LM2682 Switched Capacitor Voltage Doubling Inverter General Description Features The LM2682 is a CMOS charge-pump voltage inverter capable of converting positive voltage in the range of +2.0V to +5.5V to the corresponding doubled negative voltage of −4.0V to −11.0V respectively. The LM2682 uses three low cost capacitors to provide 10 mA of output current without the cost, size, and EMI related to inductor based circuits. With an operating current of only 150 µA and an operating efficiency greater than 90% with most loads, the LM2682 provides ideal performance for battery powered systems. The LM2682 offers a switching frequency of 6 kHz. n n n n Inverts then doubles input supply voltage Small MSOP-8 package (mini SO-8) and SO-8 package 90Ω typical output impedance 94% typical power efficiency at 10 mA Applications n n n n n LCD contrast biasing GaAs power amplifier biasing Interface power supplies Handheld instrumentation Laptop computers and PDAs Typical Operating Circuit and Pin Configuration 8-Pin MSOP or 8-Pin SOIC DS100997-2 DS100997-1 Ordering Information Order Number Package Package Number Package Marking LM2682MM MSOP-8 MUA08A S11A Tape and Reel (1000 units/reel) LM2682MMX MSOP-8 MUA08A S11A Tape and Reel (3500 units/reel) LM2682M SO-8 M08A LM2682M Rail (95 units/rail) LM2682MX SO-8 M08A LM2682M Tape and Reel (2500 units/reel) © 1999 National Semiconductor Corporation DS100997 Supplied As www.national.com LM2682 Switched Capacitor Voltage Doubling Inverter November 1999 LM2682 Absolute Maximum Ratings (Note 1) MSOP-8 300 mW If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. SO-8 470 mW Input Voltage (VIN) TJMAX Operating Ratings +5.8V VIN dV/dT 1V/µsec VOUT ESD Susceptibility (Note 3) −11.6V Continuous Human Body Model Machine Model −65˚C to +150˚C Ambient Temp. Range −40˚C to +85˚C +300˚C Junction Temp. Range −40˚C to +125˚C VOUT Short-Circuit Duration Storage Temperature +150˚C Lead Temperature Soldering 2 kV 200V Power Dissipation (Note 2) LM2682 Electrical Characteristics VIN = 5V and C1 = C2 = C3 = 3.3µF unless otherwise specified. Limits with bold typeface apply over the full operating ambient temperature range, −40˚C to +85˚C, limits with standard typeface apply for TA = 25˚C. Symbol Parameter Conditions Min Typical (Note 4) VIN Supply Voltage Range RL = 2 kΩ IIN Supply Current Open Circuit, No Load 150 ROUT VOUT Source Resistance IL = 10 mA 90 2.0 Max Units 5.5 V 300 400 µA 150 Ω 200 IL = 5 mA, VIN = 2 V 110 250 Ω fOSC Oscillator Frequency (Note 5) 12 30 kHz fSW Switching Frequency 15 kHz Power Efficiency (Note 5) RL = 2k (Note 6) 6 ηPOWER ηVOLTAGE Voltage Conversion Efficiency 90 93 % 99.9 % Note 1: Absolute Maximum Ratings are those values 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: The maximum power dissipation must be de-rated at elevated temperatures (only needed for TA > 85˚C) and is limited by TJMAX (maximum junction temperature), θJ-A (junction to ambient thermal resistance) and TA (ambient temperature). θJ-A is 140˚C/W for the SO-8 package and 220˚C/W for the MSOP-8 package. The maximum power dissipation at any temperature is: PDissMAX = (TJMAX − TA)/θJ-A up to the value listed in the Absolute Maximum Ratings. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200pF capacitor discharged directly into each pin. Note 4: Typical numbers are at 25˚C and represent the most likely norm. Note 5: The output switches operate at one half of the oscillator frequency, fOSC = 2fSW. Note 6: The minimum specification is guaranteed by design and is not tested. Pin Description www.national.com Pin Number Symbol 1 C1− Capacitor C1 negative terminal Description 2 C2+ Capacitor C2 positive terminal 3 C2− Capacitor C2 negative terminal 4 VOUT Negative output voltage (−2VIN) 5 GND Device ground 6 VIN Power supply voltage 7 C1+ Capacitor C1 positive terminal 8 NC No Connection 2 Output Resistance vs Input Voltage LM2682 Typical Performance Charactistics VIN = 5V and TA = 25˚C unless otherwise noted. Output Voltage vs Load Current DS100997-7 DS100997-6 Supply