LMC7660IMX - NATIONAL SEMICONDUCTOR

LMC7660

LMC7660 Switched Capacitor Voltage Converter

Literature Number: SNOSBZ9B

LMC7660

Switched Capacitor Voltage Converter

General Description

The LMC7660 is a CMOS voltage converter capable of

converting a positive voltage in the range of +1.5V to +10V to

the corresponding negative voltage of −1.5V to −10V. The

LMC7660 is a pin-for-pin replacement for the industry-

standard 7660. The converter features: operation over full

temperature and voltage range without need for an external

diode, low quiescent current, and high power efficiency.

The LMC7660 uses its built-in oscillator to switch 4 power

MOS switches and charge two inexpensive electrolytic ca-

pacitors.

Features

nOperation over full temperature and voltage range

without an external diode

nLow supply current, 200 µA max

nPin-for-pin replacement for the 7660

nWide operating range 1.5V to 10V

n97% Voltage Conversion Efficiency

n95% Power Conversion Efficiency

nEasy to use, only 2 external components

nExtended temperature range

nNarrow SO-8 Package

Block Diagram

00913601

Pin Configuration

00913602

Ordering Information

Package Temperature Range NSC Drawing

Industrial

−40˚C to +85˚C

8-Lead Molded DIP LMC7660IN N08E

8-Lead Molded Small Outline LMC7660IM M08A

February 2005

LMC7660 Switched Capacitor Voltage Converter

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 10.5V

Input Voltage on Pin 6, 7

(Note 2) −0.3V to (V

+ 0.3V)

for V

<5.5V

− 5.5V) to (V

+ 0.3V)

for V

>5.5V

Current into Pin 6 (Note 2) 20 µA

Output Short Circuit

Duration (V

≤5.5V) Continuous

Power Dissipation (Note 3)

Dual-In-Line Package 1.4W

Surface-Mount Package 0.6W

Max (Note 3) 150˚C

(Note 3)

Dual-In-Line Package 90˚C/W

Surface-Mount Package 160˚C/W

Storage Temp. Range −65˚C ≤T≤150˚C

Lead Temperature

(Soldering, 5 sec.) 260˚C

ESD Tolerance (Note 7) ±2000V

Electrical Characteristics (Note 4)

Symbol Parameter Conditions Typ

LMC7660IN/

Units

Limits

LMC7660IM

Limit

(Note 5)

Supply Current R

=∞120 200 µA

400 max

H Supply Voltage R

=10kΩ, Pin 6 Open 3 to 10 3 to 10 V

Range High (Note 6) Voltage Efficiency ≥90% 3to10

L Supply Voltage R

=10kΩ, Pin 6 to Gnd. 1.5 to 3.5 1.5 to 3.5 V

Range Low Voltage Efficiency ≥90% 1.5 to 3.5

out

Output Source I

=20mA 55 100 Ω

Resistance 120 max

V = 2V, I

= 3 mA 110 200 Ω

Pin 6 Short to Gnd. 300 max

osc

Oscillator 10 kHz

Frequency

eff

Power Efficiency R

=5kΩ97 95 %

90 min

o eff

Voltage Conversion R

=∞99.9 97 %

Efficiency 95 min

osc

Oscillator Sink or Pin 7 = Gnd. or V

3µA

Source Current

Note 1: Absolute Maximum ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating

the device beyond its rated operating conditions. See Note 4 for conditions.

Note 2: Connecting any input terminal to voltages greater than V+or less than ground may cause destructive latchup. It is recommended that no inputs from sources

operating from external supplies be applied prior to “power-up” of the LMC7660.

Note 3: For operation at elevated temperature, these devices must be derated based on a thermal resistance of θja and Tjmax, Tj=T

A+θja PD.

Note 4: Boldface numbers apply at temperature extremes. All other numbers apply at TA= 25˚C, V+=5V,C

osc = 0, and apply for the LMC7660 unless otherwise

specified. Test circuit is shown in Figure 1 .

Note 5: Limits at room temperature are guaranteed and 100% production tested. Limits in boldface are guaranteed over the operating temperature range (but not

100% tested), and are not used to calculate outgoing quality levels.

Note 6: The LMC7660 can operate without an external diode over the full temperature and voltage range. The LMC7660 can also be used with the external diode

Dx, when replacing previous 7660 designs.

Note 7: The test circuit consists of the human body model of 100 pF in series with 1500Ω.

