Datasheet LM2660, LM2661 Datasheet (National Semiconductor)

LM2660/LM2661
Switched Capacitor Voltage Converter

LM2660/LM2661 Switched Capacitor Voltage Converter

September 1999

General Description

The LM2660/LM2661 CMOS charge-pump voltage con­verter inverts a positive voltage in the range of 1.5V to 5.5V
to thecorresponding negative voltage. The LM2660/LM2661
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 µA
and operating efficiency greater than 90%at most loads, the
LM2660/LM2661 provides ideal performance for battery
powered systems. The LM2660/LM2661 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 LM2660/LM2661 with an external clock. For
LM2660, a frequency control (FC) pin selects the oscillator
frequency of 10 kHz or 80 kHz. For LM2661, an external
shutdown (SD) pin replaces the FC pin. The SD pin can be
used to disable the device and reduce the quiescent current
to 0.5 µA. The oscillator frequency for the LM2661 is 80 kHz.

Basic Application Circuits

Voltage Inverter

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Features

n Inverts or doubles input supply voltage
n Narrow SO-8 and Mini SO-8 Package
n 6.5Ω typical output resistance
n 88%typical conversion efficiency at 100 mA
n (LM2660) selectable oscillator frequency: 10 kHz/80 kHz
n (LM2661) low current shutdown mode

Applications

n Laptop computers
n Cellular phones
n Medical instruments
n Operational amplifier power supplies
n Interface power supplies
n Handheld instruments

Positive Voltage Doubler

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Splitting VINin Half

DS012911-26

© 1999 National Semiconductor Corporation DS012911 www.national.com

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

LM2660/LM2661

LV (OUT − 0.3V) to (GND + 3V)
FC, OSC The least negative of (OUT − 0.3V)

V+ and OUT Continuous Output Current 120 mA
Output Short-Circuit Duration to GND (Note 2) 1 sec.

or (V+ − 6V) to (V+ + 0.3V)

Power Dissipation

=

25˚C) (Note 3) 735 mW 500 mW

(T

A

Max (Note 3) 150˚C 150˚C

T

J

(Note 3) 170˚C/W 250˚C/W

θ

JA

Operating Junction
Temperature

Range −40˚C to +85˚C
Storage Temperature Range −65˚C to +150˚C
Lead Temperature 300˚C

Package

MMM

(Soldering, 10 seconds)
ESD Rating 2 kV

Electrical Characteristics

Limits in standard typeface are for T
less otherwise specified: V+=5V, FC=Open, C

Symbol Parameter Condition Min Typ Max Units

V+ Supply Voltage R

I

Q

I

SD

Supply Current No Load FC=Open (LM2660) 0.12 0.5

Shutdown Supply Current
(LM2661)

V

SD

Shutdown Pin Input Voltage Shutdown Mode 2.0 (Note 5)
(LM2661) Normal Operation 0.3

I

L

R

OUT

f

OSC

f

SW

I

OSC

P

EFF

V

OEFF

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 85˚C, 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 P
ambient temperature, and θ

Note 4: In the test circuit, capacitors C
age and efficiency.

Note 5: In doubling mode, when V
Note 6: Specified output resistance includes internal switch resistance and capacitor ESR.
Note 7: For LM2661, the oscillator frequency is 80 kHz.
Note 8: The output switches operate at one half of the oscillator frequency, f

Output Current TA≤ +85˚C, OUT ≤ −4V 100

Output Resistance (Note 6) I

Oscillator Frequency (Note 7) OSC=Open FC=Open 5 10

Switching Frequency (Note 8) OSC=Open FC=Open 2.5 5

OSC Input Current FC=Open

Power Efficiency RL(1k) between V+and OUT 96 98

Voltage Conversion Efficiency No Load 99 99.96

is the junction-to-ambient thermal resistance of the specified package.

JA

out

=

25˚C, and limits in boldface type apply over the full operating temperature range. Un-

J

=

=

C

150 µF. (Note 4)

1

2

=

1k Inverter, LV=Open 3.5 5.5

L

Inverter, LV=GND 1.5 5.5 V
Doubler, LV=OUT 2.5 5.5

SD=Ground (LM2661)

1 3

0.5 2 µA

>

+85˚C, OUT ≤ −3.8V 100

T

A

=

100 mA T

L

≤ +85˚C 6.5 10

A

>

+85˚C 12

T

A

FC=V+ 40 80

FC=V+ 20 40

±

2

±

FC=V+

R

(500) between GND and OUT 92 96

L

=

I

100 mA to GND 88

L

=

DMax

and C2are 0.2Ω maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce output volt-

1

>

5V, minimum input high for shutdown equals V

=

OSC

(T

JMax−TA

out

2f

SW

−3V.

.

