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MAX868EUB+ |MAX868EUBMAXIMN/a260avaiRegulated, Adjustable -2x Inverting Charge Pump
MAX868EUB+T |MAX868EUBTMAXIMN/a584avaiRegulated, Adjustable -2x Inverting Charge Pump
MAX868EUB-T |MAX868EUBTMAXIMN/a2940avaiRegulated, Adjustable -2x Inverting Charge Pump


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MAX868EUB+-MAX868EUB+T-MAX868EUB-T
Regulated, Adjustable -2x Inverting Charge Pump
General Description
The MAX868 inverting charge pump provides a low-cost
and compact means of generating a regulated negative
voltage up to -2 x VINfrom 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 VIN)
Ultra-Small, 10-Pin μMAX PackageOn-Demand Switching at up to 450kHz30μA Quiescent Supply Current Requires Only Four Small External Capacitors1.8V to 5.5V Input Voltage Range0.1μA Logic-Controlled ShutdownUp to 30mA Output Currentgulated, Adjustable -2x nverting Charge Pump
SHDN
C2+PGND
C1-
OUT
GND
MAX868
μMAX
TOP VIEW
C2-C1+
Configuration

MAX868
C1+
PGND
SHDN
GND
OUT
0.1μF
1μF
2.2μF
VOUT = 0V TO -2 x VIN
VIN = 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. RANGEPIN-PACKAGE

Dice*
10 μMAX
Ordering Information

*Dice are tested at TA= +25°C.
gulated, Adjustable -2xnverting Charge PumpABSOLUTE MAXIMUM RATINGS
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.)
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.
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
FB = IN
No load, VFB= -50mV= 3kΩto GND
VIN= 5.5V, SHDN= IN or GND
VIN= 1.8V to 5.5V
SHDN= GND (OUT pulls to GND)
VIN= 1.8V to 5.5V, TA= +25°C
IOUT= 5mA, FB = IN
No load, SHDN= GND
VFB= 50mV
Closed loop
VOUT= -5V
CONDITIONS
-1001100SHDNInput Bias Current0.7VINVIHSHDNInput Threshold0.3VINVIL-50150FB Input Bias Current30IOUTOutput Current125IINSupply CurrentμA30501.85.5VINSupply-Voltage Range-303050
ROUTOpen-Loop Output
ResistanceΩ1251000.11IIN,SHDNShutdown Current
kHz293450607Oscillator Frequency270630fOSC0.2ROUT,CLClosed-Loop Output
Resistance
UNITSMINTYPMAXSYMBOLPARAMETER
= +25°C= 0°C to +85°C= +25°C= 0°C to +85°C
VIN= 3.3V, VOUT= -5V
VIN= 5V, VOUT= -3.3V
VIN= 1.8V to 5.5VmV-4040FB Trip PointTA= 0°C to +85°C= +25°C
gulated, Adjustable -2nverting Charge PumpELECTRICAL CHARACTERISTICS
(VIN= +3.3V, C1 = C2 = 0.22μF, CIN= 1μF, COUT= 10μF, TA= -40°C to +85°C, unless otherwise noted. (Note 1)
Note 1:
Specifications to -40°C are guaranteed by design, not production tested.= 3kΩto GND
VFB= 50mV
No load, VFB= -50mV
No load, SHDN= GND
CONDITIONS

kHz270630fOSCOscillator Frequency1IIN,SHDNShutdown Current1.85.5VINSupply-Voltage Range55IINSupply Current
UNITSMINTYPMAXSYMBOLPARAMETER

IOUT= 5mA, FB = IN
VIN= 1.8V to 5.5V
SHDN= GND (OUTpulls to GND)
VIN= 1.8V to 5.5V-100100FB Input Bias Current-4040FB Trip Point
125Open-Loop Output
ResistanceΩ50ROUT
VIN= 5.5V, SHDN= IN or GND
VIN= 1.8V to 5.5V-100100SHDNInput Bias Current0.7VINVIHSHDNInput Threshold0.3VINVIL
__________________________________________Typical Operating Characteristics

(Circuit of Figure 5, TA= +25°C, unless otherwise noted.)
LOAD-REGULATION ERROR
vs. LOAD CURRENT
(VIN = 5V)
X868-01
LOAD CURRENT (mA)
(m
VOUT = -7.5V
VOUT = -3.3V
VOUT = -5V
LOAD-REGULATION ERROR
vs. LOAD CURRENT
(VIN = 3.3V)
X868-02
LOAD CURRENT (mA)
(m
VOUT = -3.3V
VOUT = -5V
MAXIMUM SWITCHING FREQUENCY
vs. TEMPERATURE
AX868-03
TEMPERATURE (°C)
(kFB = IN
VIN = 3.3V
VIN = 2V
VIN = 5V
gulated, Adjustable -2xnverting Charge Pump____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 5, TA= +25°C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
(VIN = 5V)
AX868-04
LOAD CURRENT (mA)
(%
VOUT = -5V
VOUT = -7.5V
VOUT = -3.3V
EFFICIENCY vs. LOAD CURRENT
(VIN = 3.3V)
AX868-05
LOAD CURRENT (mA)
(%
VOUT = -5V
VOUT = -3.3V
EFFICIENCY vs. LOAD CURRENT
(VIN = 5V)
AX868-06
LOAD CURRENT (mA)
(%
CIRCUIT OF FIGURE 6
VOUT = -2.5V
VOUT = -3.3V
OPEN-LOOP OUTPUT IMPEDANCE
vs. TEMPERATURE
(FB = IN, VOUT = -2 x VIN)
AX868-07
TEMPERATURE (°C)
IM