Current vs Input Voltage Output Resistance vs Temperature DS100997-8 DS100997-9 Output Voltage Ripple vs Load Current DS100997-10 3 www.national.com LM2682 Basic Application Circuits DS100997-3 FIGURE 1. Doubling Voltage Inverter DS100997-4 FIGURE 2. +5V to −5V Regulated Voltage Converter Output resistance is typically 90Ω with an input voltage of +5V, an operating temperature of 25˚C, and using low ESR 3.3 µF capacitors. This equation shows the importance of capacitor selection. Large value, low ESR capacitors will reduce the output resistance significantly but will also require a larger overall circuit. Smaller capacitors will take up less space but can lower efficiency greatly if the ESR is large. Also to be considered is that C1 must be rated at 6 VDC or greater while C2 and C3 must be rated at 12 VDC or greater. Application Information Voltage Doubling Inverter The main application of the LM2682 is to generate a negative voltage that is twice the positive input voltage. This circuit requires only three external capacitors and is connected as shown in Figure 1. It is important to keep in mind that the efficiency of the circuit is determined by the output resistance. A derivation of the output resistance is shown below: ROUT = 2(RSW1+RSW2+ESRC1+RSW3+RSW4+ESRC2) + The amount of output voltage ripple is determined by the output capacitor C3 and the output current as shown in this equation: VRIPPLE P-P = IOUT x (2xESRC3 + 1/[2x(fOSCxC3)]) 2(RSW1+RSW2+ESRC1+RSW3+RSW4+ESRC2) + 1/(fOSCxC1) + 1/(fOSCxC2) + ESRC3 Using the assumption that all four switches have the same ON resistance our equation becomes: ROUT = 16RSW + 4ESRC1 + 4ESRC2 + ESRC3 + Once again a larger capacitor with smaller ESR will give better results. 1/(fOSCxC1) + 1/(fOSCxC2) www.national.com 4 Paralleling Devices Any number of devices can be paralleled to reduce the output resistance. As shown in Figure 3, each device must have its own pumping capacitors, C1 and C2, but only one shared output capacitor is required. The effective output resistance is the output resistance of one device divided by the number of devices used in parallel. Paralleling devices also gives the capability of increasing the maximum output current. The maximum output current now becomes the maximum output current for one device multiplied by the number of devices used in parallel. For example, if you parallel two devices you can get 20 mA of output current and have half the output resistance of one device supplying 10 mA. (Continued) +5V to −5V Regulated Voltage Converter Another application in which the LM2682 can be used is for generating a −5V regulated supply from a +5V unregulated supply. This involves using an op-amp and a reference and is connected as shown in Figure 2. The LM358 op-amp was chosen for its low cost and versatility and the LM4040-5.0 reference was chosen for its low bias current requirement. Of course other combinations may be used at the designer’s discretion to fit accuracy, efficiency, and cost requirements. With this configuration the circuit is well regulated and is still capable of providing nearly 10 mA of output current. With a 9 mA load the circuit can typically maintain 5% regulation on the output voltage with the input varying anywhere from 4.5V to the maximum of 5.5V. With less load the results are even better. Voltage ripple concerns are reduced in this case since the ripple at the output of the LM2682 is reduced at the output by the PSRR of the op-amp used. DS100997-5 FIGURE 3. Paralleling Devices 5 www.national.com LM2682 Application Information LM2682 Physical Dimensions inches (millimeters) unless otherwise noted 8 Lead (0.150” Wide) Molded Small Outline Package, JEDEC NS Package Number M08A www.national.com 6 LM2682 Switched Capacitor Voltage Doubling Inverter Physical Dimensions inches (millimeters) unless otherwise noted (Continued) Surface Mount Mini SO-8 Package NS Package Number MUA08A 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. 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