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Final Electrical Specifications
LT1761 Series 100mA, Low Noise, Low Dropout Micropower Regulators in SOT-23
May 1999
FEATURES
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DESCRIPTION
The LT (R)1761 series are micropower, low noise, low dropout regulators. The devices are capable of supplying 100mA of output current with a dropout voltage of 300mV. Designed for use in battery-powered systems, the low quiescent current, 20A operating and < 1A in shutdown, make them an ideal choice. Quiescent current is well controlled; it does not rise in dropout as it does with many other regulators. Other features of the LT1761 regulators include low output noise. With the addition of an external 0.01F bypass capacitor, output noise drops to 20VRMS over a 10Hz to 100kHz bandwidth. The LT1761 regulators are capable of operating with small capacitors and are stable with output capacitors as low as 1F. Small ceramic capacitors can be used without the series resistance required by other regulators. Internal protection circuitry includes reverse battery protection, current limiting, thermal limiting and reverse current protection. The parts come in fixed output voltages of 2.5V, 3V, 3.3V and 5V, and as an adjustable device with a 1.22V reference voltage. The LT1761 regulators are available in the 5-lead SOT-23 package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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SOT-23 Package Low Quiescent Current: 20A Zero Quiescent Current Shutdown State Low Dropout Voltage: 300mV Output Current: 100mA Low Noise: 20VRMS (10Hz to 100kHz) No Protection Diodes Needed Fixed Output Voltages: 2.5V, 3V, 3.3V, 5V Adjustable Output from 1.22V to 20V Stable with 1F Output Capacitor Stable with Aluminum, Tantalum or Ceramic Capacitors Reverse Battery Protection No Reverse Current Overcurrent and Overtemperature Protected
APPLICATIONS
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Cellular Phones Pagers Battery-Powered Systems Frequency Synthesizers
TYPICAL APPLICATION
3.3V Low Noise Supply with Shutdown
DROPOUT VOLTAGE (mV)
400 350
VIN 3.7V TO 20V
+
IN 1F SHDN GND
OUT LT1761-3.3 BYP 0.01F
+
3.3V AT 100mA 20VRMS NOISE (TYP) 10F
1761 TA01
300 250 200 150 100 50 0 0 10 20 30 40 50 60 70 80 90 100 OUTPUT CURRENT (mA)
1761 TA02
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 its circuits as described herein will not infringe on existing patent rights.
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Dropout Voltage
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LT1761 Series
ABSOLUTE MAXIMUM RATINGS
IN Pin Voltage ........................................................ 20V OUT Pin Voltage .................................................... 20V Input to Output Differential Voltage ....................... 20V ADJ Pin Voltage ...................................................... 7V BYP Pin Voltage.................................................... 0.6V
PACKAGE/ORDER INFORMATION
TOP VIEW IN 1 GND 2 BYP 3 4 ADJ 5 OUT IN 1 GND 2 SHDN 3 4 ADJ TOP VIEW 5 OUT
S5 PACKAGE 5-LEAD PLASTIC SOT-23
S5 PACKAGE 5-LEAD PLASTIC SOT-23
TJMAX = 150C, JA = 250C/ W SEE THE APPLICATIONS INFORMATION SECTION.
TJMAX = 150C, JA = 250C/ W SEE THE APPLICATIONS INFORMATION SECTION.
