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MAX660 Switched Capacitor Voltage Converter
August 1998
MAX660 Switched Capacitor Voltage Converter
General Description
The MAX660 CMOS charge-pump voltage converter inverts a positive voltage in the range of 1.5V to 5.5V to the corresponding negative voltage. The MAX660 uses two low cost capacitors to provide 100 mA of output current without the cost, size, and EMI related to inductor based converters. With an operating current of only 120 A and operating efficiency greater than 90% at most loads, the MAX660 provides ideal performance for battery powered systems. The MAX660 may also be used as a positive voltage doubler. The oscillator frequency can be lowered by adding an external capacitor to the OSC pin. Also, the OSC pin may be used to drive the MAX660 with an external clock. A frequency control (FC) pin selects the oscillator frequency of 10 kHz or 80 kHz.
Features
n n n n n Inverts or doubles input supply voltage Narrow SO-8 Package 6.5 typical output resistance 88% typical conversion efficiency at 100 mA Selectable oscillator frequency: 10 kHz/80 kHz
Applications
n n n n n n Laptop computers Cellular phones Medical instruments Operational amplifier power supplies Interface power supplies Handheld instruments
Typical Application Circuits
Voltage Inverter
Positive Voltage Doubler
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Connection Diagram
8-Lead SO
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Top View
Ordering Information
Order Number MAX660M MAX660MX Top Mark Date Code MAX660M Date Code MAX660M Package M08A M08A Rail (95 units/rail) Tape and Reel (2500 units/rail) Supplied as
(c) 1998 National Semiconductor Corporation
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage (V+ to GND, or GND to OUT) 6V LV (OUT - 0.3V) to (GND + 3V) FC, OSC The least negative of (OUT - 0.3V) or (V+ - 6V) to (V+ + 0.3V) V+ and OUT Continuous Output Current 120 mA Output Short-Circuit Duration to GND (Note 2) 1 sec.
Power Dissipation (TA = 25C) (Note 3) TJ Max (Note 3) JA (Note 3) Operating Temperature Range Storage Temperature Range Lead Temperature (Soldering, 10 seconds) ESD Rating
735 mW 150C 170C/W -40C to +85C -65C to +150C 300C 2 kV
Electrical Characteristics
Limits in standard typeface are for TJ = 25C, and limits in boldface type apply over the full operating temperature range. Unless otherwise specified: V+ = 5V, FC = Open, C1 = C2 = 150 F. (Note 4) Symbol V+ Parameter Supply Voltage RL = 1k Condition Inverter, LV = Open (Note 5) Inverter, LV = GND Doubler, LV = OUT FC = Open FC = V+ 100 100 6.5 5 40 10 80 10 12 Min 3.5 1.5 2.5 0.12 1 Typ Max 5.5 5.5 5.5 0.5 3 mA V Units
IQ
Supply Current
No Load LV = Open
IL ROUT FOSC IOSC PEFF
Output Current Output Resistance (Note 6) Oscillator Frequency OSC Input Current Power Efficiency
TA +85C, OUT -4V TA > +85C, OUT -3.8V IL = 100 mA TA +85C OSC = Open FC = Open FC = V+ RL (1k) between V and OUT RL (500) between GND and OUT IL = 100 mA to GND
+
mA kHz A
TA > +85C FC = Open FC = V+
2 16
96 92 99 98 96 88 99.96
% %
VOEFF
Voltage Conversion Efficiency
No Load
Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device beyond its rated operating conditions. Note 2: OUT may be shorted to GND for one second without damage. However, shorting OUT to V+ may damage the device and should be avoided. Also, for temperatures above 85C, OUT must not be shorted to GND or V+, or device may be damaged. Note 3: The maximum allowable power dissipation is calculated by using PDMax = (TJMax - TA)/JA, where TJMax is the maximum junction temperature, TA is the ambient temperature, and JA is the junction-to-ambient thermal resistance of the specified package. Note 4: In the test circuit, capacitors C1 and C2 are 0.2 maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce output voltage and efficiency. Note 5: The minimum limit for this parameter is different from the limit of 3.0V for the industry-standard "660" product. For inverter operation with supply voltage below 3.5V, connect the LV pin to GND. Note 6: Specified output resistance includes internal switch resistance and capacitor ESR.
