LM2781TP

LM2781 Ultra-Low Ripple Switched Capacitor Voltage Inverter March 2003 LM2781 Ultra-Low Ripple Switched Capacitor Voltage Inverter General Description The LM2781 is a charge pump that inverts an input voltage in the range of 1.8V to 5.5V to the corresponding output voltage of -1.8V to -5.5V. With use of three or four low-cost external ceramic capacitors, the LM2781 can provide up to 50mA of output current. The LM2781 operates at a 210kHz typical switching frequency to minimize supply current (0.7mA typ.). Two charge pumps operate 180˚ out of phase to significantly reduce the output voltage ripple compared to that of traditional singlephase charge pump circuits. If larger output ripple can be tolerated, optional capacitor C2 may be omitted to reduce the cost and size of the solution. The LM2781 is available in an 8-pin Micro SMD chip-scale package. See also: LM2780, a slow clock (12kHz typ) version of this topology with extremely low supply current. Features n Inverts Input Voltage n Ultra-Low Voltage Ripple and 8Ω Typical Output Resistance with Two Flying Capacitors n Output Resistance of 18Ω Using One Flying Capacitor n 210kHz (typ.) Switching Frequency n 8-pin micro SMD Package (1.4mm x 1.4mm x 0.5mm) Applications n n n n n n Cellular Phones Pagers PDAs Op-Amp Power Supplies Interface Power Supplies Handheld Instruments Typical Application Circuit 20044701 © 2003 National Semiconductor Corporation DS200447 www.national.com LM2781 Connection Diagram 8-Bump Thin Micro SMD Package NS Package Number TPA08 20044702 Pin Description Pin A1 B1 C1 A2 C2 A3 B3 C3 Name C2+ VIN C1+ SD GND C2VOUT C1Description Secondary flying-capacitor connection Input voltage. Input range: 1.8V to 5.5V Primary flying-capacitor connection Shutdown pin logic input. Low = on, High = shutdown Ground Secondary flying-capacitor connection Output voltage. VOUT = -VIN Primary flying capacitor connection Ordering Information Order Number LM2781TP LM2781TPX Package Description TPA08: Thin Micro SMD TPA08: Thin Micro SMD Package Marking 12 12 Supplied as Tape and reel, 250 pcs. Tape and reel, 3000 pcs. www.national.com 2 LM2781 Absolute Maximum Ratings 2) (Notes 1, If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN pin: Voltage to GND SD pin: Voltage to GND (VIN Output Short-to-GND Duration (Note 3) Maximum Junction Temperature (TJ-MAX) Storage Temperature Range Pad Temperature (Soldering, 10 sec.) −0.3V to 5.8V -0.3V to + 0.3V), with 5.8V max 1 sec. 150˚C -65˚C to +150˚C 265˚C (Notes 2, 7) ESD Rating (Note 4) Human-body model Machine model 2kV 200V Operating Ratings (Notes 1, 2) Input Voltage Range Junction Temperature (TJ) Range Ambient Temperature (TA) Range (Note 5) 1.8V to 5.5V -40˚C to +90˚C -40˚C to +85˚C Thermal Properties Junction-to-Ambient Thermal Resistance (θJA), TPA08 Package (Note 6) 220˚C/W Electrical Characteristics Limits in standard typface are for TJ = 25˚C, and limits in boldface type apply over the full operating junction temperature range (-40˚C ≤ TJ ≤ +90˚C). Unless otherwise noted: specifications apply to the LM2781 Typical Application Circuit (pg. 1) with VIN = 5.0V, V(SD) = 0V, and CIN = COUT = C1 = C2 = 1µF. (Note 8) Symbol ROUT IQ ISD FSW VSD Parameter Output Resistance (Note 9) Supply Current Shutdown Supply Current Switching Frequency Shutdown Pin Logic Levels Normal Operation 1.8V ≤ VIN ≤ 5.5V Shutdown Mode 1.8V ≤ VIN ≤ 5.5V VOUTp-p PEFF Output Voltage Ripple Power Efficiency VIN = 5.0V, IOUT = 10mA VIN = 5.0V, IOUT = 10mA Conditions IOUT = 10mA, C2 = 1µF IOUT = 10mA, C2 removed No load V(SD) = 5.0V 100 0 0.5xVIN 10 92 Min Typ 8 18 0.7 0.1 210 Max 30 40 1.4 0.5 400 0.5 VIN V Units Ω Ω mA µA kHz mV % Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the Electrical Characteristics table. Note 2: All voltages are with respect to the potential at the GND pin. Note 3: VOUT may be shorted to GND for one second without damage when the ambient temperature is at or below 85˚C. Prolonged or repeated shorts of VOUT to GND may damage the device, and should be avoided. Note 4: The human-body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. The machine model is a 200pF capacitor discharged directly into each pin. Note 5: Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = 90˚C), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to-ambient thermal resistance of the part in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP - (θJA x PD-MAX). Note 6: Junction-to-ambient thermal resistance of the micro SMD package is highly application and board-layout dependent. Note 7: Max and Min limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm. Note 8: CIN, COUT, C1, and C2: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics. Note 9: Output resistance is a model for the voltage drop at the output, resulting from internal switch resistance, capacitor ESR, and charge pump charge transfer characteristics. Output voltage can be predicted with the following equation: VOUT = -[VIN - (IOUT x ROUT)] 3 www.national.com LM2781 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, CIN = COUT = C1 = C2 = 1µF. Output Voltage vs. Load Current Power Efficiency vs. Load Current 20044703 20044704 Output Resistance vs. Temperature (IOUT = 10mA) Supply Current vs. Temperature (IOUT = 0) 20044705 20044706 Switching Frequency vs. Temperature Output Voltage Ripple vs. Output Current 20044707 20044708 www.national.com 4 LM2781 Block Diagram 20044710 Operation