19-1290; Rev 1; 2/98 Regulated, Adjustable -2x Inverting Charge Pump ____________________________Features The MAX868 inverting charge pump provides a low-cost and compact means of generating a regulated negative voltage up to -2 x V IN from a positive input voltage between 1.8V and 5.5V. It uses a pulse-frequencymodulation (PFM) control scheme to generate the regulated negative output voltage. PFM operation is obtained by gating the internal 450kHz oscillator on and off as needed to maintain output voltage regulation. This unique on-demand switching scheme gives the MAX868 excellent light-load efficiency without degrading its fullload operation (up to 30mA), permitting smaller capacitors to take advantage of the oscillator’s high switching frequency. The MAX868 requires no inductors; only four capacitors are required to build a complete DC-DC converter. Output voltage regulation is achieved by adding just two resistors. The MAX868 comes in a 10-pin µMAX package, which is only 1.11mm high and occupies just half the board area of a standard 8-pin SO. ♦ Regulated Negative Output Voltage (up to -2 x VIN) ________________________Applications MAX868C/D 0°C to +70°C MAX868EUB -40°C to +85°C *Dice are tested at TA = +25°C. Small LCD Panels Cell Phones ♦ Ultra-Small, 10-Pin µMAX Package ♦ On-Demand Switching at up to 450kHz ♦ 30µA Quiescent Supply Current ♦ Requires Only Four Small External Capacitors ♦ 1.8V to 5.5V Input Voltage Range ♦ 0.1µA Logic-Controlled Shutdown ♦ Up to 30mA Output Current Ordering Information PART TEMP. RANGE PIN-PACKAGE Dice* 10 µMAX Cordless Phones Camcorders Handy-Terminals, PDAs Typical Operating Circuit Medical Instruments Battery-Operated Equipment Configuration VIN = 1.8V TO 5.5V 1µF SHDN TOP VIEW IN MAX868 GND 1 OUT C1- 10 FB 2 3 MAX868 9 SHDN 8 C2+ PGND 4 7 IN C1+ 5 6 C2- µMAX C1+ FB 0.1µF C1VOUT = 0V TO -2 x VIN C2+ OUT 0.1µF 2.2µF C2PGND GND ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 408-737-7600 ext. 3468. MAX868 General Description MAX868 Regulated, Adjustable -2x Inverting Charge Pump ABSOLUTE MAXIMUM RATINGS IN to GND .................................................................-0.3V to +6V OUT to GND ...........................................................+0.3V to -12V IN to OUT.................................................................-0.3V to -17V C1+ to GND ........................................(VIN - 12V) to (VIN + 0.3V) C1- to GND.............................................................+0.3V to -12V C2+ to GND ....................................................(VIN + 0.3V) to -6V C2- to GND...............................................................+0.3V to -6V SHDN, FB to GND .......................................-0.3V to (VIN + 0.3V) PGND to GND .......................................................-0.3V to +0.3V Output Current ....................................................................35mA Short-Circuit Duration.................................................Continuous Continuous Power Dissipation (TA = +70°C) 10-pin µMAX (derate 5.6mW/°C above +70°C) ...........444mW Operating Temperature Range MAX868EUB ....................................................-40°C to +85°C Storage Temperature Range .............................-65°C to +160°C Lead Temperature (soldering, 10sec) .............................