LM2660/LM2661
Switched Capacitor Voltage Converter
General Description
The LM2660/LM2661 CMOS charge-pump voltage con-
verter inverts a positive voltage in the range of 1.5V to 5.5V
to the corresponding 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 exter-
nal 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.
Features
nInverts or doubles input supply voltage
nNarrow SO-8 and Mini SO-8 Package
n6.5typical output resistance
n88% typical conversion efficiency at 100 mA
n(LM2660) selectable oscillator frequency: 10 kHz/80 kHz
n(LM2661) low current shutdown mode
Applications
nLaptop computers
nCellular phones
nMedical instruments
nOperational amplifier power supplies
nInterface power supplies
nHandheld instruments
Basic Application Circuits
Voltage Inverter Positive Voltage Doubler
01291103
01291104
Splitting V
IN
in Half
01291126
September 1999
LM2660/LM2661 Switched Capacitor Voltage Converter
© 2004 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
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.
Package
MMM
Power Dissipation
(T
A
= 25˚C) (Note 3) 735 mW 500 mW
T
J
Max (Note 3) 150˚C 150˚C
θ
JA
(Note 3) 170˚C/W 250˚C/W
Operating Junction
Temperature
Range −40˚C to +85˚C
Storage Temperature
Range
−65˚C to +150˚C
Lead Temperature 300˚C
(Soldering, 10 seconds)
ESD Rating 2 kV
Electrical Characteristics
Limits in standard typeface are for T
J
= 25˚C, and limits in boldface type apply over the full operating temperature range. Un-
less otherwise specified: V+ = 5V, FC = Open, C
1
=C
2
= 150 µF. (Note 4)
Symbol Parameter Condition Min Typ Max Units
V+ Supply Voltage R
L
= 1k Inverter, LV = Open 3.5 5.5
Inverter, LV = GND 1.5 5.5 V
Doubler, LV = OUT 2.5 5.5
I
Q
Supply Current No Load FC = Open (LM2660) 0.12 0.5
mALV = Open FC = V+ (LM2660) or 13
SD = Ground (LM2661)
I
SD
Shutdown Supply Current 0.5 2µA
(LM2661)
V
SD
Shutdown Pin Input Voltage Shutdown Mode 2.0 (Note 5) V
(LM2661) Normal Operation 0.3
I
L
Output Current T
A
+85˚C, OUT −4V 100 mA
T
A
>+85˚C, OUT −3.8V 100
R
OUT
Output Resistance (Note 6) I
L
= 100 mA T
A
+85˚C 6.5 10
T
A
>+85˚C 12
f
OSC
Oscillator Frequency (Note 7) OSC = Open FC = Open 510 kHz
FC=V+ 40 80
f
SW
Switching Frequency (Note 8) OSC = Open FC = Open 2.5 5kHz
FC=V+ 20 40
I
OSC
OSC Input Current FC = Open ±2µA
FC=V+ ±16
P
EFF
Power Efficiency R
L
(1k) between V
+
and OUT 96 98
R
L
(500) between GND and OUT 92 96 %
I
L
= 100 mA to GND 88
V
OEFF
Voltage Conversion Efficiency No Load 99 99.96 %
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 PDMax =(T
JMax −T
A)/θJA, where TJMax is the maximum junction temperature, TAis the
ambient temperature, and θJA is the junction-to-ambient thermal resistance of the specified package.
Note 4: In the test circuit, capacitors C1and C2are 0.2maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce output
voltage and efficiency.
Note 5: In doubling mode, when Vout >5V, minimum input high for shutdown equals Vout −3V.
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, fOSC =2f
SW.
LM2660/LM2661
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Test Circuits
Typical Performance Characteristics
(Circuit of Figure 1)
Supply Current vs
Supply Voltage
Supply Current vs
Oscillator Frequency
Output Source
Resistance vs Supply
Voltage
01291107 01291108 01291109
Output Source
Resistance vs
Temperature
Efficiency vs Load
Current
Output Voltage Drop
vs Load Current
01291110 01291111 01291112
01291105
01291106
FIGURE 1. LM2660 and LM2661 Test Circuits
LM2660/LM2661
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Typical Performance Characteristics (Circuit of Figure 1) (Continued)
Efficiency vs
Oscillator Frequency
Output Voltage vs
Oscillator Frequency
Oscillator Frequency
vs External
Capacitance
01291113 01291114 01291115
Oscillator Frequency
vs Supply Voltage
(FC = V+)
Oscillator Frequency
vs Supply Voltage
(FC = Open)
Oscillator Frequency
vs Temperature
(FC = V+)
01291116 01291117 01291118
Oscillator Frequency
vs Temperature
(FC = Open)
Shutdown Supply
Current vs
Temperature
(LM2661 Only)
01291119 01291120
LM2660/LM2661
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Connection Diagrams
8-Lead SO (M) or Mini SO (MM)
01291101 01291102
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.
LM2660/LM2661
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Pin Description
Pin Name Function
Voltage Inverter Voltage Doubler
1 FC Frequency control for internal oscillator: Same as inverter.
(LM2660) FC = open, f
OSC
= 10 kHz (typ);
FC = V+, f
OSC
= 80 kHz (typ);
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.
Same as inverter.
2 CAP+ Connect this pin to the positive terminal of
charge-pump capacitor.
Same as inverter.
3 GND Power supply ground input. Power supply positive voltage input.
4 CAP− Connect this pin to the negative terminal of
charge-pump capacitor.
Same as inverter.
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.
LV must be tied to OUT.
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.
Same as inverter except that OSC cannot be driven
by an external clock.