LMC7660

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Typical Performance Characteristics

OSC Freq. vs OSC

Capacitance V

out

vs I

out

=2V

00913618 00913619

out

vs I

out

=5V

Supply Current & Power Efficiency

vs Load Current (V

= 2V)

00913620 00913621

00913605

FIGURE 1. LMC7660 Test Circuit

LMC7660

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Typical Performance Characteristics (Continued)

Supply Current & Power Efficiency

vs Load Current (V

= 5V)

Output Source Resistiance as a

Function of Temperature

00913622 00913623

Unloaded Oscillator Frequency

as a Function of Temperature Output R vs Supply Voltage

00913624 00913625

eff

vs OSC Freq. @V

=5V

00913626

Application Information

CIRCUIT DESCRIPTION

The LMC7660 contains four large CMOS switches which are

switched in a sequence to provide supply inversion V

out

−V

. Energy transfer and storage are provided by two inex-

pensive electrolytic capacitors. Figure 2 shows how the

LMC7660 can be used to generate −V

from V

. When

switches S1 and S3 are closed, C

charges to the supply

voltage V

. During this time interval, switches S2 and S4 are

open. After C

charges to V

, S1 and S3 are opened, S2 and

S4 are then closed. By connecting S2 to ground, C

devel-

ops a voltage −V

/2 on C

. After a number of cycles C

will be

pumped to exactly −V

. This transfer will be exact assuming

no load on C

, and no loss in the switches.

LMC7660

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Application Information (Continued)

In the circuit of Figure 2, S1 is a P-channel device and S2,

S3, and S4 are N-channel devices. Because the output is

biased below ground, it is important that the p

−

wells of S3

and S4 never become forward biased with respect to either

their sources or drains. A substrate logic circuit guarantees

that these p

−

wells are always held at the proper voltage.

Under all conditions S4 p

−

well must be at the lowest poten-

tial in the circuit. To switch off S4, a level translator generates

GS4

= 0V, and this is accomplished by biasing the level

translator from the S4 p

−

well.

An internal RC oscillator and ÷ 2 circuit provide timing sig-

nals to the level translator. The built-in regulator biases the

oscillator and divider to reduce power dissipation on high

supply voltage. The regulator becomes active at about V

6.5V. Low voltage operation can be improved if the LV pin is

shorted to ground for V

≤3.5V. For V

≥3.5V, the LV pin

must be left open to prevent damage to the part.

POWER EFFICIENCY AND RIPPLE

It is theoretically possible to approach 100% efficiency if the

following conditions are met:

1. The drive circuitry consumes little power.

2. The power switches are matched and have low R

3. The impedance of the reservoir and pump capacitors

are negligibly small at the pumping frequency.

The LMC7660 closely approaches 1 and 2 above. By using

a large pump capacitor C

, the charge removed while sup-

plying the reservoir capacitor is small compared to C

’s total

charge. Small removed charge means small changes in the

pump capacitor voltage, and thus small energy loss and high

efficiency. The energy loss by C

is:

By using a large reservoir capacitor, the output ripple can be

reduced to an acceptable level. For example, if the load

current is 5 mA and the accepted ripple is 200 mV, then the

reservoir capacitor can omit approximately be calculated

from:

PRECAUTIONS

1. Do not exceed the maximum supply voltage or junction

temperature.

2. Do not short pin 6 (LV terminal) to ground for supply

voltages greater than 3.5V.

3. Do not short circuit the output to V

4. External electrolytic capacitors C

and C

should have

their polarities connected as shown in Figure 1.

REPLACING PREVIOUS 7660 DESIGNS

To prevent destructive latchup, previous 7660 designs re-

quire a diode in series with the output when operated at

elevated temperature or supply voltage. Although this pre-

vented the latchup problem of these designs, it lowered the

available output voltage and increased the output series

resistance.

The National LMC7660 has been designed to solve the

inherent latch problem. The LCM7660 can operate over the

entire supply voltage and temperature range without the

need for an output diode. When replacing existing designs,

the LMC7660 can be operated with diode Dx.

Typical Applications

CHANGING OSCILLATOR FREQUENCY

It is possible to dramatically reduce the quiescent operating

current of the LMC7660 by lowering the oscillator frequency.

The oscillator frequency can be lowered from a nominal 10

kHz to several hundred hertz, by adding a slow-down ca-

pacitor C

osc

(Figure 3). As shown in the Typical Performance

Curves the supply current can be lowered to the 10 µA

range. This low current drain can be extremely useful when

used in µPower and battery back-up equipment. It must be

understood that the lower operating frequency and supply

current cause an increased impedance of C

and C

. The

increased impedance, due to a lower switching rate, can be

offset by raising C

and C

until ripple and load current

requirements are met.

SYNCHRONIZING TO AN EXTERNAL CLOCK

Figure 4 shows an LMC7660 synchronized to an external

clock. The CMOS gate overrides the internal oscillator when

it is necessary to switch faster or reduce power supply

00913606

FIGURE 2. Idealized Voltage Converter

LMC7660

www.national.com5

Typical Applications (Continued)

interference. The external clock still passes through the ÷2

circuit in the 7660, so the pumping frequency will be

⁄

the

external clock frequency.

LOWERING OUTPUT IMPEDANCE

Paralleling two or more LMC7660’s lowers output imped-

ance. Each device must have it’s own pumping capacitor C

but the reservoir capacitor C

is shared as depicted in Figure

5. The composite output resistance is:

00913607

FIGURE 3. Reduce Supply Current by Lowering Oscillator Frequency

00913608

FIGURE 4. Synchronizing to an External Clock

LMC7660

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Typical Applications (Continued)

INCREASING OUTPUT VOLTAGE

Stacking the LMC7660s is an easy way to produce a greater

negative voltage. It should be noted that the input current

required for each stage is twice the load current on that

stage as shown in Figure 6. The effective output resistance

is approximately the sum of the individual R

out

values, and

so only a few levels of multiplication can be used.