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VIN = +3.3V, SHDN = IN, C1 = C2 = 0.22µF, CIN = 1µF, COUT = 10µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Supply-Voltage Range Supply Current Shutdown Current SYMBOL VIN IIN IIN,SHDN CONDITIONS RL = 3kΩ to GND FB = IN 5 No load, SHDN = GND VFB = 50mV Closed-Loop Output Resistance ROUT,CL VOUT = -5V VIN = 1.8V to 5.5V IOUT FB Input Bias Current SHDN Input Threshold SHDN Input Bias Current 2 IOUT = 5mA, FB = IN TA = +25°C 293 TA = 0°C to +85°C 270 Closed loop TA = +25°C VIH V 50 µA mA 0.1 1 607 630 70 TA = +25°C -30 TA = 0°C to +85°C -40 VIN = 3.3V, VOUT = -5V 12 VIN = 5V, VOUT = -3.3V 30 VIN = 1.8V to 5.5V VIN = 5.5V, SHDN = IN or GND -50 1 _______________________________________________________________________________________ Ω 50 30 mV 40 mV mA 50 0.7VIN 1 kHz 100 0.3VIN -100 µA Ω 125 15 UNITS 5.5 450 TA = 0°C to +85°C VIN = 1.8V to 5.5V, TA = +25°C VIL MAX 0.2 SHDN = GND (OUT pulls to GND) FB Trip Point Output Current 1.8 30 fOSC ROUT TYP No load, VFB = -50mV Oscillator Frequency Open-Loop Output Resistance MIN 100 nA V nA Regulated, Adjustable -2x Inverting Charge Pump MAX868 ELECTRICAL CHARACTERISTICS (VIN = +3.3V, C1 = C2 = 0.22µF, CIN = 1µF, COUT = 10µF, TA = -40°C to +85°C, unless otherwise noted. (Note 1) PARAMETER SYMBOL CONDITIONS Supply-Voltage Range VIN RL = 3kΩ to GND Supply Current IIN No load, VFB = -50mV IIN,SHDN No load, SHDN = GND Shutdown Current Oscillator Frequency fOSC Open-Loop Output Resistance ROUT MIN TYP MAX 1.8 VFB = 50mV 270 UNITS 5.5 V 55 µA 1 µA 630 kHz IOUT = 5mA, FB = IN 125 SHDN = GND (OUT pulls to GND) 50 Ω FB Trip Point VIN = 1.8V to 5.5V -40 40 mV FB Input Bias Current VIN = 1.8V to 5.5V -100 100 nA VIL SHDN Input Threshold VIH SHDN Input Bias Current 0.3VIN VIN = 1.8V to 5.5V V 0.7VIN VIN = 5.5V, SHDN = IN or GND -100 100 nA Note 1: Specifications to -40°C are guaranteed by design, not production tested. __________________________________________Typical Operating Characteristics (Circuit of Figure 5, TA = +25°C, unless otherwise noted.) VOUT = -5V -5 -10 VOUT = -7.5V -15 -20 VOUT = -3.3V -25 VOUT = -5V 0 -3 -6 -9 VOUT = -3.3V -12 -30 -35 5 10 15 20 25 30 35 40 45 50 LOAD CURRENT (mA) FB = IN 480 VIN = 5V 470 460 450 440 VIN = 3.3V 430 420 VIN = 2V 410 400 -15 0 500 490 MAX868-03 0 MAXIMUM SWITCHING FREQUENCY vs. TEMPERATURE MAX868-02 MAX868-01 3 LOAD-REGULATION ERROR (mV) LOAD-REGULATION ERROR (mV) 5 LOAD-REGULATION ERROR vs. LOAD CURRENT (VIN = 3.3V) MAXIMUM SWITCHING FREQUENCY (kHz) LOAD-REGULATION ERROR vs. LOAD CURRENT (VIN = 5V) 0 5 10 15 LOAD CURRENT (mA) 20 25 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) _______________________________________________________________________________________ 3 ____________________________Typical Operating Characteristics (continued) (Circuit of Figure 5, TA = +25°C, unless otherwise noted.) VOUT = -3.3V 10 50 40 30 1 10 10 10 100 1 10 100 OPEN-LOOP OUTPUT IMPEDANCE vs. TEMPERATURE (FB = IN, VOUT = -2 x VIN) OPEN-LOOP OUTPUT IMPEDANCE vs. TEMPERATURE (FB = IN, VOUT = -VIN) OUTPUT IMPEDANCE (Ω) 140 VIN = 2V 120 100 VIN = 3.3V 80 60 120 80 20mV/div VIN = 3.3V 60 0 60 VIN = 2V 100 20 40 100 10mA/div 140 0 20 10 40 VIN = 5V 0 1 MAX868-08 160 20 -20 0.1 LOAD-TRANSIENT RESPONSE CIRCUIT OF FIGURE 6 180 0.01 LOAD CURRENT (mA) 200 MAX868-07 160 40 80 VIN = 5V -40 100 -20 0 20 40 60 80 100 200µs/div VIN = 5V, VOUT = -5V, IOUT = 1mA TO 11mA STEP TEMPERATURE (°C) TEMPERATURE (°C) OUTPUT VOLTAGE RIPPLE (COUT = 10µF CERAMIC) OUTPUT VOLTAGE RIPPLE MAX868-09 MAX868-10 OUTPUT VOLTAGE RIPPLE (COUT = 10µF TANTALUM) 20mV/div 20mV/div 20µs/div VIN = 3.3V, VOUT = -3.3V, ILOAD = 5mA, VOUT AC COUPLED (20mV/div), COUT = 10µF (AVX TPS) 4 0.1 LOAD CURRENT (mA) 180 -40 0 0.01 LOAD CURRENT (mA) 200 30 MAX868-11 0.1 40 20 0 0.01 VOUT = -2.5V 50 20 0 20mV/div VOUT = -3.3V MAX868-12 30 60 EFFICIENCY (%) VOUT = -5V 20 VOUT = -3.3V VOUT = -5V 50 40 CIRCUIT OF FIGURE 6 70 60 EFFICIENCY (%) EFFICIENCY (%) 70 VOUT = -7.5V 60 80 MAX868-05 70 EFFICIENCY vs. LOAD CURRENT (VIN = 5V) 80 MAX868-04 80 EFFICIENCY vs. LOAD CURRENT (VIN = 3.3V) MAX868-06 EFFICIENCY vs. LOAD CURRENT (VIN = 5V) OUTPUT IMPEDANCE (Ω) MAX868 Regulated, Adjustable -2x Inverting Charge Pump 20µs/div VIN = 3.3V, VOUT = -3.3V, ILOAD = 5mA, VOUT AC COUPLED (20mV/div), COUT = 10µF CERAMIC 20µs/div VIN = 3.3V, VOUT = -3.3V, ILOAD = 5mA, VOUT AC COUPLED (20mV/div), COUT = 2.2µF CERAMIC _______________________________________________________________________________________ Regulated, Adjustable -2x Inverting Charge Pump PIN NAME FUNCTION 1 GND Analog Ground 2 OUT Charge-Pump Output 3 C1- Negative Terminal of Flying Capacitor C1 4 PGND 5 C1+ Positive Terminal of Flying Capacitor C1 6 C2- Negative Terminal of Flying Capacitor C2 7 IN 8 C2+ 9 SHDN 10 FB Power Ground Supply-Voltage Input. Input voltage range is 1.8V to 5.5V. Positive Terminal of Flying Capacitor C2 Active-Low Shutdown Input. Connect SHDN to GND to put the MAX868 in shutdown mode and reduce supply current to 0.1µA. Connect to IN for normal operation. OUT is actively pulled to GND in shutdown. Feedback Input. Connect FB to a resistor divider for a regulated output voltage. Connect to IN to generate an unregulated -2 x VIN output voltage. Detailed Description The MAX868 inverting charge pump uses pulsefrequency-modulation (PFM) control to generate a regulated negative output voltage up to -2 x V IN. PFM operation is obtained by enabling the internal 450kHz oscillator as needed to maintain output voltage regulation. This control scheme reduces supply current at light loads and permits the use of small capacitors. The functional diagram shown in Figure 1 indicates the two phases of MAX868 operation: charge phase (Φ1) and discharge phase (Φ2). In charge phase, the switches on the left-hand side close, and the switches on the right-hand side open. In the discharge phase, the inverse occurs. Figure 2 illustrates that in charge phase, both flying capacitors are charged in parallel. The load is serviced entirely by the charge stored in the output capacitor. Figure 3 demonstrates the series connection of the flying capacitors in the discharge phase. The series combination of the flying capacitors, when connected to the output capacitor, transfers charge to the output in order to maintain output voltage regulation. In normal operation, the MAX868 operates predominantly in charge phase, switching to discharge phase only as needed to maintain a regulated output. C2+ IN C2C1+ OUT C1Φ1 Φ2 FB SHDN COUT OSCILLATOR VREF Figure 1. Functional Diagram _______________________________________________________________________________________ 5 MAX868 Pin Description MAX868 Regulated, Adjustable -2x Inverting Charge Pump (a) C2+ (a) IN C2+ IN C2C1+ C2C1+ VOUT C1- VOUT C1- COUT COUT (b) C2+ C2- (b) C1+ C1+ C2+ VOUT IN COUT GND C1- C1VOUT COUT C2- Figure 2. a) In charge phase, the left-hand switches are closed and the right-hand switches are open, charging the flying capacitors (C1 and C2) while the output capacitor (COUT) services the load. b) The equivalent circuit of the charge phase of operation. Figure 3. a) In discharge phase, the left-hand switches are open and the right-hand switches are closed, transferring energy from the flying capacitors (C1 and C2) to the output capacitor (COUT). b) The equivalent circuit of the discharge phase of operation. __________________Design Procedure When the MAX868 is powered by an unregulated supply, such as when operating directly from a battery, use any available positive reference voltage in the system. Note that due to the MAX868’s doubling and inverting charge-pump action, the output voltage is limited to -2 x VIN. Alternatively, to configure the MAX868 as a simple, unregulated doubler-inverter (VOUT = -2 x VIN), connect FB to IN. In this