)/θJA, where T

is the maximum junction temperature, TAis the

JMax

16

mALV=Open FC=V+ (LM2660) or

V

mA

Ω

kHz

kHz

µA

%

%

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Test Circuits

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FIGURE 1. LM2660 and LM2661 Test Circuits

Typical Performance Characteristics

(Circuit of

Figure 1

LM2660/LM2661

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)

Supply Current vs
Supply Voltage

Output Source
Resistance vs
Temperature

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DS012911-10

Supply Current vs
Oscillator Frequency

Efficiency vs Load
Current

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Output Source
Resistance vs Supply
Voltage

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Output Voltage Drop
vs Load Current

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Typical Performance Characteristics (Circuit of

Figure 1

) (Continued)

Efficiency vs
Oscillator Frequency

LM2660/LM2661

Oscillator Frequency
vs Supply Voltage
(FC=V+)

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DS012911-16

Output Voltage vs
Oscillator Frequency

Oscillator Frequency
vs Supply Voltage
(FC=Open)

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Oscillator Frequency
vs External
Capacitance

DS012911-15

Oscillator Frequency
vs Temperature
(FC=V+)

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Oscillator Frequency
vs Temperature
(FC=Open)

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Shutdown Supply
Current vs
Temperature
(LM2661 Only)

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Connection Diagrams

LM2660/LM2661

8-Lead SO (M) or Mini SO (MM)

DS012911-1

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Top View

Order Number LM2660M, LM2661M, LM2660MM or LM2661MM

See NS Package Number M08A and MUA08A

Ordering Information

Order Number Package Number Package Marking Supplied As

LM2660M M08A Datecode Rail (95 units/rail)

LM26
60M

LM2660MX M08A Datecode Tape and Reel (2500 units/rail)

LM26

60M
LM2660MM MUA08A S01A (Note 9) Tape and Reel (250 units/rail)
LM2660MMX MUA08A S01A (Note 9) Tape and Reel (3500 units/rail)
LM2661M M08A Datecode Rail (95 units/rail)

LM26

61M
LM2661MX M08A Datecode Tape and Reel (2500 units/rail)

LM26

61M
LM2661MM MUA08A S02A (Note 9) Tape and Reel (250 units/rail)
LM2661MMX MUA08A S02A (Note 9) Tape and Reel (3500 units/rail)

Note 9: The first letter “S” identifies the part as a switched capacitor converter. The next two numbers are the device number: “01” for a LM2660 device, and “02”
for a LM2661 device. The fourth letter “A” indicates the grade. Only one grade is available. Larger quantity reels are available upon request.

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Pin Description

Pin Name Function

Voltage Inverter Voltage Doubler

1 FC Frequency control for internal oscillator: Same as inverter.

LM2660/LM2661

(LM2660) FC=open, f

FC=V+, f
FC has no effect when OSC pin is driven externally.

1SD

(LM2661)

Shutdown control pin, tie this pin to the ground in
normal operation, and to V+ for shutdown.

2 CAP+ Connect this pin to the positive terminal of

charge-pump capacitor.
3 GND Power supply ground input. Power supply positive voltage input.
4 CAP− Connect this pin to the negative terminal of

charge-pump capacitor.
5 OUT Negative voltage output. Power supply ground input.
6 LV 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.
7 OSC 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.
8 V+ Power supply positive voltage input. Positive voltage output.

OSC

OSC

=

10 kHz (typ);

=

80 kHz (typ);

Same as inverter.

Same as inverter.

Same as inverter.

LV must be tied to OUT.

Same as inverter except that OSC cannot be driven
by an external clock.

Circuit Description

The LM2660/LM2661 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.
scheme. When S
ply voltage V+. During this time interval switches S
are open. In the second time interval, S1and S3are open
and S

2

of cycles, the voltage across C
the anode of C
cathode of C
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.

Figure 2

illustrates the voltage conversion

and S3are closed, C1charges to the sup-

1

2

and S

and S4are closed, C1is charging C2. After a number

will be pumped to V+. Since

is connected to ground, the output at the

2

equals −(V+) assuming no load on C2, no loss

2

2

DS012911-21

FIGURE 2. Voltage Inverting Principle

Application Information

SIMPLE NEGATIVE VOLTAGE CONVERTER

The main application of LM2660/LM2661 is to generate a
negative supply voltage. The voltage inverter circuit uses
only two external capacitors as shown in the Basic Applica­tion Circuits. The range of the input supply voltage is 1.5V to

4

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,LVmay be
connected to ground or left open. The choice of leaving LV
open simplifies the direct substitution of the LM2660/
LM2661 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 R
function of the ON resistance of the internal MOS switches,
the oscillator frequency, and the capacitance and ESR of C
and C2. A good approximation is:

where RSWis the sum of the ON resistance of the internal
MOS switches shown in

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/(f
Once this term is trivial compared with R
ther increasing in oscillator frequency and capacitance will
become ineffective.

Figure 2

.

oscxC1

and ESRs, fur-

SW

out

) term.

is a

1

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Application Information (Continued)

The peak-to-peak output voltage ripple is determined by the
oscillator frequency, and the capacitance and ESR of the
output capacitor C

Again, using a low ESR capacitor will result in lower ripple.

POSITIVE VOLTAGE DOUBLER

The LM2660/LM2661 can operate as a positive voltage dou­bler (as shown in the Basic 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 D

’s forward drop.

