Datasheet

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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

from a positive input voltage

between 1.8V and 5.5V. It uses a pulse-frequency-

modulation (PFM) control scheme to generate the regulat-

ed 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 full-

load operation (up to 30mA), permitting smaller capaci-

tors 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 pack-

age, which is only 1.11mm high and occupies just half

the board area of a standard 8-pin SO.

________________________Applications

Small LCD Panels

Cell Phones

Cordless Phones

Camcorders

Handy-Terminals, PDAs

Medical Instruments

Battery-Operated Equipment

____________________________Features

♦ Regulated Negative Output Voltage

(up to -2 x V

)

♦ 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

MAX868

Regulated, Adjustable -2x

Inverting Charge Pump

________________________________________________________________

Maxim Integrated Products

SHDN

C2+

INPGND

C1-

OUT

GND

MAX868

µMAX

TOP VIEW

C2-C1+

Configuration

MAX868

C1+

PGND

SHDN

GND

OUT

0.1µF

1µF

2.2µF

OUT

= 0V TO -2 x V

= 1.8V TO 5.5V

0.1µF

C1-

C2+

C2-

Typical Operating Circuit

19-1290; Rev 1; 2/98

PART

MAX868C/D

MAX868EUB -40°C to +85°C

0°C to +70°C

TEMP. RANGE PIN-PACKAGE

Dice*

10 µMAX

Ordering Information

Dice are tested at T

= +25°C.

Summary of content (12 pages)

PAGE 1
9-1290; 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.
PAGE 2
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.
PAGE 3
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.
PAGE 4
____________________________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.
PAGE 5
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.
PAGE 6
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.
PAGE 7
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.
PAGE 8
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.
PAGE 9
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.
PAGE 10
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.
PAGE 11
Regulated, Adjustable -2x Inverting Charge Pump 10LUMAXB.
PAGE 12
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.