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LT1123 Low Dropout Regulator Driver
FEATURES

DESCRIPTIO
Extremely Low Dropout Low Cost Fixed 5V Output, Trimmed to 1% 700A Quiescent Current 1mV Line Regulation 5mV Load Regulation Thermal Limit 4A Output Current Guaranteed Available in a 3-Pin TO-92 Package
The LT(R)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 700A. 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
+
10F*
620
20 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)
10F
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 WW U W
(Note 1)
Drive Pin Voltage (VDRIVE to Ground) ..................... 30V Feedback Pin Voltage (VFB to Ground) .................... 30V Operating Junction Temperature Range ... 0C to 125C
Storage Temperature Range ................. -65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
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 JA AT TAB 20C/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 TA = 25C.
PARAMETER Feedback Voltage CONDITIONS IDRIVE = 10mA, TJ = 25C 5mA IDRIVE 100mA 3V VDRIVE 20V Feedback Pin Bias Current Drive Current VFB = 5.00V, 2V VDRIVE 15V VFB = 5.20V, 2V VDRIVE 15V VFB = 4.80V, VDRIVE = 3V VFB = 0.5V, VDRIVE = 3V, 0C TJ 100C IDRIVE = 10mA, VFB = 4.5V IDRIVE = 125mA, VFB = 4.5V 5V < VDRIVE < 20V IDRIVE = 10 to 100mA

Drive Pin Saturation Voltage Line Regulation Load Regulation Temperature Coefficient of VOUT
Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired.
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BOTTOM VIEW
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ORDER PART NUMBER LT1123CZ
DRIVE
FB
GND
Z PACKAGE 3-LEAD TO-92 PLASTIC TJMAX = 125C, JA = 220C/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 A 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 (A)
VFB = 5V
FEEDBACK PIN BIAS CURRENT (A)
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 (A)
400
DRIVE PIN VOLTAGE (V)
OUTPUT VOLTAGE (V)
300 TJ = 125C TJ = 25C 100 TJ = 0C 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 (450A 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. 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
UW
LT1123 G01
Minimum Drive Pin Current vs Temperature
200 VDRIVE = 3V 500 150 400 300 200 100 0 0 25 50 75 100 TEMPERATURE (C) 125 0
Drive Current vs Feedback Pin Voltage
VDRIVE = 3V TJ = 125C
TJ = 25C
100 TJ = -50C 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
VFB = 4.5V 2.0 TJ = 0C 1.5 TJ = 125C 1.0 TJ = 25C
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 VBE 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 (VFB): 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 (300A 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 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
4
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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 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.
1123fb
LT1123
APPLICATIO S I FOR ATIO U
VIN RB 620 MJE1123 RD DRIVE FB LT1123 VOUT = 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 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 620. For circuits that are required to run at junction temperatures in excess of 100C the recommended value of RB is 300.
W
UU
+
GND 10F ALUM
LT1123 F01
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 25C, Dropout Voltage vs Temperature (Figure 3) clearly shows that the dropout voltage is nearly constant over a wide temperature range.
1123fb
5
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 0.13V 0.25V 0.2V 0.45V 0.3V 0.25V 0.4V 0.35V 0.75V
1.0 BASED ON MJE1123 SPECS
DROPOUT VOLTAGE (V)
0.75 IDRIVE = 120mA 0.50 IDRIVE = 20mA 0.25 IDRIVE = 50mA 0
0
1
2 3 OUTPUT CURRENT (A)
4
LT1123 F02
Figure 2. Maximum Dropout Voltage
0.75 0.65
DROPOUT VOLTAGE (V)
0.55 IC = 4A, IB = 0.12A 0.45 0.35 IC = 2A, IB = 0.05A 0.25 0.15 0.05 20 60 80 40 100 CASE TEMPERATURE (C) 120 IC = 1A, IB = 0.02A
RD
LT1123 F03
Figure 3. Dropout Voltage vs Temperature
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
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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 100 IDRIVE = 50mA IDRIVE = 120mA 10 5 6 7 8 9 10 11 12 13 14 15 VIN
LT1123 F04
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UU
Figure 4. RD Resistor Value
1123fb
LT1123
APPLICATIO S I FOR ATIO U
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 transistors is plotted. The spread of current limit is reasonably well controlled.