ORDER PART NUMBER LT1761CS5-BYP
S5 PART MARKING LTGC
ORDER PART NUMBER LT1761CS5-SD
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. (Note 2)
PARAMETER Regulated Output Voltage (Note 4) CONDITIONS LT1761-2.5 VIN = 3V, ILOAD = 1mA 3.5V < VIN < 20V, 1mA < ILOAD < 50mA 3.5V < VIN < 20V, 1mA < ILOAD < 100mA LT1761-3 VIN = 3.5V, ILOAD = 1mA 4V < VIN < 20V, 1mA < ILOAD < 50mA 4V < VIN < 20V, 1mA < ILOAD < 100mA
q q q q q q q q q q
LT1761-3.3 VIN = 3.8V, ILOAD = 1mA 4.3V < VIN < 20V, 1mA < ILOAD < 50mA 4.3V < VIN < 20V, 1mA < ILOAD < 100mA LT1761-5 VIN = 5.5V, ILOAD = 1mA 6V < VIN < 20V, 1mA < ILOAD < 50mA 6V < VIN < 20V, 1mA < ILOAD < 100mA VIN = 2V, ILOAD = 1mA, TJ = 25C 2V < VIN < 20V, 1mA < ILOAD < 50mA 2V < VIN < 20V, 1mA < ILOAD < 100mA
ADJ Pin Voltage (Note 3, 4)
LT1761
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(Note 1)
SHDN Pin Input Voltage ....................................... 20V Output Short-Circut Duration .......................... Indefinite Operating Junction Temperature Range .... 0C to 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
TOP VIEW IN 1 GND 2 SHDN 3 4 BYP 5 OUT
S5 PACKAGE 5-LEAD PLASTIC SOT-23
TJMAX = 150C, JA = 250C/ W SEE THE APPLICATIONS INFORMATION SECTION.
S5 PART MARKING LTGH
ORDER PART NUMBER LT1761CS5-2.5 LT1761CS5-3 LT1761CS5-3.3 LT1761CS5-5
S5 PART MARKING LTGD LTGE LTGF LTGG
MIN 2.465 2.435 2.415 2.960 2.930 2.900 3.250 3.230 3.190 4.935 4.900 4.850 1.205 1.190 1.170
TYP 2.5 2.5 2.5 3 3 3 3.3 3.3 3.3 5 5 5 1.220 1.220 1.220
MAX 2.535 2.565 2.575 3.040 3.070 3.090 3.350 3.370 3.400 5.065 5.100 5.120 1.235 1.250 1.260
UNITS V V V V V V V V V V V V V V V
LT1761 Series
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. (Note 2)
PARAMETER Line Regulation CONDITIONS LT1761-2.5 LT1761-3 LT1761-3.3 LT1761-5 LT1761(Note 3) LT1761-2.5 VIN = 3V to 20V, ILOAD = 1mA VIN = 3.5V to 20V, ILOAD = 1mA VIN = 3.8V to 20V, ILOAD = 1mA VIN = 5.5V to 20V, ILOAD = 1mA VIN = 2V to 20V, ILOAD = 1mA VIN = 3.5V, ILOAD = 1mA to 50mA VIN = 3.5V, ILOAD = 1mA to 50mA VIN = 3.5V, ILOAD = 1mA to 100mA VIN = 3.5V, ILOAD = 1mA to 100mA VIN = 4V, ILOAD = 1mA to 50mA VIN = 4V, ILOAD = 1mA to 50mA VIN = 4V, ILOAD = 1mA to 100mA VIN = 4V, ILOAD = 1mA to 100mA VIN = 4.3V, ILOAD = 1mA to 50mA VIN = 4.3V, ILOAD = 1mA to 50mA VIN = 4.3V, ILOAD = 1mA to 100mA VIN = 4.3V, ILOAD = 1mA to 100mA VIN = 6V, ILOAD = 1mA to 50mA VIN = 6V, ILOAD = 1mA to 50mA VIN = 6V, ILOAD = 1mA to 100mA VIN = 6V, ILOAD = 1mA to 100mA VIN = 2V, ILOAD = 1mA to 50mA VIN = 2V, ILOAD = 1mA to 50mA VIN = 2V, ILOAD = 1mA to 100mA VIN = 2V, ILOAD = 1mA to 100mA
q q q q q q
MIN
TYP 1 1 1 1 1 10 20
MAX 10 10 10 10 10 20 35 40 80 20 40 40 90 20 40 40 100 30 60 65 150 6 12 12 50 0.15 0.19 0.22 0.29 0.28 0.38 0.35 0.45 45 100 400 2 4 100 2.3 2 0.5 3 0.1
UNITS mV mV mV mV mV mV mV mV mV mV mV mV mV mV mV mV mV mV mV mV mV mV mV mV mV V V V V V V V V A A A mA mA VRMS nA V V V A A A dB
Load Regulation
q
LT1761-3
10
q
20
q
LT1761-3.3
10
q
20
q
LT1761-5
15
q
25
q
LT1761 (Note 3)