It is possible to generate −15V from +5V by connecting the

second 7660’s pin 8 to +5V instead of ground as shown in

Figure 7. Note that the second 7660 sees a full 20V and the

input supply should not be increased beyond +5V.

SPLIT V

IN HALF

Figure 8 is one of the more interesting applications for the

LMC7660. The circuit can be used as a precision voltage

divider (for very light loads), alternately it is used to generate

⁄

supply point in battery applications. In the

⁄

cycle when

S1 and S3 are closed, the supply voltage divides across the

capacitors in a conventional way proportional to their value.

In the

⁄

cycle when S2 and S4 are closed, the capacitors

switch from a series connection to a parallel connection. This

forces the capacitors to have the same voltage; the charge

00913609

FIGURE 5. Lowering Output Resistance by Paralleling Devices

00913610

FIGURE 6. Higher Voltage by Cascade

00913611

FIGURE 7. Getting −15V from +5V

LMC7660

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Typical Applications (Continued)

redistributes to maintain precisely V

/2, across C

and C

.In

this application all devices are only V

/2, and the supply

voltage can be raised to 20V giving exactly 10V at V

out

GETTING UP … AND DOWN

The LMC7660 can also be used as a positive voltage multi-

plier. This application, shown in Figure 9, requires 2 addi-

tional diodes. During the first

⁄

cycle S2 charges C

through D1; D2 is reverse biased. In the next

⁄

cycle S2 is

open and S1 is closed. Since C

1 is charged to V

−V

and

is referenced to V

through S1, the junction of D1 and D2 is

at V

+(V

−V

). D1 is reverse biased in this interval. This

application uses only two of the four switches in the 7660.

The other two switches can be put to use in performing a

negative conversion at the same time as shown in Figure 10.

In the

⁄

cycle that D1 is charging C

1, C

2 is connected

from ground to −V

out

via S2 and S4, and C

2 is storing C

2’s

charge. In the interval that S1 and S3 are closed, C

1 pumps

the junction of D1 and D2 above V

, while C

2 is refreshed

from V

THERMOMETER SPANS 180˚C

Using the combined negative and positive multiplier of Fig-

ure 11 with an LM35 it is possible to make a µPower ther-

mometer that spans a 180˚C temperature range. The LM35

temperature sensor has an output sensitivity of 10 mV/˚C,

while drawing only 50 µA of quiescent current. In order for

the LM35 to measure negative temperatures, a pull down to

a negative voltage is required. Figure 11 shows a thermom-

eter circuit for measuring temperatures from −55˚C to

+125˚C and requiring only two 1.5V cells. End of battery life

can be extended by replacing the up converter diodes with

Schottky’s.

00913612

FIGURE 8. Split V

in Half

00913613

FIGURE 9. Positive Voltage Multiplier

00913614

FIGURE 10. Combined Negative Converter and Positive Multiplier

LMC7660

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Typical Applications (Continued)

REGULATING −V

OUT

It is possible to regulate the output of the LMC7660 and still

maintain µPower performance. This is done by enclosing the

LMC7660 in a loop with a LP2951. The circuit of Figure 12

will regulate V

out

to −5V for I

= 10 mA, and V

= 6V. For V

>7V, the output stays in regulation up to I

= 25 mA. The

error flag on pin 5 of the LP2951 sets low when the regulated

output at pin 4 drops by about 5%. The LP2951 can be

shutdown by taking pin 3 high; the LMC7660 can be shut-

down by shorting pin 7 and pin 8.

The LP2951 can be reconfigured to an adjustable type regu-

lator, which means the LMC7660 can give a regulated output

from −2.0V to −10V dependent on the resistor ratios R1 and

R2, as shown in Figure 13,V

ref

= 1.235V:

00913615

*For lower voltage operation, use Schottky rectifiers

FIGURE 11. µPower Thermometer Spans 180˚C, and Pulls Only 150 µA

00913616

FIGURE 12. Regulated −5V with 200 µA Standby Current

LMC7660

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Typical Applications (Continued)

00913617

Vref = 1.235V

*Low voltage operation

FIGURE 13. LMC7660 and LP2951 Make a Negative Adjustable Regulator

LMC7660

www.national.com 10

Physical Dimensions inches (millimeters)

unless otherwise noted

Molded Small Outline Package (M)

Order Number LMC7660IM

NS Package Number M08A

LMC7660

www.national.com11

Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

Molded Dual-In-Line Package (N)

Order Number LMC7660IN

NS Package Number N08E

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.

For the most current product information visit us at www.national.com.

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(b) support or sustain life, and whose failure to perform when

properly used in accordance with instructions for use

provided in the labeling, can be reasonably expected to result

in a significant injury to the user.

2. A critical component is any component of a life support

device or system whose failure to perform can be reasonably

expected to cause the failure of the life support device or

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LMC7660 Switched Capacitor Voltage Converter

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