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 4.00 4.25 4.50 4.75 5.00 5.25 5.50 5.75 6.00 OUTPUT CURRENT (A)
LT1123 F05
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: VBE = 0.95V at IC = 1A VDRIVE = 1.75V For 50mA drive current assume: VBE = 1.2V at IC = 2A VDRIVE = 1.9V For 120mA drive current assume: VBE = 1.4V at IC = 4A VDRIVE = 2.1V The RD Selection Chart lists the recommended values for RD for the most useful range of input voltage and output current. The chart includes a number for power dissipation for the LT1123 and RD.
RD Selection Chart
INPUT VOLTAGE 5.5V OUTPUT CURRENT: DROPOUT VOLTAGE: RD Power (LT1123) Power (RD) RD Power (LT1123) Power (RD) RD Power (LT1123) Power (RD) RD Power (LT1123) Power (RD) RD Power (LT1123) Power (RD) RD Power (LT1123) Power (RD) 0A to 1A 0.3V 120 0.05W 0.12W 150 0.05W 0.13W 180 0.06W 0.16W 240 0.06W 0.17W 270 0.20W 0.07W 330 0.22W 0.07W 0A to 2A 0.4V 43 0.14W 0.32W 51 0.15W 0.35W 75 0.14W 0.36W 91 0.15W 0.42W 110 0.16W 0.47W 130 0.17W 0.52W 0A to 4A 0.75V -- -- -- 20 0.37W 0.76W 27 0.38W 0.89W 36 0.38W 0.97W 43 0.41W 1.11W 51 0.43W 1.25W
6.0V
7.0V
8.0V
Figure 5. Short-Circuit Current for 30 Random Devices
9 8 7 6
9.0V
IC (A)
10.0V
NUMBER OF UNITS
Note that in some conditions RD 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.
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5 4 3 2 1 0 0 0.05 0.10 0.15
LT1123 F06
IB (A)
Figure 6. MJE1123 IC vs IB
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LT1123
APPLICATIO S I FOR ATIO U
For some operating conditions RD 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 RD = 0 the following formula may be used to calculate the maximum power dissipation in the LT1123. PD = (VIN - VBE)(IDRIVE) where: VIN = maximum input voltage VBE = emitter/base voltage of PNP IDRIVE = 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.
THERMAL RESISTANCE Package alone ............................................................................. 220C/W Package soldered into PC board with plated through holes only ................................................................................ 175C/W Package soldered into PC board with 1/4 sq. in. of copper trace per lead .................................................................................... 145C/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 .................................................... 160C/W Aavid type 5754 ................................................................ 135C/W
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 IC 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 RD. 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 RD. 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 RD and VIN. The graph was generated using the following formula:
PD
( VIN - VBE )2 ;R =
4RD
D
> 10
where: VBE = the emitter/base voltage of the PNP pass transistor (assumed to be 0.6V)
1k 0.1W 0.2W
RD ()
100 0.3W 0.4W 0.5W 0.7W 10 5 6 7 8 9 10 11 12 13 14 15 VIN (V)
LT1123 F07
Figure 7. Power in LT1123
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The maximum operating junction temperature of the LT1123 is 125C. The maximum operating ambient temperature will be equal to 125C minus the maximum junction temperature rise above ambient. The worst-case power dissipation in RD 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 RD versus supply
1123fb
LT1123
APPLICATIO S I FOR ATIO U
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 150C. The maximum operating ambient temperature for the MJE1123 will be equal to 150C 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 50C/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 Topside* 2500 sq. mm 1000 sq. mm 225 sq. mm 100 sq. mm 1000 sq. mm 1000 sq. mm Backside 2500 sq. mm 2500 sq. mm 2500 sq. mm 2500 sq. mm 1000 sq. mm 0 Board Area 2500 sq. mm 2500 sq. mm 2500 sq. mm 2500 sq. mm 1000 sq. mm 1000 sq. mm Thermal Resistance (Junction to Ambient) 50C/W 50C/W 58C/W 64C/W 57C/W 60C/W * Tab of device attached to topside copper
1k 0.25W 0.5W 1W RD () 100 2W
voltage and resistor value. Power dissipation can be calculated using the following formula: PRD where: VBE = emitter/base voltage of the PNP pass transistor VDRIVE = voltage at the drive pin of the LT1123 = VSAT of the drive pin in the worst case The worst-case power dissipation in the PNP pass transistor is simply equal to: PMAX = (VIN - VOUT)(IOUT) where VIN = Maximum VIN IOUT = Maximum IOUT The thermal resistance of the MJE1123 is equal to: 70C/W Junction to Ambient (no heat sink) 1.67C/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.