1
q
1
q
Dropout Voltage VIN = VOUT(NOMINAL) (Notes 5, 6)
ILOAD = 1mA ILOAD = 1mA ILOAD = 10mA ILOAD = 10mA ILOAD = 50mA ILOAD = 50mA ILOAD = 100mA ILOAD = 100mA
0.10
q
0.17
q
0.24
q
0.30
q q q q q q
GND Pin Current VIN = VOUT(NOMINAL) (Notes 5, 7)
ILOAD = 0mA ILOAD = 1mA ILOAD = 10mA ILOAD = 50mA ILOAD = 100mA COUT = 10F, CBYP = 0.01F, ILOAD = 100mA, BW = 10Hz to 100kHz (Notes 3, 8) ILOAD = 100mA VOUT = Off to On VOUT = On to Off VSHDN = 0V VSHDN = 20V VIN = 6V, VSHDN = 0V VIN - VOUT = 1V (Avg), VRIPPLE = 0.5VP-P, fRIPPLE = 120Hz, ILOAD = 50mA
20 55 230 1 2.2 20 30
Output Voltage Noise ADJ Pin Bias Current Minimum Input Voltage (Note 3) Shutdown Threshold SHDN Pin Current (Note 9) Quiescent Current in Shutdown Ripple Rejection
q q q q q
1.8 0.25 0.8 0.65 0 1 0.01 55 65
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LT1761 Series
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. (Note 2)
PARAMETER Current Limit Input Reverse Leakage Current Reverse Output Current (Note 10) CONDITIONS VIN = 7V, VOUT = 0V VIN = VOUT(NOMINAL) + 1V, VOUT = - 5% VIN = - 20V, VOUT = 0V LT1761-2.5 LT1761-2 LT1761-3.3 LT1761-5 LT1761 (Note 3) VOUT = 2.5V, VIN < 2.5V VOUT = 3V, VIN < 3V VOUT = 3.3V, VIN < 3.3V VOUT = 5V, VIN < 5V VOUT = 1.22V, VIN < 1.22V
q q
MIN 110
TYP 200
MAX
UNITS mA mA
1 10 10 10 10 5 20 20 20 20 10
mA A A A A A
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LT1761 regulators are tested and specified under pulse load conditions such that TJ TA. Note 3: The LT1761 (adjustable version) is tested and specified for these conditions with the ADJ pin connected to the OUT pin. Note 4: Operating conditions are limited by maximum junction temperature. The regulated output voltage specification will not apply for all possible combinations of input voltage and output current. When operating at maximum input voltage, the output current range must be limited. When operating at maximum output current, the input voltage range must be limited. Note 5: To satisfy requirements for minimum input voltage, the LT1761 (adjustable version) is tested and specified for these conditions with an
external resistor divider (two 250k resistors) for an output voltage of 2.44V. The external resistor divider will add a 5A DC load on the otuput. Note 6: Dropout voltage is the minimum input to output voltage differential needed to maintain regulation at a specified output current. In dropout, the output voltage will be equal to: VIN - VDROPOUT. Note 7: GND pin current is tested with VIN = VOUT(NOMINAL) and a current source load. This means the device is tested while operating in its dropout region. This is the worst-case GND pin current. The GND pin current will decrease slightly at higher input voltages. Note 8: ADJ pin bias current flows into the ADJ pin. Note 9: SHDN pin current flows into the SHDN pin. Note 10: Reverse output current is tested with the IN pin grounded and the OUT pin forced to the rated output voltage. This current flows into the OUT pin and out the GND pin.