configuration, the MAX868 runs at its maximum oscillator frequency, operating as a conventional, open-loop charge pump. If multiple oscillator cycles are required to regulate the output, reduce the values for R1 and R2, or parallel a small capacitor (C C ) across R1 to compensate the feedback loop and ensure stability. Choose the lowest capacitor value that ensures stability; values up to 47pF are adequate for most applications. Setting the Output Voltage Set the output voltage using two external resistors, R1 and R2, as shown in Figure 4. Since the input bias current at FB has a 50nA maximum, large resistor values in the feedback loop do not significantly degrade accuracy. Begin by selecting R2 in the 100kΩ to 500kΩ range, and calculate R1 using the following equation: R1 = R2 x | VOUT | VREF where VOUT is the desired output voltage, and VREF is any available regulated positive voltage. When the MAX868 is powered by a regulated voltage, VIN can be used as the reference for setting the output voltage. 6 _______________________________________________________________________________________ Regulated, Adjustable -2x Inverting Charge Pump VREF VOUT R2 R1 FB OPTIONAL CONNECTION MAX868 IN VIN OUT *OPTIONAL FEED-FORWARD CAPACITOR Figure 4. Setting the Output Voltage Using Two External Resistors Capacitor Selection Choosing the Flying Capacitors Proper choice of the flying capacitors is dependent primarily upon the desired output current. For flying capacitors in the 0.1µF to 0.33µF range, the maximum output current can be approximated by the following equation: 2 x VIN − IOUT(MAX) = ( 4 ) fMAX x C1 + C2 | VOUT | + R OUT x ( VRIPPLE = 2 x VIN − | VOUT | ) RESR 1 + x ROUT 1 + 4 x COUT C1 + C2 where C1 and C2 are the flying capacitors, RESR is the output capacitor’s ESR, and R OUT is the MAX868’s open-loop output impedance, typically 70Ω. 10V VIN + Choosing the Output Capacitor The output capacitor stores the charge transferred from the flying capacitors and services the load between oscillator cycles. A good general rule is to make the output capacitance at least ten times greater than that of the flying capacitors. The output voltage ripple is dependent upon the capacitance of the flying capacitor and upon the output capacitor’s capacitance and ESR. When operating in closed-loop mode (when the MAX868 is generating a regulated output voltage), use the following equation to approximate peak-to-peak output voltage ripple: | VOUT | where fMAX is the maximum oscillator frequency (typically 450kHz), R OUT is the MAX868 open-loop output impedance (typically 70Ω), and C1 and C2 are the flyingcapacitor values. As a general rule, choose the lowestvalue flying capacitors that provide the desired output current in order to minimize output voltage ripple (see the section Choosing the Output Capacitor). Choose a low-ESR output capacitor for minimum output ripple. Surface-mount ceramic capacitors are preferred for their small size, low cost, and low ESR; low-ESR tantalum electrolytic capacitors are also acceptable. When using a ceramic output capacitor, ensure proper operation over the entire temperature range by choosing a capacitor with X7R (or equivalent) low tempco dielectric. See Table 1 for a list of suggested capacitor suppliers. Table 1. Manufacturers of Surface-Mount, Low-ESR Capacitors TYPE MANUFACTURER PART PHONE FAX AVX TPS series (803) 946-0690 (803) 626-3123 Surface-Mount Tantalum Matsuo 267 series (714) 969-2491 (714) 960-6492 Sprague 593D, 595D series (603) 224-1961 (603) 224-1430 AVX X7R type (803) 946-0690 (803) 626-3123 Matsuo X7R type (714) 969-2491 (714) 960-6492 Surface-Mount Ceramic _______________________________________________________________________________________ 