LM2991 Negative Low Dropout Adjustable Regulator General Description Features The LM2991 is a low dropout adjustable negative regulator with a output voltage range between -3V to -24V. The LM2991 provides up to 1A of load current and features a On /Off pin for remote shutdown capability. The LM2991 uses new circuit design techniques to provide a low dropout voltage, low quiescent current and low temperature coefficient precision reference. The dropout voltage at 1A load current is typically 0.6V and a guaranteed worst-case maximum of 1V over the entire operating temperature range. The quiescent current is typically 1 mA with a 1A load current and an input-output voltage differential greater than 3V. A unique circuit design of the internal bias supply limits the quiescent current to only 9 mA (typical) when the regulator is in the dropout mode (VOUT - VIN 3V). The LM2991 is short-circuit proof, and thermal shutdown includes hysteresis to enhance the reliability of the device when inadvertently overloaded for extended periods. The LM2991 is available in 5-lead TO-220 and TO-263 packages and is rated for operation over the automotive temperature range of -40C to +125C. Mil-Aero versions are also available. Output voltage adjustable from -3V to -24V, typically -2V to -25V Output current in excess of 1A Dropout voltage typically 0.6V at 1A load Low quiescent current Internal short circuit current limit Internal thermal shutdown with hysteresis TTL, CMOS compatible ON/OFF switch Functional complement to the LM2941 series Applications Post switcher regulator Local, on-card, regulation Battery operated equipment Typical Application 1126001 VOUT = VREF (1 + R2/R1) *Required if the regulator is located further than 6 inches from the power supply filter capacitors. A 1 F solid tantalum or a 10 F aluminum electrolytic capacitor is recommended. **Required for stability. Must be at least a 10 F aluminum electrolytic or a 1 F solid tantalum to maintain stability. May be increased without bound to maintain regulation during transients. Locate the capacitor as close as possible to the regulator. The equivalent series resistance (ESR) is critical, and should be less than 10 over the same operating temperature range as the regulator. (c) 2010 National Semiconductor Corporation 11260 www.national.com LM2991 Negative Low Dropout Adjustable Regulator May 18, 2010 LM2991 Connection Diagrams 1126009 1126002 Front View TO-220, 5-Lead, Straight See NS Package Number T05A Front View TO-220 Flow LB03, 5-Lead, Staggered Bend See NS Package Number T05D 1126011 Top View TO263, 5-Lead, Surface-Mount See NS Package TS5B 1126029 1126030 Top View 16-Lead, Ceramic, Dual-in-Line See NS Package Number J16A Top View 16-Lead, Ceramic, Surface-Mount See NS Package Number WG16A Ordering Information Order Number Package Type NSC Package Package Marking Supplied As LM2991S 5-Lead TO-263 TS5B LM2991S Rail of 45 LM2991SX 5-Lead TO-263 TS5B LM2991S Reel of 500 LM2991T 5-Pin TO-220 T05A LM2991T Rail of 45 LM2991T / LB03 5-Pin TO-220, Stagger T05D LM2991T Rail of 45 LM2991J-QML 16-Pin CERDIP J16A 5962-9650501QEA - LM2991J-QMLV 16-Pin CERDIP J16A 5962-9650501VEA - LM2991WG-QML 16-Lead CERPACK WG16A 5962-9650501QXA Tray of 42 www.national.com 2 If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Input Voltage ESD Susceptibility (Note 2) Power Dissipation (Note 3) Operating Ratings -26V to +0.3V 2 kV Internally limited 125C -65C to +150C 230C (Note 1) Junction Temperature Range (TJ) Maximum Input Voltage (Operational) -40C to +125C -26V Electrical Characteristics VIN = -10V, VO = -3V, IO = 1A, CO = 47 F, R1 = 2.7 k, TJ = 25C, unless otherwise specified. Boldface limits apply over the entire operating junction temperature range. Parameter Conditions 5 mA IO 1A Reference Voltage Typical (Note 4) Min Max -1.210 -1.234 -1.186 -1.27 -1.15 5 mA IO 1A, VO - 1V VIN -26V Output Voltage Range -2 VIN = -26V -25 -3 -24 Units V V Line Regulation IO = 5 mA, VO - 