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19-1290; Rev 1; 2/98
Regulated, Adjustable -2x Inverting Charge Pump MAX868
General Description
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 MAX package, which is only 1.11mm high and occupies just half the board area of a standard 8-pin SO.
____________________________Features
o Regulated Negative Output Voltage (up to -2 x VIN) o Ultra-Small, 10-Pin MAX Package o On-Demand Switching at up to 450kHz o 30A Quiescent Supply Current o Requires Only Four Small External Capacitors o 1.8V to 5.5V Input Voltage Range o 0.1A Logic-Controlled Shutdown o Up to 30mA Output Current
Ordering Information
PART TEMP. RANGE PIN-PACKAGE Dice* 10 MAX MAX868C/D 0C to +70C MAX868EUB -40C to +85C *Dice are tested at TA = +25C.
________________________Applications
Small LCD Panels Cell Phones Cordless Phones Camcorders Handy-Terminals, PDAs Medical Instruments Battery-Operated Equipment
Typical Operating Circuit Configuration
VIN = 1.8V TO 5.5V 1F
TOP VIEW
SHDN
IN
MAX868
GND 1 OUT C1PGND C1+ 2 3 4 5 10 FB 9 SHDN C2+ IN C2C2+ 0.1F C2PGND GND 2.2F OUT C1+ 0.1F C1VOUT = 0V TO -2 x VIN FB
MAX868
8 7 6
MAX
________________________________________________________________ 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.
Regulated, Adjustable -2x Inverting Charge Pump MAX868
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 = +70C) 10-pin MAX (derate 5.6mW/C above +70C) ...........444mW Operating Temperature Range MAX868EUB ....................................................-40C to +85C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
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 = 0.22F, CIN = 1F, COUT = 10F, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Supply-Voltage Range Supply Current Shutdown Current Oscillator Frequency Closed-Loop Output Resistance Open-Loop Output Resistance FB Trip Point Output Current FB Input Bias Current SHDN Input Threshold SHDN Input Bias Current VIL VIH IOUT SYMBOL VIN IIN IIN,SHDN fOSC ROUT,CL RL = 3k to GND No load, VFB = -50mV FB = IN No load, SHDN = GND VFB = 50mV VOUT = -5V IOUT = 5mA, FB = IN TA = +25C TA = 0C to +85C 15 -30 -40 12 30 -50 0.3VIN 0.7VIN -100 1 100 1 50 TA = +25C TA = 0C to +85C VIN = 3.3V, VOUT = -5V VIN = 5V, VOUT = -3.3V TA = +25C TA = 0C to +85C 293 270 0.2 70 100 125 50 30 40 mV mV mA nA V nA CONDITIONS MIN 1.8 30 5 0.1 450 1 607 630 TYP MAX 5.5 50 UNITS V A mA A kHz
ROUT
SHDN = GND (OUT pulls to GND) VIN = 1.8V to 5.5V Closed loop
VIN = 1.8V to 5.5V, TA = +25C VIN = 1.8V to 5.5V VIN = 5.5V, SHDN = IN or GND
2
_______________________________________________________________________________________
Regulated, Adjustable -2x Inverting Charge Pump
ELECTRICAL CHARACTERISTICS
(VIN = +3.3V, C1 = C2 = 0.22F, CIN = 1F, COUT = 10F, TA = -40C to +85C, unless otherwise noted. (Note 1) PARAMETER Supply-Voltage Range Supply Current Shutdown Current Oscillator Frequency Open-Loop Output Resistance FB Trip Point FB Input Bias Current SHDN Input Threshold SHDN Input Bias Current VIL VIH SYMBOL VIN IIN IIN,SHDN fOSC ROUT RL = 3k to GND No load, VFB = -50mV No load, SHDN = GND VFB = 50mV IOUT = 5mA, FB = IN SHDN = GND (OUT pulls to GND) VIN = 1.8V to 5.5V VIN = 1.8V to 5.5V VIN = 1.8V to 5.5V VIN = 5.5V, SHDN = IN or GND -40 -100 0.3VIN 0.7VIN -100 100 270 CONDITIONS MIN 1.8 TYP MAX 5.5 55 1 630 125 50 40 100 UNITS V A A kHz mV nA V nA
MAX868
Note 1: Specifications to -40C are guaranteed by design, not production tested.
