LM828 Switched Capacitor Voltage Converter General Description Features The LM828 CMOS charge-pump voltage converter inverts a positive voltage in the range of +1.8V to +5.5V to the corresponding negative voltage of −1.8V to −5.5V. The LM828 uses two low cost capacitors to provide up to 25 mA of output current. The LM828 operates at 12 kHz switching frequency to reduce output resistance and voltage ripple. With an operating current of only 40 µA (operating efficiency greater than 96% with most loads), the LM828 provides ideal performance for battery powered systems. The device is in a tiny SOT-23-5 package. n n n n Inverts Input Supply Voltage SOT-23-5 Package 20Ω Typical Output Impedance 97% Typical Conversion Efficiency at 5 mA Applications n n n n n n Cellular Phones Pagers PDAs Operational Amplifier Power Supplies Interface Power Supplies Handheld Instruments Basic Application Circuits Voltage Inverter DS100137-1 +5V to −10V Converter DS100137-2 © 1999 National Semiconductor Corporation DS100137 www.national.com LM828 Switched Capacitor Voltage Converter March 1999 Absolute Maximum Ratings (Note 1) TJMax (Note 3) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. θJA (Note 3) Supply Voltage (V+ to GND, or GND to OUT) 150˚C 300˚C/W Operating Junction Temperature Range Storage Temperature Range 5.8V V+ and OUT Continuous Output Current 50 mA Lead Temp. (Soldering, 10 seconds) Output Short-Circuit Duration to GND (Note 2) 1 sec. ESD Rating (Note 7) Continuous Power Dissipation (TA = 25˚C)(Note 3) −40˚C to 85˚C −65˚C to +150˚C 300˚C 2kV 240 mW Electrical Characteristics Limits in standard typeface are for TJ = 25˚C, and limits in boldface type apply over the full operating temperature range. Unless otherwise specified: V+ = 5V, C1 = C2 = 10 µF. (Note 4) V+ Symbol Supply Voltage Parameter Condition RL = 10kΩ Min Typ IQ Supply Current No Load 40 ROUT Output Resistance (Note 5) IL = 5 mA 20 65 Ω fOSC Oscillator Frequency (Note 6) Internal 12 24 56 kHz fSW Switching Frequency (Note 6) 12 28 kHz Power Efficiency Measured at CAP+ IL = 5 mA 6 PEFF VOEFF Voltage Conversion Efficiency No Load 95 1.8 Max Units 5.5 V 75 µA 115 97 % 99.96 % Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device beyond its rated operating conditions. Note 2: OUT may be shorted to GND for one second without damage. However, shorting OUT to V+ may damage the device and should be avoided. Also, for temperatures above 85˚C, OUT must not be shorted to GND or V+, or the device may be damaged. Note 3: The maximum allowable power dissipation is calculated by using PDMax = (TJMax − TA)/θJA, where TJMax is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction-to-ambient thermal resistance of the package. Note 4: In the test circuit, capacitors C1 and C2 are 10 µF, 0.3Ω maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce output voltage and efficiency. Note 5: Specified output resistance includes internal switch resistance and capacitor ESR. See the details in the application information. Note 6: The output switches operate at one half of the oscillator frequency, fOSC = 2fSW. Note 7: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. www.national.com 2 Test Circuit DS100137-3 *C1 and C2 are 10 µF capacitors. FIGURE 1. LM828 Test Circuit Typical Performance Characteristics (Circuit of Figure 1, V+ = 5V unless otherwise specified) Supply Current vs Supply Voltage Supply Current vs Temperature DS100137-29 Output Source Resistance vs Supply Voltage DS100137-30 Output Source Resistance vs Temperature DS100137-31 DS100137-32 3 www.national.com Typical Performance Characteristics (Circuit of Figure 1, V+ = 5V unless otherwise specified) (Continued) Output Voltage vs Load Current Efficiency vs Load Current DS100137-33 Switching Frequency vs Supply Voltage DS100137-34 Switching Frequency vs Temperature DS100137-35 DS100137-36 Connection Diagram 5-Lead Small Outline Package (M5) DS100137-14 Actual Size DS100137-13 Top View With Package Marking Ordering Information Order Number Package Number Package Marking Supplied as LM828M5 MA05B S08A (Note 8) Tape and Reel (250 units/rail) LM828M5X MA05B S08A (Note 8) Tape and Reel (3000 units/rail) Note 8: The first letter ″S″ identifies the part as a switched capacitor converter. The next two numbers are the device number. Larger quantity reels are available upon request. www.national.com 4 Pin Description Pin Name 1 OUT Function Negative voltage output. 2 V+ 3 CAP− Connect this pin to the negative terminal of the charge-pump capacitor. Power supply positive input. 4 GND Power supply ground input. 5 CAP+ Connect this pin to the positive terminal of the charge-pump capacitor. a function of the ON resistance of the internal MOSFET switches, the oscillator frequency, the capacitance and the ESR of both C1 and C2. Since the switching current charging and discharging C1 is approximately twice as the output current, the effect of the ESR of the pumping capacitor C1 will be multiplied by four in the output resistance. The output capacitor C2 is charging and discharging at a current approximately equal to the output current, therefore, this ESR term only counts once in the output resistance. A good approximation of Rout is: Circuit Description The LM828 contains four large CMOS switches which are switched in a sequence to invert the input supply voltage. Energy transfer and storage are provided by external capacitors. Figure 2 illustrates the voltage conversion scheme. When S1 and S3 are closed, C1 charges to the supply voltage V+. During this time interval, switches S2 and S4 are open. In the second time interval, S1 and S3 are open; at the same time, S2 and S4 are