1

The Schottky diode D
nal 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, D
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 D

should have enough current carrying capability to

1

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 LM2660/LM2661 as a precision volt­age divider. Since the off-voltage across each switch equals
V

/2, the input voltage can be raised to +11V.

IN

CHANGING OSCILLATOR FREQUENCY

For the LM2660, 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 allows 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.

:

2

is only needed for start-up. The inter-

1

1

TABLE 1. LM2660 Oscillator Frequency Selection

FC OSC Oscillator

Open Open 10 kHz
V+ Open 80 kHz
Open or V+ External Capacitor See Typical

N/A External Clock External Clock

(inverter mode only) Frequency

TABLE 2. LM2661 Oscillator Frequency Selection

OSC Oscillator

Open 80 kHz
External Capacitor See Typical Performance

Characteristics

External Clock External Clock Frequency

(inverter mode only)

SHUTDOWN MODE

For the LM2661, a shutdown (SD) pin is available to disable
the device and reduce the quiescent current to 0.5 µA. Ap­plying a voltage greater than 2V to the SD pin will bring the

is

device into shutdown mode. While in normal operating
mode, the SD pin is connected to ground.

CAPACITOR SELECTION

As discussed in the

Simple Negative Voltage Converter

tion, 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

Where IQ(V+) is the quiescent power loss of the IC device,

2

and I

R

is the conversion loss associated with the

L

OUT

switch on-resistance, the two external capacitors and their
ESRs.

Since the switching current charging and discharging C
approximately twice as the output current, the effect of the
ESR of the pumping capacitor C
output resistance. The output capacitor C

is multiplied by four in the

1

discharging at a current approximately equal to the output
current, therefore, its ESR only counts once in the output re­sistance. However, the ESR of C
voltage ripple. Therefore, low ESR capacitors (

directly affects the output

2

recommended for both capacitors to maximize efficiency, re­duce the output voltage drop and voltage ripple. For conve­nience, C

and C2are usually chosen to be the same.

1

The output resistance varies with the oscillator frequency
and the capacitors. In

Figure 3

, 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 µF 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

Performance
Characteristics

is charging and

2

Table 3

sec-

1

) are

LM2660/LM2661

is

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Application Information (Continued)

size capacitor usually has a higher ESR compared with a
bigger size capacitor of the same type. For lower ESR, use
ceramic capacitors.

LM2660/LM2661

FIGURE 3. Output Source Resistance vs Oscillator Frequency

TABLE 3. Low ESR Capacitor Manufacturers

Manufacturer Phone FAX Capacitor Type

Nichicon Corp. (708)-843-7500 (708)-843-2798 PL, PF series, through-hole aluminum electrolytic
AVX Corp. (803)-448-9411 (803)-448-1943 TPS series, surface-mount tantalum
Sprague (207)-324-4140 (207)-324-7223 593D, 594D, 595D series, surface-mount tantalum
Sanyo (619)-661-6835 (619)-661-1055 OS-CON series, through-hole aluminum electrolytic

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Other Applications

PARALLELING DEVICES

Any number of LM2660s (or LM2661s) can be paralleled to reduce the output resistance. Each device must have its own pumping
capacitor C

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, while only one output capacitor C

1

FIGURE 4. Lowering Output Resistance by Paralleling Devices

is needed as shown in

out

Figure 4

. The composite output resistance is:

DS012911-22

Other Applications (Continued)

CASCADING DEVICES

Cascading the LM2660s (or LM2661s) is an easy way to produce a greater negative voltage (as shown in
integer representing the number of devices cascaded, the unloaded output voltage V
is equal to the weighted sum of each individual device:

is (−nVin). The effective output resistance

out

Figure 5

LM2660/LM2661

). If n is the

A three-stage cascade circuit shown in
Cascading is also possible when devices are operating in doubling mode. In
An example of using the circuit in

Figure 6

generates −3Vin, from Vin.

Figure 6orFigure 7

Figure 7

, two devices are cascaded to generate 3Vin.

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.

DS012911-23

FIGURE 5. Increasing Output Voltage by Cascading Devices

FIGURE 6. Generating −3Vinfrom +V

FIGURE 7. Generating +3Vinfrom +V

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in

DS012911-25

in

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Other Applications (Continued)

REGULATING V

It is possible to regulate the output of the LM2660/LM2661 by use of a low dropout regulator (such as LP2951). The whole con­verter is depicted in

LM2660/LM2661

out

Figure 8

. This converter can give a regulated output from −1.5V to −5.5V by choosing the proper resistor ratio:

where, V

=

V

1.235

ref

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.

DS012911-27

FIGURE 8. Combining LM2660/LM2661 with LP2951 to Make a Negative Adjustable Regulator

Also, as shown in

Figure 9

by operating LM2660/LM2661 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.

FIGURE 9. Generating +5V from +3V Input Voltage

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DS012911-28

Physical Dimensions inches (millimeters) unless otherwise noted

LM2660/LM2661

Order Number LM2660M or LM2661M

8-Lead SO (M)

NS Package Number M08A

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

LM2660/LM2661 Switched Capacitor Voltage Converter

Order Number LM2660MM or LM2661MM

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8-Lead Mini SO (MM)

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