( VIN - VBE - VDRIVE )2 =
R
10 5
6
7
8
9
10 11 12 13 14 15 VIN (V)
LT1123 F08
Figure 8. Power in RD
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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 accomplished 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
1123fb
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LT1123
APPLICATIO S I FOR ATIO U
Assuming a thermal resistance of 150C/W, the maximum junction temperature rise above ambient will be equal to (PMAX)(150C/W) = 33C. The maximum operating junction temperature will be equal to the maximum ambient temperature plus the junction temperature rise above ambient. In this case we have (maximum ambient = 70C) plus (junction temperature rise = 33C) is equal to 103C. This is well below the maximum operating junction temperature of 125C for the LT1123. The power rating for RD can be found from the plot of Figure 8 using VIN = 7V and RD = 47. From the plot, RD 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: (VINMAX - VOUT)(IOUTMAX) = (7V - 5V)(1.5A) = 3W The maximum operating junction temperature of the MJE1123 is 150C. The difference between the maximum operating junction temperature of 150C and the maximum ambient temperature of 70C is 80C. 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 80C/3W = 26.7C/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 temperature 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 Manufacturers of Specialized Components for information on where to find the required heat sinks, resistors and capacitors. This listing is available through Linear Technology's marketing department.
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PNP. If the output of the regulator circuit can be shorted, heat sinking must be adequate to limit the rate of temperature rise of the power device to approximately 50C/ 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 < VIN < 7V IOUTMAX = 1.5A Max ambient temperature = 70C VOUT = 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 dropout at an output current of 2A. This satisfies our requirements. IDRIVE = 50mA 2. The next step is to determine the value of RD. Based on 50mA of drive current and a minimum input voltage of 5.5V, we can select RD from the graph of Figure 4. From the graph the value of RD is equal to 50, so we should use the next lowest 5% value which is 47. RD = 47 3. We can now look at the thermal requirements of the circuit. Worst-case power in the LT1123 will be equal to:
(VIN(MAX) - VBE )2
4RD
Given: VIN(MAX) = 7V, VBE = 0.6V, RD = 47 Then: PMAX (LT1123) = 0.22W.
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LT1123
TYPICAL APPLICATIO S
Isolated Feedback for Switching Regulators
VIN
600 MJE1123 75 7V
SWITCHING REGULATOR
DRIVE LT1123 FB GND
LT1123 TA03
U
5V/2A Regulator with Remote Sensing
DRIVE LT1123 FB
100 REMOTE LOAD
+
100F OR LARGER
1k
GND
100
LT1123 TA08
5V OUTPUT
5V Regulator with Antisat Miminizes Ground Pin Current in Dropout
MJE1123 620 VIN 2N2907 1k 1N4148 1N4148
DRIVE LT1123 FB GND 5V OUTPUT
+
10F ALUM
LT1123 TA04
1123fb
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LT1123
TYPICAL APPLICATIO S
5V/1A Regulator with Shutdown
6V GEL CELL
12
U
+
50k 620 MJE1123