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Test Circuit
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FIGURE 1. MAX660 Test Circuit
Typical Performance Characteristics
Supply Current vs Supply Voltage
(Circuit of Figure 1) Output Source Resistance vs Supply Voltage
Supply Current vs Oscillator Frequency
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Output Source Resistance vs Temperature
Efficiency vs Load Load Current
Output Voltage Drop vs Load Current
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Typical Performance Characteristics
Efficiency vs Oscillator Frequency
(Circuit of Figure 1) (Continued)
Output Voltage vs Oscillator Frequency
Oscillator Frequency vs External Capacitance
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Oscillator Frequency Supply Voltage (FC = V+)
Oscillator Frequency vs Supply Voltage (FC = Open)
Oscillator Frequency vs Temperature (FC = V+)
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Oscillator Frequency vs Temperature (FC = Open)
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Pin Description
Pin 1 Name Voltage Inverter FC Frequency control for internal oscillator: FC = open, fOSC = 10 kHz (typ); FC = V+, fOSC = 80 kHz (typ); FC has no effect when OSC pin is driven externally. 2 3 4 5 6 CAP+ GND CAP- OUT LV Connect this pin to the positive terminal of charge-pump capacitor. Power supply ground input. Connect this pin to the negative terminal of charge-pump capacitor. Negative voltage output. Low-voltage operation input. Tie LV to GND when input voltage is less than 3.5V. Above 3.5V, LV can be connected to GND or left open. When driving OSC with an external clock, LV must be connected to GND. Oscillator control input. OSC is connected to an internal 15 pF capacitor. An external capacitor can be connected to slow the oscillator. Also, an external clock can be used to drive OSC. Power supply positive voltage input. Same as inverter. Power supply positive voltage input. Same as inverter. Power supply ground input. LV must be tied to OUT. Function Voltage Doubler Same as inverter.
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OSC
Same as inverter except that OSC cannot be driven by an external clock.
8
V+
Positive voltage output.
Circuit Description
The MAX660 contains four large CMOS switches which are switched in a sequence to invert the input supply voltage. Energy transfer and storage are provided by external capacitors. Figure 2 illustrates the voltage conversion scheme. When S1 and S3 are closed, C1 charges to the supply voltage V+. During this time interval switches S2 and S4 are open. In the second time interval, S1 and S3 are open and S2 and S4 are closed, C1 is charging C2. After a number of cycles, the voltage across C2 will be pumped to V+. Since the anode of C2 is connected to ground, the output at the cathode of C2 equals -(V+) assuming no load on C2, no loss in the switches, and no ESR in the capacitors. In reality, the charge transfer efficiency depends on the switching frequency, the on-resistance of the switches, and the ESR of the capacitors.
Application Information
SIMPLE NEGATIVE VOLTAGE CONVERTER The main application of MAX660 is to generate a negative supply voltage. The voltage inverter circuit uses only two external capacitors as shown in the Typical Application Circuits. The range of the input supply voltage is 1.5V to 5.5V. For a supply voltage less than 3.5V, the LV pin must be connected to ground to bypass the internal regulator circuitry. This gives the best performance in low voltage applications. If the supply voltage is greater than 3.5V, LV may be connected to ground or left open. The choice of leaving LV open simplifies the direct substitution of the MAX660 for the LMC7660 Switched Capacitor Voltage Converter. The output characteristics of this circuit can be approximated by an ideal voltage source in series with a resistor. The voltage source equals -(V+). The output resistance Rout is a function of the ON resistance of the internal MOS switches, the oscillator frequency, and the capacitance and ESR of C1 and C2. A good approximation is:
where RSW is the sum of the ON resistance of the internal MOS switches shown in Figure 2.