PIN FUNCTIONS
IN (Pin 1): Input. Power is supplied to the device through the IN pin. A bypass capacitor is required on this pin if the device is more than six inches away from the main input filter capacitor. In general, the output impedance of a battery rises with frequency, so it is advisable to include a bypass capacitor in battery-powered circuits. A bypass capacitor in the range of 1F to 10F is sufficient. The LT1761 regulators are designed to withstand reverse voltages on the IN pin with respect to ground and the OUT pin. In the case of a reverse input, which can happen if a battery is plugged in backwards, the device will act as if there is a diode in series with its input. There will be no reverse current flow into the regulator and no reverse voltage will appear at the load. The device will protect both itself and the load. GND (Pin 2): Ground. SHDN (Pin 3, Fixed/-SD Devices): Shutdown. The SHDN pin is used to put the LT1761 regulators into a low power shutdown state. The output will be off when the SHDN pin is pulled low. The SHDN pin can be driven either by 5V logic or open-collector logic with a pull-up resistor. The pull-up resistor is required to supply the pull-up current of the open-collector gate, normally several microamperes, and the SHDN pin current, typically 1A. If unused, the SHDN pin must be connected to VIN. The device will not function if the SHDN pin is not connected. For the LT1761-BYP, the SHDN pin is internally connected to VIN. BYP (Pins 3/4, Fixed/-BYP Devices): Bypass. The BYP pin is used to bypass the reference of the LT1761 regulators to achieve low noise performance from the regulator. The BYP pin is clamped internally to 0.6V (one VBE) from ground. A small capacitor from the output to this pin will bypass the reference to lower the output voltage noise. A maximum value of 0.01F can be used for reducing output voltage noise to a typical 20VRMS over a 10Hz to 100kHz bandwidth. If not used, this pin must be left unconnected. ADJ (Pin 4, Adjustable Devices Only): Adjust Pin. For the adjustable LT1761, this is the input to the error amplifier. This pin is internally clamped to 7V. It has a bias current of 30nA which flows into the pin. The ADJ pin voltage is
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LT1761 Series
PIN FUNCTIONS
1.22V referenced to ground and the output voltage range is 1.22V to 20V. OUT (Pin 5): Output. The output supplies power to the load. A minimum output capacitor of 1F is required to prevent oscillations. Larger output capacitors will be required for applications with large transient loads to limit peak voltage transients. See the Applications Information section for more information on output capacitance and reverse output characteristics.
APPLICATIONS INFORMATION
The LT1761 series are 100mA low dropout regulators with micropower quiescent current and shutdown. The devices are capable of supplying 100mA at a dropout voltage of 300mV. Output voltage noise can be lowered to 20VRMS over a 10Hz to 100kHz bandwidth with the addition of a 0.01F reference bypass capacitor. Additionally, the reference bypass capacitor will improve transient response of the regulator, lowering the settling time for transient load conditions. The low operating quiescent current (20A) drops to less than 1A in shutdown. In addition to the low quiescent current, the LT1761 regulators incorporate several protection features which make them ideal for use in battery-powered systems. The devices are protected against both reverse input and reverse output voltages. In battery backup applications where the output can be held up by a backup battery when the input is pulled to ground, the LT1761-X acts like it has a diode in series with its output and prevents reverse current flow. Additionally, in dual supply applications where the regulator load is returned to a negative supply, the output can be pulled below ground by as much as 20V and still allow the device to start and operate. Adjustable Operation The adjustable version of the LT1761 has an output voltage range of 1.22V to 20V. The output voltage is set by the ratio of two external resistors as shown in Figure 1. The device servos the output to maintain the voltage at the ADJ pin at 1.22V referenced to ground. The current in R1 is then equal to 1.22V/R1 and the current in R2 is the current in R1 plus the ADJ pin bias current. The ADJ pin bias current, 30nA at 25C, flows through R2 into the ADJ pin. The output voltage can be calculated using the formula in Figure 1. The value of R1 should be less than 250k to minimize errors in the output voltage caused by the ADJ pin bias current. Note that in shutdown the output is turned off and the divider current will be zero.
