CMOS Monolithic Voltage Converter
Combined Positive Supply Multiplication
and Negative Voltage Conversion
This dual function is illustrated in Figure 5. In this cir-cuit, capacitors C1 and C3 perform the pump andreservoir functions respectively for generation of thenegative voltage. Capacitors C2 and C4 are respec-tively pump and reservoir for the multiplied positivevoltage. This circuit configuration, however, leads tohigher source impedances of the generated supplies.This is due to the finite impedance of the commoncharge-pump driver.
+VIN82MAX660D1D1, D2 = 1N414835VC1OUT = -VINC246D2VOUT = (2VIN) -C3C4(VFD1) - (VFD2)Figure 5. Combined Positive Multiplier and Negative Converter
1M1M3V LITHIUM BATTERYOPEN-DRAINDURACELL DL123ALOW-BATTERY OUTPUTLBI388INOUT25V/100mA2150μFMAX660MAX667LBO7150150μF46μFDD1620k1MSET6GNDSHDN545220kNOTE: ALL 150μF CAPACITORS ARE MAXC001, AVAILABLE FROM MAXIM.Figure 6. MAX660 generates a +5V regulated output from a 3Vlithium battery and operates for 16 hours with a 40mA load.
MAX660CMOS Monolithic Voltage ConverterMAX660______________Detailed Description
The MAX660 capacitive charge-pump circuit eitherinverts or doubles the input voltage (see TypicalOperating Circuits). For highest performance, loweffective series resistance (ESR) capacitors should beused. See Capacitor Selectionsection for more details.When using the inverting mode with a supply voltageless than 3V, LV must be connected to GND. Thisbypasses the internal regulator circuitry and providesbest performance in low-voltage applications. Whenusing the inverter mode with a supply voltage above3V, LV may be connected to GND or left open. The partis typically operated with LV grounded, but since LVmay be left open, the substitution of the MAX660 for theICL7660 is simplified. LV must be grounded when over-driving OSC (see Changing Oscillator Frequencysec-tion). Connect LV to OUT (for any supply voltage) whenusing the doubling mode.
one-half of the charge-pump cycle. This introduces apeak-to-peak ripple of:
VRIPPLE = IOUT+IOUT(ESRC2)
2(fPUMP) (C2)
For a nominal fPUMPof 5kHz (one-half the nominal10kHz oscillator frequency) and C2 = 150μF with anESR of 0.2?, ripple is approximately 90mV with a100mA load current. If C2 is raised to 390μF, the rippledrops to 45mV.
Positive Voltage Doubler
The MAX660 operates in the voltage-doubling mode asshown in the Typical Operating Circuit.The no-loadoutput is 2 x VIN.
Other Switched-Capacitor Converters
Please refer to Table 1, which shows Maxim’s charge-pump offerings.
__________Applications Information
Negative Voltage Converter
The most common application of the MAX660 is as acharge-pump voltage inverter. The operating circuituses only two external capacitors, C1 and C2 (seeTypical Operating Circuits).
Even though its output is not actively regulated, theMAX660 is very insensitive to load current changes. Atypical output source resistance of 6.5?means thatwith an input of +5V the output voltage is -5V underlight load, and decreases only to -4.35V with a load of100mA. Output source resistance vs. temperature andsupply voltage are shown in the Typical OperatingCharacteristicsgraphs.
Output ripple voltage is calculated by noting the outputcurrent supplied is solely from capacitor C2 during
Changing Oscillator Frequency
Four modes control the MAX660’s clock frequency, aslisted below:FCOpenFC = V+Open orFC = V+Open
OSCOpenOpenExternal CapacitorExternal Clock
Oscillator Frequency10kHz80kHz
See Typical OperatingCharacteristics
External Clock Frequency
When FC and OSC are unconnected (open), the oscil-lator runs at 10kHz typically. When FC is connected toV+, the charge and discharge current at OSC changesfrom 1.0μA to 8.0μA, thus increasing the oscillator
Table 1. Single-Output Charge Pumps
MAX828PackageOp. Current(typ, mA)Output ?(typ)Pump Rate(kHz)Input (V)SOT 23-50.0620121.25 to 5.5MAX829SOT 23-50.1520351.25 to 5.5MAX860SO-8,μMAXMAX861SO-8,μMAXMAX660SO-8MAX1044SO-8,μMAX0.036.551.5 to 10ICL7662SO-80.25125101.5 to 10ICL7660SO-8,μMAX0.0855101.5 to 100.2 at 6kHz,0.3 at 13kHz,0.12 at 5kHz,0.6 at 50kHz,1.1 at 100kHz,1 at 40kHz1.4 at 130kHz2.5 at 250kHz126, 50, 1301.5 to 5.51213, 100, 1501.5 to 5.56.55, 401.5 to 5.5
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