__________________________________________Typical Operating Characteristics
(Circuit of Figure 5, TA = +25C, unless otherwise noted.)
LOAD-REGULATION ERROR vs. LOAD CURRENT (VIN = 5V)
MAX868-01
LOAD-REGULATION ERROR vs. LOAD CURRENT (VIN = 3.3V)
MAX868-02
MAXIMUM SWITCHING FREQUENCY vs. TEMPERATURE
MAXIMUM SWITCHING FREQUENCY (kHz) 490 480 470 460 450 440 430 420 410 400 -40 -20 VIN = 2V 0 20 40 60 80 100 VIN = 3.3V VIN = 5V FB = IN
MAX868-03
5 LOAD-REGULATION ERROR (mV) 0 -5 -10 VOUT = -7.5V -15 -20 VOUT = -3.3V -25 -30 -35 0 5 VOUT = -5V
3 LOAD-REGULATION ERROR (mV) 0 -3 -6 -9 -12 -15 VOUT = -5V
500
VOUT = -3.3V
10 15 20 25 30 35 40 45 50 LOAD CURRENT (mA)
0
5
10
15
20
25
LOAD CURRENT (mA)
TEMPERATURE (C)
_______________________________________________________________________________________
3
Regulated, Adjustable -2x Inverting Charge Pump MAX868
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 5, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (VIN = 5V)
MAX868-04
EFFICIENCY vs. LOAD CURRENT (VIN = 3.3V)
MAX868-05
EFFICIENCY vs. LOAD CURRENT (VIN = 5V)
CIRCUIT OF FIGURE 6 70 60 EFFICIENCY (%) 50 40 30 20 10 0 VOUT = -2.5V VOUT = -3.3V
MAX868-06
80 70 60 EFFICIENCY (%) 50 40 30 20 10 0 0.01 0.1 1 LOAD CURRENT (mA) 10 VOUT = -3.3V VOUT = -5V VOUT = -7.5V
80 70 VOUT = -5V 60 EFFICIENCY (%) 50 40 30 20 10 0 VOUT = -3.3V
80
100
0.01
0.1
1 LOAD CURRENT (mA)
10
100
0.01
0.1
1 LOAD CURRENT (mA)
10
100
OPEN-LOOP OUTPUT IMPEDANCE vs. TEMPERATURE (FB = IN, VOUT = -2 x VIN)
MAX868-07
OPEN-LOOP OUTPUT IMPEDANCE vs. TEMPERATURE (FB = IN, VOUT = -VIN)
MAX868-08
LOAD-TRANSIENT RESPONSE
MAX868-12
200 180 OUTPUT IMPEDANCE () 160 140 120 100 80 60 40 20 0 -40 -20 0 20 40 60 80 VIN = 5V VIN = 2V VIN = 3.3V
200 180 OUTPUT IMPEDANCE () 160 140 120 100 80 60 40 20 0 VIN = 5V -40 -20 0 20 40 60 80 VIN = 3.3V VIN = 2V CIRCUIT OF FIGURE 6
10mA/div
20mV/div
100
100
TEMPERATURE (C)
TEMPERATURE (C)
200s/div VIN = 5V, VOUT = -5V, IOUT = 1mA TO 11mA STEP
OUTPUT VOLTAGE RIPPLE (COUT = 10F TANTALUM)
MAX868-09
OUTPUT VOLTAGE RIPPLE (COUT = 10F CERAMIC)
MAX868-10
OUTPUT VOLTAGE RIPPLE
MAX868-11
20mV/div
20mV/div
20mV/div
20s/div VIN = 3.3V, VOUT = -3.3V, ILOAD = 5mA, VOUT AC COUPLED (20mV/div), COUT = 10F (AVX TPS)
20s/div VIN = 3.3V, VOUT = -3.3V, ILOAD = 5mA, VOUT AC COUPLED (20mV/div), COUT = 10F CERAMIC
20s/div VIN = 3.3V, VOUT = -3.3V, ILOAD = 5mA, VOUT AC COUPLED (20mV/div), COUT = 2.2F CERAMIC
4
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Regulated, Adjustable -2x Inverting Charge Pump
Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 NAME GND OUT C1PGND C1+ C2IN C2+ SHDN FB Analog Ground Charge-Pump Output Negative Terminal of Flying Capacitor C1 Power Ground Positive Terminal of Flying Capacitor C1 Negative Terminal of Flying Capacitor C2 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 0.1A. 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. FUNCTION
MAX868
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 (1) and discharge phase (2). 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 C11 SHDN 2 FB OSCILLATOR COUT
VREF
Figure 1. Functional Diagram
_______________________________________________________________________________________
5
Regulated, Adjustable -2x Inverting Charge Pump MAX868
(a) (a) C2+ IN C2C1+ C2+ IN
C2C1+
VOUT VOUT C1COUT (b) C2+ C2(b) C1+ IN GND C1C2C2+ VOUT COUT C1+ C1VOUT COUT C1COUT
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
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 100k to 500k 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.