7 MAX868 Surface-mount ceramic capacitors are preferred, due to their small size, low cost, and low equivalent series resistance (ESR). To ensure proper operation over the entire temperature range, choose ceramic capacitors with X7R (or equivalent) low temperature-coefficient (tempco) dielectrics. See Table 1 for a list of suggested capacitor suppliers. CC* MAX868 Regulated, Adjustable -2x Inverting Charge Pump __________Applications Information unconnected. Furthermore, doubling the flying capacitor to provide the same flying capacitance as the standard configuration (i.e., setting CF = C1 + C2) provides the same load-current capability as the standard configuration and reduces the MAX868’s open-loop output resistance by a factor of two, due to the reduction in the number of switches in the current path. Low-Output-Voltage Operation Since the difference between the voltage of the seriesconnected flying capacitors and the output voltage must be dissipated within the device, the MAX868’s efficiency is very similar to that of a linear regulator. Estimate efficiency using the following equation: η = Layout and Grounding | VOUT | Proper layout is important to obtain optimal performance. Connect GND to PGND together using the shortest trace possible, and similarly connect these pins to the ground plane. Mount all capacitors as close to the MAX868 as possible, keeping traces short to minimize parasitics. Keep all connections to the FB pin as short as possible. Specifically, locate R1 and R2 next to FB (Figures 7 and 8). Should it become necessary in the final layout, leave room to parallel a feedforward capacitor across R1. k x VIN where k is a constant equal to 2 for the standard configuration of Figure 5 and equal to 1 for the circuit of Figure 6. This equation’s denominator is the voltage resulting from the series connection of the flying capacitors (-2 x VIN, as shown in Figure 3b), while its numerator is simply the regulated output voltage. For applications in which the output voltage will not be more negative than -|VIN|, the efficiency can be doubled using the circuit of Figure 6, as compared to the circuit of Figure 5. In Figure 6, a single flying capacitor is connected between C2+ and C1-, with C2- and C1+ left Chip Information TRANSISTOR COUNT: 96 SUBSTRATE CONNECTED TO IN VIN = 5V VIN = 5V 1µF 1µF SHDN SHDN IN R2 500k IN R2 500k MAX868 MAX868 C1+ C2+ FB FB 0.1µF R1 750k C1C2+ OUT 0.1µF C2- * C1+ R1 330k CF = 0.2µF VOUT = -7.5V OUT PGND PGND GND GND *C1+ AND C2- MUST BE LEFT UNCONNECTED. Figure 6. Alternative Configuration for |VOUT| ≤ VIN Figure 5. Standard Configuration for Generating an Output Voltage up to -2 x VIN 8 VOUT = -3.3V AT 20mA 10µF C1- 10µF C2- * ___________________________________ Regulated, Adjustable -2x Inverting Charge Pump MAX868 COMPONENT PLACEMENT GUIDE PC BOARD LAYOUT 0.5" 0.5" Figure 7a. Suggested Layout for Circuit of Figure 5 Figure 7b. Suggested Layout for Circuit of Figure 5 _______________________________________________________________________________________ 9 MAX868 Regulated, Adjustable -2x Inverting Charge Pump COMPONENT PLACEMENT GUIDE 0.5" Figure 8a. Suggested Layout for External Reference Applications 10 PC BOARD LAYOUT 0.5" Figure 8b. Suggested Layout for External Reference Applications ______________________________________________________________________________________ Regulated, Adjustable -2x Inverting Charge Pump 10LUMAXB.EPS ______________________________________________________________________________________ 11 MAX868 Package Information MAX868 Regulated, Adjustable -2x Inverting Charge Pump NOTES Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 1998 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.