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FIGURE 2. Voltage Inverting Principle
High value, low ESR capacitors will reduce the output resistance. Instead of increasing the capacitance, the oscillator frequency can be increased to reduce the 2/(fosc x C1) term. Once this term is trivial compared with RSW and ESRs, further increasing in oscillator frequency and capacitance will become ineffective. The peak-to-peak output voltage ripple is determined by the oscillator frequency, and the capacitance and ESR of the output capacitor C2:
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Application Information
(Continued)
lows smaller capacitors to be used for equivalent output resistance and ripple, but increases the typical supply current from 0.12 mA to 1 mA. The oscillator frequency can be lowered by adding an external capacitor between OSC and GND. (See Typical Performance Characteristics.) Also, in the inverter mode, an external clock that swings within 100 mV of V+ and GND can be used to drive OSC. Any CMOS logic gate is suitable for driving OSC. LV must be grounded when driving OSC. The maximum external clock frequency is limited to 150 kHz. The switching frequency of the converter (also called the charge pump frequency) is half of the oscillator frequency. Note: OSC cannot be driven by an external clock in the voltage-doubling mode. TABLE 1. MAX660 Oscillator Frequency Selection FC Open V+ Open or V+ N/A OSC Open Open External Capacitor External Clock (inverter mode only) Oscillator 10 kHz 80 kHz See Typical Performance Characteristics External Clock Frequency
Again, using a low ESR capacitor will result in lower ripple. POSITIVE VOLTAGE DOUBLER The MAX660 can operate as a positive voltage doubler (as shown in the Typical Application Circuits). The doubling function is achieved by reversing some of the connections to the device. The input voltage is applied to the GND pin with an allowable voltage from 2.5V to 5.5V. The V+ pin is used as the output. The LV pin and OUT pin must be connected to ground. The OSC pin can not be driven by an external clock in this operation mode. The unloaded output voltage is twice of the input voltage and is not reduced by the diode D1's forward drop. The Schottky diode D1 is only needed for start-up. The internal oscillator circuit uses the V+ pin and the LV pin (connected to ground in the voltage doubler circuit) as its power rails. Voltage across V+ and LV must be larger than 1.5V to insure the operation of the oscillator. During start-up, D1 is used to charge up the voltage at V+ pin to start the oscillator; also, it protects the device from turning-on its own parasitic diode and potentially latching-up. Therefore, the Schottky diode D1 should have enough current carrying capability to charge the output capacitor at start-up, as well as a low forward voltage to prevent the internal parasitic diode from turning-on. A Schottky diode like 1N5817 can be used for most applications. If the input voltage ramp is less than 10V/ms, a smaller Schottky diode like MBR0520LT1 can be used to reduce the circuit size. SPLIT V+ IN HALF Another interesting application shown in the Basic Application Circuits is using the MAX660 as a precision voltage divider. Since the off-voltage across each switch equals VIN/2, the input voltage can be raised to +11V.
CAPACITOR SELECTION As discussed in the Simple Negative Voltage Converter section, the output resistance and ripple voltage are dependent on the capacitance and ESR values of the external capacitors. The output voltage drop is the load current times the output resistance, and the power efficiency is
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FIGURE 3. Splitting VIN in Half CHANGING OSCILLATOR FREQUENCY The internal oscillator frequency can be selected using the Frequency Control (FC) pin. When FC is open, the oscillator frequency is 10 kHz; when FC is connected to V+, the frequency increases to 80 kHz. A higher oscillator frequency al-
Where IQ(V+) is the quiescent power loss of the IC device, and IL2ROUT is the conversion loss associated with the switch on-resistance, the two external capacitors and their ESRs. Since the switching current charging and discharging C1 is approximately twice as the output current, the effect of the ESR of the pumping capacitor C1 is multiplied by four in the output resistance. The output capacitor C2 is charging and discharging at a current approximately equal to the output current, therefore, its ESR only counts once in the output resistance. However, the ESR of C2 directly affects the output voltage ripple. Therefore, low ESR capacitors (Table 2) are recommended for both capacitors to maximize efficiency, reduce the output voltage drop and voltage ripple. For convenience, C1 and C2 are usually chosen to be the same. The output resistance varies with the oscillator frequency and the capacitors. In Figure 4, the output resistance vs. oscillator frequency curves are drawn for three different tantalum capacitors. At very low frequency range, capacitance plays the most important role in determining the output resistance. Once the frequency is increased to some point (such as 20 kHz for the 150 F capacitors), the output resistance is dominated by the ON resistance of the internal switches and the ESRs of the external capacitors. A low value, smaller size capacitor usually has a higher ESR compared with a bigger size capacitor of the same type. For lower ESR, use ceramic capacitors.