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.
6
_______________________________________________________________________________________
Regulated, Adjustable -2x Inverting Charge Pump
CC*
VREF R2 FB OPTIONAL CONNECTION VIN IN R1
VOUT
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.
MAX868
MAX868
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 0.1F to 0.33F range, the maximum output current can be approximated by the following equation:
2 x VIN - fMAX x C1 + C2
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:
RESR 1 + x ROUT 1 + 4 x COUT C1 + C2
VRIPPLE = 2 x VIN -
(
| VOUT |
)
IOUT(MAX) =
| VOUT |
10V VIN +
(
4
)
+ R OUT x
| VOUT |
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 70. 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.
where fMAX is the maximum oscillator frequency (typically 450kHz), R OUT is the MAX868 open-loop output impedance (typically 70), 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).
Table 1. Manufacturers of Surface-Mount, Low-ESR Capacitors
TYPE MANUFACTURER AVX Surface-Mount Tantalum Matsuo Sprague Surface-Mount Ceramic AVX Matsuo PART TPS series 267 series 593D, 595D series X7R type X7R type PHONE (803) 946-0690 (714) 969-2491 (603) 224-1961 (803) 946-0690 (714) 969-2491 FAX (803) 626-3123 (714) 960-6492 (603) 224-1430 (803) 626-3123 (714) 960-6492 7
_______________________________________________________________________________________
Regulated, Adjustable -2x Inverting Charge Pump MAX868
__________Applications Information
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: 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.
Layout and Grounding
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.
=
| VOUT |
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 1F 1F SHDN IN R2 500k SHDN IN R2 500k
MAX868
C2+ FB R1 330k OUT
MAX868
C1+ 0.1F C1C2+ 0.1F C2PGND GND OUT FB R1 750k VOUT = -7.5V 10F
*
CF = 0.2F
C2C1+ C1PGND GND
VOUT = -3.3V AT 20mA 10F
*
*C1+ AND C2- MUST BE LEFT UNCONNECTED.
Figure 5. Standard Configuration for Generating an Output Voltage up to -2 x VIN
Figure 6. Alternative Configuration for |VOUT| VIN
8
___________________________________
Regulated, Adjustable -2x Inverting Charge Pump MAX868
COMPONENT PLACEMENT GUIDE
PC BOARD LAYOUT
0.5"
Figure 7a. Suggested Layout for Circuit of Figure 5
0.5"
Figure 7b. Suggested Layout for Circuit of Figure 5
_______________________________________________________________________________________
9
Regulated, Adjustable -2x Inverting Charge Pump MAX868
COMPONENT PLACEMENT GUIDE
PC BOARD LAYOUT
0.5"
Figure 8a. Suggested Layout for External Reference Applications
0.5"
Figure 8b. Suggested Layout for External Reference Applications
10
______________________________________________________________________________________
Regulated, Adjustable -2x Inverting Charge Pump
Package Information
10LUMAXB.EPS
MAX868
______________________________________________________________________________________
11
Regulated, Adjustable -2x Inverting Charge Pump MAX868
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 (c) 1998 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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