiple Battery Packs*
5V/1A OUTPUT
LT1123 TA10
620
1M
6V
GEL CELL
10µF
ALUM
Si9400DY*
HI = ON
LO = OFF 1/6
MM74C906
(OPEN COLLECTOR
OUTPUT) 68
*P-CHANNEL, LOGIC LEVEL
DRIVE
LT1123
GND
FB
+
5-CELL NiCd
BATTERY PACK
(6V)
R1
1.5k
R2
820
5V/1A
OUTPUT
MJE1123MJE1123MJE1123
10µF
10V
10µF
10V
LT1123 TA06
*PACKS WILL SHARE CURRENT
DRIVE
LT1123
GND
FB
+
10µF
10V R5
1.5k
R3
1.5k
R6
820
R4
820
+
10µF
10V
+
+
RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS
LT1083/4/5 7.5A, 5A, 3A Low Dropout Positive Regulators 1.5V Dropout Voltage, 0.1% Load Regulator, 1.25V
REF
LT1117 800mA Low Dropout Regulator SOT-223 Package, 0.4% Load Regulator
LT1121 150mA, Low Dropout LDO 0.4V Dropout Voltage, I
Q
= 30µA
LT1761 100mA, Low Noise LDO 300mV Dropout Voltage, I
Q
= 20µA
LT1763 1.5A, Low Noise, Fast Transient Response LDO Optimized for Hot Response

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