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Application Information
(Continued)
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FIGURE 4. Output Source Resistance vs Oscillator Frequency TABLE 2. Low ESR Capacitor Manufacturers Manufacturer Nichicon Corp. AVX Corp. Sprague Sanyo Phone (708)-843-7500 (803)-448-9411 (207)-324-4140 (619)-661-6835 FAX (708)-843-2798 (803)-448-1943 (207)-324-7223 (619)-661-1055 Capacitor Type PL, PF series, through-hole aluminum electrolytic TPS series, surface-mount tantalum 593D, 594D, 595D series, surface-mount tantalum OS-CON series, through-hole aluminum electrolytic
Other Applications
PARALLELING DEVICES Any number of MAX660s can be paralleled to reduce the output resistance. Each device must have its own pumping capacitor C1, while only one output capacitor Cout is needed as shown in Figure 5. The composite output resistance is:
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FIGURE 5. Lowering Output Resistance by Paralleling Devices CASCADING DEVICES Cascading the is an easy way to produce a greater negative voltage (as shown in Figure 6). If n is the integer representing the number of devices cascaded, the unloaded output voltage Vout is (-nVin). The effective output resistance is equal to the weighted sum of each individual device:
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Other Applications
(Continued)
A three-stage cascade circuit shown in Figure 7 generates -3Vin, from Vin. Cascading is also possible when devices are operating in doubling mode. In Figure 8, two devices are cascaded to generate 3Vin. An example of using the circuit in Figure 7 or Figure 8 is generating +15V or -15V from a +5V input. Note that the number of n is practically limited since the increasing of n significantly reduces the efficiency and increases the output resistance and output voltage ripple.
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FIGURE 6. Increasing Output Voltage by Cascading Devices
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FIGURE 7. Generating -3Vin from +Vin
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FIGURE 8. Generating +3Vin from +Vin REGULATING Vout It is possible to regulate the output of the MAX660 by use of a low dropout regulator (such as LP2951). The whole converter is depicted in Figure 9. This converter can give a regulated output from -1.5V to -5.5V by choosing the proper resistor ratio:
where Vref = 1.235V.
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Other Applications
(Continued)
The error flag on pin 5 of the LP2951 goes low when the regulated output at pin 4 drops by about 5%. The LP2951 can be shutdown by taking pin 3 high.
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FIGURE 9. Combining MAX660 with LP2951 to Make a Negative Adjustable Regulator Also, as shown in Figure 10 by operating MAX660 in voltage doubling mode and adding a linear regulator (such as LP2981) at the output, we can get +5V output from an input as low as +3V.
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FIGURE 10. Generating +5V from +3V Input Voltage
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Other Applications
(Continued)
OTHER SWITCHED-CAPACITOR CONVERTERS Please refer to Table 3, which shows National's Switched-Capacitor Converter products. TABLE 3. Switched-Capacitor Converters LM2664 Package Supply Current (typ., mA) Output (typ.) Oscillator (kHz) Input (V) Output Mode(s) SOT23-6 0.22 12 80 1.8 to 5.5 Invert LM2660 Package Supply Current (typ., mA) Output (typ.) Oscillator (kHz) Input (V) Output Mode(s) Mini SO-8, SO-8 0.12 at 10kHz, 1.0 at 80kHz 6.5 10, 80 1.8 to 5.5 Invert, Double LM2665 SOT23-6 0.22 12 80 1.8 to 5.5 Double LM2661 Mini SO-8, SO-8 1.0 6.5 80 1.8 to 5.5 Invert, Double LM3350 Mini SO-8 3.75 4.2 800 2.5 to 6.25 3/2, 2/3 LM2662 SO-8 0.3 at 10kHz, 1.3 at 70kHz 3.5 10, 70 1.8 to 5.5 Invert, Double LM3351 Mini SO-8 1.1 4.2 200 2.5 to 6.25 3/2, 2/3 LM2663 SO-8 1.3 3.5 70 1.8 to 5.5 Invert, Double MAX660 SO-8 0.12 at 10kHz, 1.0 at 80kHz 6.5 10, 80 1.8 to 5.5 Invert, Double
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MAX660 Switched Capacitor Voltage Converter
Physical Dimensions
inches (millimeters) unless otherwise noted
8-Lead SO (M) Order Number MAX660M NS Package Number M08A
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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

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