IN VIN ADJ GND R1
1761 F01
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OUT
VOUT
+
R2
R2 VOUT = 1.22V 1+ + (IADJ )(R2) R1 VADJ = 1.22V IADJ = 30nA AT 25C OUTPUT RANGE = 1.22V TO 20V
Figure 1. Adjustable Operation
The adjustable device is tested and specified with the ADJ pin tied to the OUT pin for an output voltage of 1.22V. Specifications for output voltages greater than 1.22V will be proportional to the ratio of the desired output voltage to 1.22V: VOUT/1.22V. For example, load regulation for an output current change of 1mA to 100mA is -1mV typical at VOUT = 1.22V. At VOUT = 12V, load regulation is: (12V/1.22V)(-1mV) = - 9.8mV Bypass Capacitance and Low Noise Performance The LT1761 regulators may be used with the addition of a bypass capacitor from VOUT to the BYP pin to lower output voltage noise. A good quality low leakage capacitor is recommended. This capacitor will bypass the reference of the regulator, providing a low frequency noise pole. The noise pole provided by this bypass capacitor will lower the output voltage noise to as low as 20VRMS with the addition of a 0.01F bypass capacitor. Using a bypass capacitor has the added benefit of improving transient response. With no bypass capacitor and a 10F output capacitor, a 10mA to 100mA load step will settle to within 1% of its final value in less than 100s. With the addition of a 0.01F bypass capacitor, the output will stay within
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LT1761 Series
APPLICATIONS INFORMATION
1% for a 10mA to 100mA load step. However, regulator start-up time is inversely proportional to the size of the bypass capacitor, slowing to 15ms with a 0.01F bypass capacitor and 10F output capacitor. Output Capacitance and Transient Response The LT1761 regulators are designed to be stable with a wide range of output capacitors. The ESR of the output capacitor affects stability, most notably with small capacitors. A minimum output capacitor of 1F with an ESR of 3 or less is recommended to prevent oscillations. The LT1761-X is a micropower device and output transient response will be a function of output capacitance. Larger values of output capacitance decrease the peak deviations and provide improved transient response for larger load current changes. Bypass capacitors, used to decouple individual components powered by the LT1761-X, will increase the effective output capacitor value. With larger capacitors used to bypass the reference (for low noise operation), larger values of output capacitors are needed. For 100pF of bypass capacitance, 2.2F of output capacitor is recommended. With a 330pF bypass capacitor or larger, a 3.3F output capacitor is recommended. Extra consideration must be given to the use of ceramic capacitors. Ceramic capacitors are manufactured with a variety of dielectrics, each with different behavior across temperature and applied voltage. The most common dielectrics used are Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are good for providing high capacitances in a small package, but exhibit strong voltage and temperature coefficients as shown in Figures 2 and 3. When used with a 5V regulator, a 10F Y5V capacitor can exhibit an effective value as low as 1F to 2F over the operating temperature range. The X5R and X7R dielectrics result in more stable characteristics and are more suitable for use as the output capacitor. The X7R type has better stability across temperature, while the X5R is less expensive and is available in higher values. Voltage and temperature coefficients are not the only sources of problems. Some ceramic capacitors have a piezoelectric response. A piezoelectric device generates voltage across its terminals due to mechanical stress, similar to the way a piezoelectric accelerometer or microphone works. For a ceramic capacitor the stress can be induced by vibrations in the system or thermal transients. The resulting voltages produced can cause appreciable amounts of noise, especially when a ceramic capacitor is used for noise bypassing. A ceramic capacitor produced Figure 4's trace in response to light tapping from a pencil. Similar vibration induced behavior can masquerade as increased output voltage noise.