VIN = 2V
VIN = 3.3V
VIN = 5V
20μs/div
20mV/div
VIN = 3.3V, VOUT = -3.3V, ILOAD = 5mA,
VOUT AC COUPLED (20mV/div), COUT = 10μF CERAMIC
OUTPUT VOLTAGE RIPPLE
(COUT = 10μF CERAMIC)

AX868-10
OPEN-LOOP OUTPUT IMPEDANCE
vs. TEMPERATURE
(FB = IN, VOUT = -VIN)
TEMPERATURE (°C)
IM

VIN = 2V
CIRCUIT OF FIGURE 6
VIN = 3.3V
VIN = 5V
20μs/div
20mV/div
VIN = 3.3V, VOUT = -3.3V, ILOAD = 5mA,
VOUT AC COUPLED (20mV/div), COUT = 10μF (AVX TPS)
OUTPUT VOLTAGE RIPPLE
(COUT = 10μF TANTALUM)

AX868-09
20μs/div
20mV/div
VIN = 3.3V, VOUT = -3.3V, ILOAD = 5mA,
VOUT AC COUPLED (20mV/div), COUT = 2.2μF CERAMIC
OUTPUT VOLTAGE RIPPLE

200μs/div
10mA/div
20mV/div
VIN = 5V, VOUT = -5V, IOUT = 1mA TO 11mA STEP
LOAD-TRANSIENT RESPONSE

868-12
Detailed Description
The MAX868 inverting charge pump uses pulse-
frequency-modulation (PFM) control to generate a reg-
ulated negative output voltage up to -2 x VIN. PFM
operation is obtained by enabling the internal 450kHz
oscillator as needed to maintain output voltage regula-
tion. 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 fly-
ing capacitors in the discharge phase. The series com-
bination of the flying capacitors, when connected to the
output capacitor, transfers charge to the output in order
to maintain output voltage regulation. In normal opera-
tion, the MAX868 operates predominantly in charge
phase, switching to discharge phase only as needed to
maintain a regulated output.gulated, Adjustable -2nverting Charge Pump
Pin Description

Active-Low Shutdown Input. Connect SHDNto GND to put the MAX868 in shutdown mode and reduce sup-
ply current to 0.1μA. Connect to IN for normal operation. OUT is actively pulled to GND in shutdown.SHDN9
Feedback Input. Connect FB to a resistor divider for a regulated output voltage. Connect to IN to generate
an unregulated -2 x VINoutput voltage.FB10
Positive Terminal of Flying Capacitor C1C1+5
Negative Terminal of Flying Capacitor C2C2-6
Supply-Voltage Input. Input voltage range is 1.8V to 5.5V.IN7
Positive Terminal of Flying Capacitor C2C2+8
Power GroundPGND4
Negative Terminal of Flying Capacitor C1C1-3
PIN

Charge-Pump OutputOUT2
Analog GroundGND1
FUNCTIONNAME

VREF
COUT
OUT
C2-
C1+
C1-Φ2
OSCILLATOR
C2+
SHDN
Figure 1. Functional Diagram
__________________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 cur-
rent at FB has a 50nA maximum, large resistor values in
the feedback loop do not significantly degrade accura-
cy. Begin by selecting R2 in the 100kΩto 500kΩrange,
and calculate R1 using the following equation:
where VOUTis the desired output voltage, and VREFis
any available regulated positive voltage. When the
MAX868 is powered by a regulated voltage, VINcan be
used as the reference for setting the output voltage.
When the MAX868 is powered by an unregulated sup-
ply, 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), con-
nect FB to IN. In this configuration, the MAX868 runs at
its maximum oscillator frequency, operating as a con-
ventional, 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 (CC) 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.RxV
OUT
REF2 ||=gulated, Adjustable -2xnverting Charge Pump
COUT
VOUT
C2-
C1+
C1-
C2+
GND
(a)
(b)

COUT
VOUT
C2-
C2+
C1-
C1+
Figure 2. a)In charge phase, the left-hand switches are
closed and the right-hand switches are open, charging the fly-
ing capacitors (C1 and C2) while the output capacitor (COUT)
services the load. b) The equivalent circuit of the charge phase
of operation.
C2-
C1+
C1-
C2+
(a)
(b)

COUT
VOUT
COUT
VOUT
C2+
C2-
C1+
C1-
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.
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