Description The LM2781 is a switched capacitor voltage inverter that produces a negative output voltage from a positive input voltage. To minimize output voltage ripple, the LM2781 contains two charge pump inverters that operate 180˚ out of phase. These two charge pumps can be seen in the block diagram above. A basic switched capacitor inverter is a two-phase charge pump circuit that transfers charge from the input to the output using a flying capacitor. Charge Pump 1 in the block diagram will be used to illustrate the operation of a single charge pump. Disregard Charge Pump 2 in the diagram for now. In phase 1 (φ1), the CLK1A signal of Charge Pump 1 is high, and MOSFET switches S1A and S1B are ON. For Charge Pump 1, this is the charge phase, as the flying capacitor C1 is charged to the input voltage. In the second phase (φ2), the positive-charged plate of C1 is connected to GND through switch S1C, and the negative-charged plate is connected to the output through switch S1D. This results in the output voltage being approximately equal in magnitude to the input voltage, but opposite in sign (VOUT ~= - VIN). In this pump phase, charge stored on the flying capacitor supplies the current to the load and transfers charge to the output capacitor. This basic inverter has ripple in the output voltage. During the charge phase, the load current is supplied by the charge on the output capacitor. As charge is pulled off COUT, the voltage across the capacitor drops, and the magnitude of the output voltage ( |VOUT| ) falls. During the pump phase, the 5 flying capacitor transfers charge to the COUT, elevating both the voltage across the COUT and the magnitude of the output voltage. The LM2781 contains two charge pump inverters running 180˚ out of phase. Notice in the block diagram that the clock connections on Charge Pump 2 are opposite of the clock connections to Charge Pump 1. Thus, when Charge Pump 1 is in the charge phase, Charge Pump 2 is in the pump phase, and vice-versa. With this double-pump technology, there is always a flying capacitor connected to the output. This results in significantly lower output voltage ripple than that of a single-pump inverter. OUTPUT VOLTAGE RIPPLE Several factors influence the magnitude of voltage ripple on the output of the LM2781. Primary influences are output capacitance, flying capacitance, load current, input voltage, and output capacitor ESR. The magnitude of the output voltage ripple is difficult to predict with equations when the LM2781 is double pumping. Refer to the Typical Performance Characteristic curves for this information. In general, ripple decreases with increased output capacitance, decreased load current, a lower input voltage, a smaller flying capacitance and/or a smaller output capacitor ESR. When only one charge pump is active (C2 removed), the output ripple is more predictable, and can be estimated with the following equation: VRIPPLE = [(0.5xIOUT) / (fSWxCOUT) ] + (2xIOUTxESRCout) www.national.com LM2781 Operation Description OUTPUT RESISTANCE (Continued) Output resistance (ROUT) models the internal non-idealities of a charge pump that result in droop in the output voltage. The effect of output resistance on output voltage is expressed in the following equation: VOUT = - [VIN - ( |IOUT| x ROUT )] Output resistance is a function of the internal switch resistance (RSW) , flying capacitance (C1), flying capacitor equivalent series resistance (ESRC1), output capacitor ESR (ESRCout), and internal switching frequency (fSW). When the LM2781 operates as a single charge pump (C2 removed), the output resistance can be approximated by the following equation: 1 Fly Cap: ROUT1 ~= 2RSW1+[1/(fSW*C1)]+4ESRC1+ESRCout RSW1 is the sum of the ON resistance of the four internal MOSFET switches of Charge Pump 1. With both C1 and C2 in place, the double pumping topology of the LM2781 cuts the output resistance in half: 2 Fly Caps: ROUT2 ~= 0.5xROUT1 Recommendations for minimizing output resistance include increasing flying capacitance and minimizing output and flying capacitor ESR. CAPACITOR SELECTION Surface-mount multi-layer ceramic capacitors are recommended for use with the LM2781. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR, < 15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally are not recommended for use with the LM2781 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM2781. These capacitors have tight capacitance tolerance (as good as ± 10%), hold their value over temperature (X7R: ± 15% over -55˚C to 125˚C; X5R: ± 15% over -55˚C to 85˚C), and typically have little voltage coefficient. Capacitors with Y5V and/or Z5U temperature characteristic are generally not recommended for use with the LM2781. These types of capacitors typically have wide capacitance tolerance (+80%, -20%), vary significantly over temperature (Y5V: +22%, -82% over -30˚C to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C range), and have poor voltage coefficients. Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only 0.1µF. Such deviation would severely affect performance of the LM2781 circuit. www.national.com 6 LM2781 Ultra-Low Ripple Switched Capacitor Voltage Inverter Physical Dimensions inches (millimeters) unless otherwise noted TPA08: 8-Bump Thin Micro SMD X1 = X2 = 1.361mm ± 0.030mm X3 = 0.500mm ± 0.075mm LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 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