20 0
CHANGE IN VALUE (%)
CHANGE IN VALUE (%)
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BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10F X5R
-20 -40 -60 Y5V -80
-100
0
2
4
8 6 10 12 DC BIAS VOLTAGE (V)
14
16
1761 F02
Figure 2. Ceramic Capacitor DC Bias Characteristics
40 20 0 -20 -40 -60 -80 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10F 50 25 75 0 TEMPERATURE (C) 100 125 Y5V X5R
-100 -50 -25
1761 G03
Figure 3. Ceramic Capacitor Temperature Characteristics
LT1761-5 COUT = 10F CBYP = 0.01F ILOAD = 100mA VOUT 500V/DIV
100ms/DIV
1761 F04
Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor
LT1761 Series
APPLICATIONS INFORMATION
Thermal Considerations The power handling capability of the device will be limited by the maximum rated junction temperature (125C). The power dissipated by the device will be made up of two components: 1. Output current multiplied by the input/output voltage differential: (IOUT)(VIN - VOUT), and 2. GND pin current multiplied by the input voltage: (IGND)(VIN). The GND pin current can be estimated using the GND Pin Current specification in the Electrical Characteristics table. Power dissipation will be equal to the sum of the two components listed above. The LT1761 series regulators have internal thermal limiting designed to protect the device during overload conditions. For continuous normal conditions, the maximum junction temperature rating of 125C must not be exceeded. It is important to give careful consideration to all sources of thermal resistance from junction to ambient. Additional heat sources mounted nearby must also be considered. For surface mount devices, heat sinking is accomplished by using the heat spreading capabilities of the PC board and its copper traces. Copper board stiffeners and plated through-holes can also be used to spread the heat generated by power devices. The following table lists thermal resistance for several different board sizes and copper areas. All measurements were taken in still air on 3/32" FR-4 board with one ounce copper.
Table 1. Measured Thermal Resistance
COPPER AREA TOPSIDE* 2500mm 1000mm 225mm 100mm
2 2
THERMAL RESISTANCE BOARD AREA 2500mm 2500mm 2500mm 2500mm
2 2 2 2
BACKSIDE 2500mm 2500mm 2500mm 2500mm
2 2 2 2
(JUNCTION-TO-AMBIENT) 125C/W 125C/W 130C/W 135C/W 150C/W
2 2
50mm2
2500mm2
2500mm2
*Device is mounted on topside.
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Calculating Junction Temperature Example: Given an output voltage of 3.3V, an input voltage range of 4V to 6V, an output current range of 0mA to 50mA and a maximum ambient temperature of 50C, what will the maximum junction temperature be? The power dissipated by the device will be equal to: IOUT(MAX)(VIN(MAX) - VOUT) + IGND(VIN(MAX)) where, IOUT(MAX) = 50mA VIN(MAX) = 6V IGND at (IOUT = 50mA, VIN = 6V) = 1mA So, P = 50mA(6V - 3.3V) + 1mA(6V) = 0.14W The thermal resistance will be in the range of 125C/W to 150C/W depending on the copper area. So the junction temperature rise above ambient will be approximately equal to: 0.14W(150C/W) = 21.2C The maximum junction temperature will then be equal to the maximum junction temperature rise above ambient plus the maximum ambient temperature or: TJMAX = 50C + 21.2C = 71.2C Protection Features The LT1761 regulators incorporate several protection features which make them ideal for use in battery-powered circuits. In addition to the normal protection features associated with monolithic regulators, such as current limiting and thermal limiting, the devices are protected against reverse input voltages, reverse output voltages and reverse voltages from output to input. Current limit protection and thermal overload protection are intended to protect the device against current overload conditions at the output of the device. For normal operation, the junction temperature should not exceed 125C. The input of the device will withstand reverse voltages of 20V. Current flow into the device will be limited to less than 1mA (typically less than 100A) and no negative voltage will appear at the output. The device will protect both itself