8 V+ Power supply positive voltage input. Positive voltage output.
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 exter-
nal capacitors. Figure 2 illustrates the voltage conversion
scheme. When S
1
and S
3
are closed, C
1
charges to the
supply voltage V+. During this time interval switches S
2
and
S
4
are open. In the second time interval, S
1
and S
3
are open
and S
2
and S
4
are closed, C
1
is charging C
2
. After a number
of cycles, the voltage across C
2
will be pumped to V+. Since
the anode of C
2
is connected to ground, the output at the
cathode of C
2
equals −(V+) assuming no load on C
2
, no loss
in the switches, and no ESR in the capacitors. In reality, the
charge transfer efficiency depends on the switching fre-
quency, the on-resistance of the switches, and the ESR of
the capacitors.
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 Applica-
tion 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 cir-
cuitry. This gives the best performance in low voltage appli-
cations. 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 LM2660/
LM2661 for the LMC7660 Switched Capacitor Voltage Con-
verter.
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
out
is a
function of the ON resistance of the internal MOS switches,
the oscillator frequency, and the capacitance and ESR of C
1
and C
2
. A good approximation is:
where R
SW
is the sum of the ON resistance of the internal
MOS switches shown in Figure 2.
High value, low ESR capacitors will reduce the output resis-
tance. Instead of increasing the capacitance, the oscillator
frequency can be increased to reduce the 2/(f
osc
xC
1
) term.
Once this term is trivial compared with R
SW
and ESRs,
further increasing in oscillator frequency and capacitance will
become ineffective.
01291121
FIGURE 2. Voltage Inverting Principle
LM2660/LM2661
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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
2
:
Again, using a low ESR capacitor will result in lower ripple.
POSITIVE VOLTAGE DOUBLER
The LM2660/LM2661 can operate as a positive voltage dou-
bler (as shown in the Basic Application Circuits). The dou-
bling function is achieved by reversing some of the connec-
tions 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
1
’s forward drop.
The Schottky diode D
1
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,
D
1
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 D
1
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 Applica-
tion Circuits is using the LM2660/LM2661 as a precision
voltage divider. Since the off-voltage across each switch
equals V
IN
/2, the input voltage can be raised to +11V.
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 con-
nected 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 exter-
nal capacitor between OSC and GND. (See Typical Perfor-
mance Characteristics.) Also, in the inverter mode, an exter-
nal 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. LM2660 Oscillator Frequency Selection
FC OSC Oscillator
Open Open 10 kHz
V+ Open 80 kHz
Open or V+ External Capacitor See Typical
Performance
Characteristics
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 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
section, the output resistance and ripple voltage are depen-
dent 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 I
Q
(V+) is the quiescent power loss of the IC device,
and I
L2
R
OUT
is the conversion loss associated with the
switch on-resistance, the two external capacitors and their
ESRs.
Since the switching current charging and discharging C
1
is
approximately twice as the output current, the effect of the
ESR of the pumping capacitor C
1
is multiplied by four in the
output resistance. The output capacitor C
2
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 C
2
directly affects the
output voltage ripple. Therefore, low ESR capacitors (Table
3) are recommended for both capacitors to maximize effi-
ciency, reduce the output voltage drop and voltage ripple.
For convenience, C
1
and C
2
are usually chosen to be the
same.
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 tan-
talum capacitors. At very low frequency range, capacitance
plays the most important role in determining the output re-
sistance. 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
LM2660/LM2661
www.national.com7
Application Information (Continued)
value, smaller size capacitor usually has a higher ESR com-
pared with a bigger size capacitor of the same type. For
lower ESR, use ceramic capacitors.
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
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
1
, while only one output capacitor C
out
is
needed as shown in Figure 4. The composite output resis-
tance is:
01291132
FIGURE 3. Output Source Resistance vs Oscillator Frequency
01291122
FIGURE 4. Lowering Output Resistance by Paralleling Devices
LM2660/LM2661
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Other Applications (Continued)
CASCADING DEVICES
Cascading the LM2660s (or LM2661s) is an easy way to
produce a greater negative voltage (as shown in Figure 5). If
n is the integer representing the number of devices cas-
caded, the unloaded output voltage V
out
is (−nV
in
). The
effective output resistance is equal to the weighted sum of
each individual device:
A three-stage cascade circuit shown in Figure 6 generates
−3V
in
, from V
in
.
Cascading is also possible when devices are operating in
doubling mode. In Figure 7, two devices are cascaded to
generate 3V
in
.
An example of using the circuit in Figure 6 or Figure 7 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 in-
creases the output resistance and output voltage ripple.
01291123
FIGURE 5. Increasing Output Voltage by Cascading Devices
01291124
FIGURE 6. Generating −3V
in
from +V
in
LM2660/LM2661
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Other Applications (Continued)
REGULATING V
out
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 Figure 8. This converter can
give a regulated output from −1.5V to −5.5V by choosing the
proper resistor ratio:
where, V
ref
= 1.235V
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.
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.
01291125
FIGURE 7. Generating +3V
in
from +V
in
01291127
FIGURE 8. Combining LM2660/LM2661 with LP2951 to Make a Negative Adjustable Regulator
LM2660/LM2661
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Other Applications (Continued)
01291128
FIGURE 9. Generating +5V from +3V Input Voltage
LM2660/LM2661
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Physical Dimensions inches (millimeters) unless otherwise noted
8-Lead SO (M)
Order Number LM2660M or LM2661M
NS Package Number M08A
8-Lead Mini SO (MM)
Order Number LM2660MM or LM2661MM
NS Package Number MUA08A
LM2660/LM2661
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Notes
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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LM2660/LM2661 Switched Capacitor Voltage Converter