closed, C1 is charging C2. After a number of cycles, the voltage across C2 will be pumped to V+. Since the anode of C2 is connected to ground, the output at the cathode of C2 equals −(V+) when there is no load current. The output voltage drop when a load is added is determined by the parasitic resistance (Rds(on) of the MOSFET switches and the ESR of the capacitors) and the charge transfer loss between capacitors. where RSW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 2. High capacitance, low ESR capacitors will reduce the output resistance. The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the output capacitor C2: Again, using a low ESR capacitor will result in lower ripple. Capacitor Selection The output resistance and ripple voltage are dependent on the capacitance and ESR values of the external capacitors. The output voltage drop is the load current times the output resistance, and the power efficiency is DS100137-26 FIGURE 2. Voltage Inverting Principle Application Information Simple Negative Voltage Converter The main application of LM828 is to generate a negative supply voltage. The voltage inverter circuit uses only two external capacitors as shown in the Basic Application Circuits. The range of the input supply voltage is 1.8V to 5.5V. The output characteristics of this circuit can be approximated by an ideal voltage source in series with a resistance. The voltage source equals −(V+). The output resistance, Rout , is Where IQ(V+) is the quiescent power loss of the IC device, and IL2Rout is the conversion loss associated with the switch on-resistance, the two external capacitors and their ESRs. The selection of capacitors is based on the specifications of the dropout voltage (which equals Iout Rout), the output voltage ripple, and the converter efficiency. Low ESR capacitors (following table) are recommended to maximize efficiency, reduce the output voltage drop and voltage ripple. Low ESR Capacitor Manufacturers Manufacturer Phone Capacitor Type Nichicon Corp. (708)-843-7500 PL & PF series, through-hole aluminum electrolytic AVX Corp. (803)-448-9411 TPS series, surface-mount tantalum Sprague (207)-324-4140 593D, 594D, 595D series, surface-mount tantalum Sanyo (619)-661-6835 OS-CON series, through-hole aluminum electrolytic 5 www.national.com Application Information (Continued) Low ESR Capacitor Manufacturers Manufacturer (Continued) Phone Capacitor Type Murata (800)-831-9172 Ceramic chip capacitors Taiyo Yuden (800)-348-2496 Ceramic chip capacitors Tokin (408)-432-8020 Ceramic chip capacitors Other Applications Paralleling Devices Any number of LM828s can be paralleled to reduce the output resistance. Each device must have its own pumping capacitor C1, while only one output capacitor Cout is needed as shown in Figure 3. The composite output resistance is: DS100137-9 FIGURE 3. Lowering Output Resistance by Paralleling Devices Cascading Devices Cascading the LM828s is an easy way to produce a greater negative voltage (e.g. A two-stage cascade circuit is shown in Figure 4). If n is the integer representing the number of devices cascaded, the unloaded output voltage Vout is (-nVin). The effective output resistance is equal to the weighted sum of each individual device: Rout = nRout_1 + n/2 Rout_2 + ... + Rout_n This can be seen by first assuming that each device is 100 percent efficient. Since the output voltage is different on each device the output current is as well. Each cascaded device sees less current at the output than the previous so the ROUT voltage drop is lower in each device added. Note that, the number of n is practically limited since the increasing of n significantly reduces the efficiency, and increases the output resistance and output voltage ripple. DS100137-10 FIGURE 4. Increasing Output Voltage by Cascading Devices www.national.com 6 Other Applications (Continued) Combined Doubler and Inverter In Figure 5, the LM828 is used to provide a positive voltage doubler and a negative voltage converter. Note that the total current drawn from the two outputs should not exceed 40 mA. DS100137-11 FIGURE 5. Combined Voltage Doubler and Inverter Regulating VOUT Note that the following conditions must be satisfied simultaneously for worst case design: Vin_min > Vout_min +Vdrop_max (LP2980) It is possible to regulate the negative output of the LM828 by use of a low dropout regulator (such as the LP2980). The whole converter is depicted in Figure 6. This converter can give a regulated output from −1.8V to −5.5V by choosing the proper resistor ratio: Vout = Vref (1 + R1/R2) where, Vref = 1.23V + Iout_max x Rout_max (LM828) Vin_max < Vout_max +Vdrop_min (LP2980) + Iout_min x Rout_min (LM828) DS100137-12 FIGURE 6. Combining LM828 with LP2980 to Make a Negative Adjustable Regulator 7 www.national.com LM828 Switched Capacitor Voltage Converter Physical Dimensions inches (millimeters) unless otherwise noted 5-Lead Small Outline Package (M5) NS Package Number MA05B For Order Numbers, refer to the table in the ″Ordering Information″ section of this document. 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 OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 2. A critical component is any component of a life support 1. Life support devices or systems are devices or sysdevice or system whose failure to perform can be reatems which, (a) are intended for surgical implant into sonably expected to cause the failure of the life support the body, or (b) support or sustain life, and whose faildevice or system, or to affect its safety or effectiveness. ure 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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