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LT1761 Series
APPLICATIONS INFORMATION
and the load. This provides protection against batteries which can be plugged in backward. The output of the LT1761-X can be pulled below ground without damaging the device. If the input is left open circuit or grounded, the output can be pulled below ground by 20V. For fixed voltage versions, the output will act like a large resistor, typically 500k or higher, limiting current flow to typically less than 100A. For adjustable versions, the output will act like an open circuit; no current will flow out of the pin. If the input is powered by a voltage source, the output will source the short-circuit current of the device and will protect itself by thermal limiting. In this case, grounding the SHDN pin will turn off the device and stop the output from sourcing the short-circuit current. The ADJ pin of the adjustable device can be pulled above or below ground by as much as 7V without damaging the device. If the input is left open circuit or grounded, the ADJ pin will act like an open circuit when pulled below ground and like a large resistor (typically 100k) in series with a diode when pulled above ground. In situations where the ADJ pin is connected to a resistor divider that would pull the ADJ pin above its 7V clamp voltage if the output is pulled high, the ADJ pin input current must be limited to less than 5mA. For example, a resistor divider is used to provide a regulated 1.5V output from the 1.22V reference when the output is forced to 20V. The top resistor of the resistor divider must be chosen to limit the current into the ADJ pin to less than 5mA when the ADJ pin is at 7V. The 13V difference between output and ADJ pin divided by the 5mA maximum current into the ADJ pin yields a minimum top resistor value of 2.6k. In circuits where a backup battery is required, several different input/output conditions can occur. The output voltage may be held up while the input is either pulled to ground, pulled to some intermediate voltage or is left open circuit. Current flow back into the output will follow the curve shown in Figure 5. When the IN pin of the LT1761-X is forced below the OUT pin or the OUT pin is pulled above the IN pin, input current will typically drop to less than 2A. This can happen if the input of the device is connected to a discharged (low voltage) battery and the output is held up by either a backup battery or a second regulator circuit. The state of the SHDN pin will have no effect on the reverse output current when the output is pulled above the input.
100 90 80 70 60 50 40 30 20 10 0 0 1 2 LT1761-5 345678 OUTPUT VOLTAGE (V) 9 10 LT1761-3.3 TJ = 25C vIN = 0V CURRENT FLOWS INTO OUTPUT PIN VOUT = VADJ (LT1761-BYP, -SD)
REVERSE OUTPUT CURRENT (A)
RELATED PARTS
PART NUMBER LT1120 LT1121 LT1129 LT1175 LT1521 LT1529 LT1611 LT1613 LTC1627 DESCRIPTION 125mA Low Dropout Regulator with 20A IQ 150mA Micropower Low Dropout Regulator 700mA Micropower Low Dropout Regulator 500mA Negative Low Dropout Micropower Regulator 300mA Low Dropout Micropower Regulator with Shutdown 3A Low Dropout Regulator with 50A IQ Inverting 1.4MHz Switching Regulator 1.4MHz Single-Cell Micropower DC/DC Converter High Efficiency Synchronous Step-Down Switching Regulator COMMENTS Includes 2.5V Reference and Comparator 30A IQ, SOT-223 Package 50A Quiescent Current 45A IQ, 0.26V Dropout Voltage, SOT-223 Package 15A IQ, Reverse Battery Protection 500mV Dropout Voltage 5V to - 5V at 150mA, Low Output Noise, SOT-23 Package SOT-23 Package, Internally Compensated Burst ModeTM Operation, Monolithic, 100% Duty Cycle
Burst Mode is a trademark of Linear Technology Corporation.
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
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LT1761-BYP LT1761-SD
LT1761-2.5 LT1761-3
1761 F05
Figure 5. Reverse Output Current
1761IA LT/TP 0599 2K REV A * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1999

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