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MAX865EUA
Compact / Dual-Output Charge Pump
_______________General DescriptionThe MAX865 is a CMOS charge-pump DC-DC convert-
er in an ultra-small µMAX package. It produces positive
and negative outputs from a single positive input, and
requires only four capacitors. The charge pump first
doubles the input voltage, then inverts the doubled volt-
age. The input voltage ranges from +1.5V to +6.0V.
The internal oscillator is guaranteed to be between
20kHz and 38kHz, keeping noise above the audio
range while consuming minimal supply current. A 75Ω
output impedance permits useful output currents up to
20mA.
The MAX865 comes in a 1.11mm-high, 8-pin µMAX
package that occupies half the board area of a stan-
dard 8-pin SOIC. For a device with selectable frequen-
cies and logic-controlled shutdown, refer to the MAX864
data sheet.
________________________ApplicationsLow-Voltage GaAsFET Bias in Wireless Handsets
VCO and GaAsFET Supplies
Split Supply from 3 Ni Cells or 1 Li+ Cell
Low-Cost Split Supply for Low-Voltage
Data-Acquisition Systems
Split Supply for Analog Circuitry
LCD Panels
____________________________Features1.11mm-High µMAX PackageCompact: Circuit Fits in 0.08in2Requires Only Four CapacitorsDual Outputs (positive and negative)+1.5V to +6.0V Input Voltage20kHz (min) Frequency (above the audio range)
MAX865
Compact, Dual-Output Charge Pump
__________________Pin Configuration
__________Typical Operating Circuit19-0472; Rev 1; 7/97
__________________________________________Typical Operating Characteristics(Circuit of Figure 1, VIN= 5V, TA= +25°C, unless otherwise noted.)
MAX865
Compact, Dual-Output Charge Pump
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VIN= 5V, C1 = C2 = C3 = C4 = 3.3µF, TA= TMINto TMAX, 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.
V+ to GND.................................................................+12V, -0.3V
IN to GND.................................................................+6.2V, -0.3V
V- to GND..................................................................-12V, +0.3V
V- Output Current .............................................................100mA
V- Short-Circuit to GND ................................................Indefinite
Continuous Power Dissipation (TA= +70°C)
µMAX (derate 4.1mW/°C above +70°C).......................330mW
Operating Temperature Range
MAX865EUA.....................................................-40°C to +85°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10sec).............................+300°C
Note 1:These specifications are guaranteed by design and are not production tested.
MAX865
Compact, Dual-Output Charge Pump
____________________________Typical Operating Characteristics (continued)(Circuit of Figure 1, VIN= 5V, TA= +25°C, unless otherwise noted.)
_____________________Pin DescriptionFigure 1. Test Circuit
MAX865
Compact, Dual-Output Charge Pump
____________________________Typical Operating Characteristics (continued)(Circuit of Figure 1, VIN= 5V, TA= +25°C, unless otherwise noted.)
MAX865
Compact, Dual-Output Charge Pump
_______________Detailed DescriptionThe MAX865 contains all the circuitry needed to imple-
ment a voltage doubler/inverter. Only four external
capacitors are needed. These may be polarized elec-
trolytic or ceramic capacitors with values ranging from
1µF to 100µF.
Figure 2a shows the ideal operation of the positive volt-
age doubler. The on-chip oscillator generates a 50%
duty-cycle clock signal. During the first half cycle,
switches S2 and S4 open, switches S1 and S3 close,
and capacitor C1 charges to the input voltage (VIN).
During the second half cycle, switches S1 and S3
open, switches S2 and S4 close, and capacitor C1 is
level shifted upward by VIN. Assuming ideal switches
and no load on C3, charge transfers into C3 from C1
such that the voltage on C3 will be 2VIN, generating the
positive supply output (V+).
Figure 2b illustrates the ideal operation of the negative
converter. The switches of the negative converter are
out of phase with the positive converter. During the
second half cycle, switches S6 and S8 open and
switches S5 and S7 close, charging C2 from V+
(pumped up to 2VINby the positive charge pump) to
and S7 open, switches S6 and S8 close, and the
charge on capacitor C2 transfers to C4, generating the
negative supply. The eight switches are CMOS power
MOSFETs. Switches S1, S2, S4, and S5 are P-channel
devices, while switches S3, S6, S7, and S8 are N-chan-
nel devices.
Charge-Pump OutputThe MAX865 is not a voltage regulator: the output
source resistance of either charge pump is approxi-
mately 150Ωat room temperature with VIN= +5V, and
V+ and V- will approach +10V and -10V, respectively,
when lightly loaded. Both V+ and V- will droop toward
GND as the current draw from either V+ or V- increas-
es, since V- is derived from V+. Treating each convert-
er separately, the droop of the negative supply
(VDROOP-) is the product of the current draw from V-
(IV-) and the source resistance of the negative convert-
er (RS-):
The droop of the positive supply (VDROOP+) is the
product of the current draw from the positive supply
converter (RS+), where ILOAD+is the combination of IV-
and the external load on V+ (IV+):
Determine V+ and V- as follows:
The output resistance for the positive and negative
charge pumps are tested and specified separately. The
positive charge pump is tested with V- unloaded. The
negative charge pump is tested with V+ supplied from
an external source, isolating the negative charge
pump.
Current draw from either V+ or V- is supplied by the
reservoir capacitor alone during one half cycle of the
clock. Calculate the resulting ripple voltage on either
output as follows:
where ILOADis the load on either V+ or V-. For the typi-
cal fPUMPof 30kHz with 3.3µF reservoir capacitors, the
ripple is 25mV when ILOADis 5mA. Remember that, in
most applications, the total load on V+ is the V+ load
current (IV+) and the current taken by the negative
charge pump (IV-).
Efficiency ConsiderationsTheoretically, a charge-pump voltage multiplier can
approach 100% power efficiency under the following
conditions:The charge-pump switches have virtually no offset
and extremely low on-resistance.The drive circuitry consumes minimal power. The impedances of the reservoir and pump capaci-
tors are negligible.
For the MAX865, the energy loss per clock cycle is the
sum of the energy loss in the positive and negative
converters, as follows:
The average power loss is simply:
Resulting in an efficiency of:
Figure 3. Paralleling MAX865s
MAX865
Compact, Dual-Output Charge Pump