10F ALUM
HI = ON LO = OFF 1/6 MPSA12 MM74C906 (OPEN COLLECTOR OUTPUT)
DRIVE LT1123 FB GND 5V/1A OUTPUT
+
10F ALUM
LT1123 TA09
Undervoltage Indicator On for VIN < (VZ +5V)
1k
2.4k VIN DRIVE LT1123 FB GND
LT1123 TA12
VZ
470k
5V Shunt Regulator or Voltage Clamp
1k IRL510 DRIVE LT1123 FB GND
LT1123 TA11
+
10F ALUM
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LT1123
TYPICAL APPLICATIO S
Battery Backup Regulator +
10F ALUM 620 MJE1123 1N4148 1N4148 620 MJE1123 EXTERNAL POWER 10F ALUM INTERNAL BATTERY 6V GEL CELL
U
+
20 DRIVE LT1123 FB GND 5V OUTPUT
+
10F ALUM
LT1123 TA07
Adjusting VOUT
620 VIN > VOUT RD DRIVE LT1123 FB GND IFB MJE1123 VOUT*
+
RX
10F ALUM
LT1123 TA14
*VOUT = (5V + (IFB * RX)) IFB 300A
Adjusting VOUT
620 VIN > VOUT RD DRIVE LT1123 FB GND *VOUT = (5V + VZ) VZ MJE1123 VOUT*
+
10F ALUM
LT1123 TA13
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LT1123
PACKAGE DESCRIPTIO U
ST Package 3-Lead Plastic SOT-223
(Reference LTC DWG # 05-08-1630)
.129 MAX .059 MAX .248 BSC .039 MAX .059 MAX .090 BSC .181 MAX .0905 (2.30) BSC .033 - .041 (0.84 - 1.04) RECOMMENDED SOLDER PAD LAYOUT 10 - 16 .071 (1.80) MAX 10 MAX .010 - .014 (0.25 - 0.36) 10 - 16 .024 - .033 (0.60 - 0.84) .181 (4.60) BSC .012 (0.31) MIN .0008 - .0040 (0.0203 - 0.1016)
ST3 (SOT-233) 0502
.248 - .264 (6.30 - 6.71) .114 - .124 (2.90 - 3.15)
.264 - .287 (6.70 - 7.30) .130 - .146 (3.30 - 3.71)
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LT1123
PACKAGE DESCRIPTIO U
Z Package 3-Lead Plastic TO-92 (Similar to TO-226)
(Reference LTC DWG # 05-08-1410)
.90 (2.286) NOM 5 NOM .015 .002 (0.381 0.051)
Z3 (TO-92) 0801
.060 .005 (1.524 0.127) DIA
.180 .005 (4.572 0.127)
.180 .005 (4.572 0.127)
.500 (12.70) MIN
.050 UNCONTROLLED (1.270) LEAD DIMENSION MAX
.050 (1.27) BSC
.016 .003 (0.406 0.076)
.060 .010 (1.524 0.254)
.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
3
2
1
.140 .010 (3.556 0.127)
10 NOM
1123fb
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 representation that the interconnection of circuits as described herein will not infringe on existing patent rights.
15
LT1123
TYPICAL APPLICATIO S
5V/1A Regulator with Shutdown 5V Regulator Powered by Multiple Battery Packs*
6V GEL CELL
1M
620
HI = ON LO = OFF
Si9400DY* 1/6 MM74C906 (OPEN COLLECTOR OUTPUT)
68
DRIVE LT1123 FB GND *P-CHANNEL, LOGIC LEVEL 5V/1A OUTPUT
RELATED PARTS
PART NUMBER LT1083/4/5 LT1117 LT1121 LT1761 LT1763 DESCRIPTION 7.5A, 5A, 3A Low Dropout Positive Regulators 800mA Low Dropout Regulator 150mA, Low Dropout LDO 100mA, Low Noise LDO 1.5A, Low Noise, Fast Transient Response LDO COMMENTS 1.5V Dropout Voltage, 0.1% Load Regulator, 1.25VREF SOT-223 Package, 0.4% Load Regulator 0.4V Dropout Voltage, IQ = 30A 300mV Dropout Voltage, IQ = 20A Optimized for Hot Response
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
U
5-CELL NiCd BATTERY PACK (6V)
+
R1 1.5k
10F 10V
+
R3 1.5k
10F 10V
+
R5 1.5k
10F 10V
MJE1123 R2 820 R4 820
MJE1123 R6 820
MJE1123
+
10F ALUM
LT1123 TA10
DRIVE LT1123 FB GND
+
10F 10V
LT1123 TA06
5V/1A OUTPUT
*PACKS WILL SHARE CURRENT
1123fb LT/LWI/LT 0505 REV B * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 1992

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