1V VIN -26V 0.004 0.04 %/V Load Regulation 50 mA IO 1A 0.04 0.4 % IO = 0.1A, VO 100 mV 0.1 0.2 0.3 V IO = 1A, VO 100 mV 0.6 0.8 1 V IO 1A 0.7 5 mA 16 50 mA Dropout Voltage Quiescent Current Dropout Quiescent Current VIN = VO, IO 1A Ripple Rejection Vripple = 1 Vrms, f ripple = 1 kHz, IO = 5 mA 60 Output Noise 10 Hz - 100 kHz, IO = 5 mA 200 (VOUT: ON) 1.2 (VOUT: OFF) 1.3 VON/OFF = 0.8V (VOUT: ON) 0.1 10 VON/OFF = 2.4V (VOUT: OFF) 40 100 Output Leakage Current VIN = -26V, VON/OFF = 2.4V, VOUT = 0V 60 Current Limit VOUT = 0V 2 ON /OFF Input Voltage ON /OFF Input Current 50 dB 450 0.8 2.4 250 1.5 V V A A A Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: Human body model, 100 pF discharged through a 1.5 k resistor. Note 3: The maximum power dissipation is a function of TJmax, JA and TA. The maximum allowable power dissipation at any ambient temperature is PD = (TJmax - TA)/JA. If this dissipation is exceeded, the die temperature will rise above 125C and the LM2991 will go into thermal shutdown. For the LM2991, the junction-to-ambient thermal resistance is 53C/W for the TO-220, 73C/W for the TO-263, and junction-to-case thermal resistance is 3C. If the TO-263 package is used, the thermal resistance can be reduced by increasing the PC board copper area thermally connected to the package. Using 0.5 square inches of copper area, JA is 50C/W; with 1 square inch of copper area, JA is 37C/W; and with 1.6 or more square inches of copper area, JA is 32C/W. Note 4: Typicals are at TJ = 25C and represent the most likely parametric norm. 3 www.national.com LM2991 Junction Temperature (TJmax) Storage Temperature Range Lead Temperature (Soldering, 10 sec.) Absolute Maximum Ratings (Note 1) LM2991 Typical Performance Characteristics Dropout Voltage Normalized Output Voltage 1126013 1126014 Output Voltage Output Noise Voltage 1126015 1126016 Quiescent Current Maximum Output Current 1126017 www.national.com 1126018 4 LM2991 Line Transient Response Load Transient Response 1126019 1126020 Maximum Output Current Ripple Rejection 1126022 1126021 Output Impedance ON /OFF Control Voltage 1126023 1126024 5 www.national.com LM2991 Adjust Pin Current Low Voltage Behavior 1126026 1126025 Maximum Power Dissipation (TO-220) Maximum Power Dissipation (TO-263) (See Note 3) 1126028 1126027 www.national.com 6 EXTERNAL CAPACITORS Like any low-dropout regulator, external capacitors are required to stabilize the control loop. These capacitors must be correctly selected for proper performance. CERAMIC BYPASS CAPACITORS Many designers place distributed ceramic capacitors whose value is in the range of 1000 pF to 0.1 F at the power input pins of the IC's across a circuit board. These can cause reduced phase margin or oscillations in LDO regulators. The advent of multi-layer boards with dedicated power and ground planes has removed the trace inductance that (previously) provided the necessary "de-coupling" to shield the output of the LDO from the effects of bypass capacitors. These capacitors should be avoided if possible, and kept as far away from the LDO output as is practical. INPUT CAPACITOR An input capacitor is required if the regulator is located more than 6 inches from the input power supply filter capacitor (or if no other input capacitor is present). A solid Tantalum or ceramic capacitor whose value is at least 1 F is recommended, but an aluminum electrolytic ( 10 F) may be used. However, aluminum electrolytic types should not be used in applications where the ambient temperature can drop below 0C because their internal impedance increases significantly at cold temperatures. MINIMUM LOAD A minimum load current of 500 A is required for proper operation. The external resistor divider can provide the minimum load, with the resistor from the adjust pin to ground set to 2.4 k. OUTPUT CAPACITOR The output capacitor must meet the ESR limits shown in Figure 1, which means it must have an ESR between about 25 m and 10. SETTING THE OUTPUT