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MAX868EUB
Regulated / Adjustable -2x Inverting Charge Pump
General DescriptionThe 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.
________________________ApplicationsSmall LCD Panels
Cell Phones
Cordless Phones
Camcorders
Handy-Terminals, PDAs
Medical Instruments
Battery-Operated Equipment
____________________________FeaturesRegulated 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 Current
MAX868
Regulated, Adjustable -2x
Inverting Charge Pump
Configuration
Typical Operating Circuit19-1290; Rev 1; 2/98
MAX868
Regulated, Adjustable -2x
Inverting Charge Pump
ABSOLUTE 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 valuesare 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 -6VSHDN, 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
MAX868
Regulated, Adjustable -2x
Inverting Charge Pump
ELECTRICAL 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.
__________________________________________Typical Operating Characteristics(Circuit of Figure 5, TA= +25°C, unless otherwise noted.)
Regulated, Adjustable -2x
Inverting Charge Pump
____________________________Typical Operating Characteristics (continued)
Detailed DescriptionThe 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 (F1)
and discharge phase (F2). 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.
MAX868
Regulated, Adjustable -2x
Inverting Charge Pump
Pin Description
MAX868
__________________Design Procedure
Setting the Output VoltageSet 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 100kWto 500kWrange,
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.
Regulated, Adjustable -2x
Inverting Charge PumpFigure 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.
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.