VOLTAGE The output voltage of the LM2991 is set externally by a resistor divider using the following equation: VOUT = VREF x (1 + R2/R1) - (IADJ x R2) where VREF = -1.21V. The output voltage can be programmed within the range of -3V to -24V, typically an even greater range of -2V to -25V. The adjust pin current is about 60 nA, causing a slight error in the output voltage. However, using resistors lower than 100 k makes the error due to the adjust pin current negligible. For example, neglecting the adjust pin current, and setting R2 to 100 k and VOUT to -5V, results in an output voltage error of only 0.16%. ON/OFF PIN The LM2991 regulator can be turned off by applying a TTL or CMOS level high signal to the ON/OFF pin. The impedance of the voltage source driving the ON/OFF pin should be low enough to source the ON/OFF pin input current to meet the OFF threshold voltage level, 100 A maximum at 2.4V. If the ON/OFF function is not needed, the pin should be connected to Ground. The ON/OFF pin should not be left floating, as this is not a guaranteed operating condition. See the Adjustable Current Sink Application, Figure 3 1126005 FIGURE 1. Output Capacitor ESR Range A solid Tantalum (value 1 F) is the best choice for the output capacitor. An aluminum electrolytic ( 10 F) may be used if the ESR is in the stable range. It should be noted that the ESR of a typical aluminum electrolytic will increase by as much as 50X as the temperature is reduced from 25C down to -40C, while a Tantalum will exhibit an ESR increase of about 2X over the same range. For this and other reasons, aluminum electrolytics should not be used in applications where low operating temperatures occur. The lower stable ESR limit of 25 m means that ceramic capacitors can not be used directly on the output of an LDO. A ceramic ( 2.2 F) can be used on the output if some external resistance is placed in series with it (1 recommended). Di- FORCING THE OUTPUT POSITIVE Due to an internal clamp circuit, the LM2991 can withstand positive voltages on its output. If the voltage source pulling the output positive is DC, the current must be limited to 1.5A. A current over 1.5A fed back into the LM2991 could damage the device. The LM2991 output can also withstand fast positive voltage transients up to 26V, without any current limiting of the source. However, if the transients have a duration of over 1 ms, the output should be clamped with a Schottky diode to ground. 7 www.national.com LM2991 electric types X7R or X5R must be used if the temperature range of the application varies more than 25 from ambient to assure the amount of capacitance is sufficient. Application Hints LM2991 Typical Applications 1126006 FIGURE 2. Fully Isolated Post-Switcher Regulator 1126010 FIGURE 3. Adjustable Current Sink www.national.com 8 LM2991 1126008 Equivalent Schematic 9 www.national.com LM2991 Physical Dimensions inches (millimeters) unless otherwise noted 5-Lead T0-220, Straight Leads (T) Order Number LM2991T NS Package Number T05A 5-Lead T0-220, Bent, Staggered Leads (T) Order Number LM2991T Flow LBO3 NS Package Number T05D www.national.com 10 LM2991 TO-263 5-Lead Plastic Surface Mount Package Ordering Number LM2991S NS Package Number TS5B 16-Lead Ceramic Dual-in-Line Package Order Number LM2991J-QMLV 5962-9650501VEA Order Number LM2991J-QML 5962-9650501QEA NS Package Number J16A 11 www.national.com LM2991 16-Lead Ceramic Surface-Mount Package Order Number LM2991WG-QML 5962-9650501QXA NS Package Number WG16A www.national.com 12 LM2991 Notes 13 www.national.com LM2991 Negative Low Dropout Adjustable Regulator Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH(R) Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise(R) Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagicTM www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise(R) Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION ("NATIONAL") PRODUCTS. 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