LM2681
LM2681 Switched Capacitor Voltage Converter
Literature Number: SNVS042A
LM2681
Switched Capacitor Voltage Converter
General Description
The LM2681 CMOS charge-pump voltage converter oper-
ates as a voltage doubler for an input voltage in the range of
+2.5V to +5.5V. Two low cost capacitors and a diode
(needed during start-up) is used in this circuit to provide up
to 20 mA of output current. The LM2681 can also work as a
voltage divider to split a voltage in the range of +1.8V to
+11V in half.
The LM2681 operates at 160 kHz oscillator frequency to
reduce output resistance and voltage ripple. With an operat-
ing current of only 550 µA (operating efficiency greater than
90% with most loads) the LM2681 provides ideal perfor-
mance for battery powered systems. The device is in SOT-
23-6 package.
Features
nDoubles or Splits Input Supply Voltage
nSOT23-6 Package
n15ΩTypical Output Impedance
n90% Typical Conversion Efficiency at 20 mA
Applications
nCellular Phones
nPagers
nPDAs
nOperational Amplifier Power Suppliers
nInterface Power Suppliers
nHandheld Instruments
Basic Application Circuits
Voltage Doubler
10096501
Splitting V
in
in Half
10096502
January 2003
LM2681 Switched Capacitor Voltage Converter
© 2003 National Semiconductor Corporation DS100965 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.
V+ to GND Voltage: 5.8V
OUT to GND Voltage: 11.6V
OUT to V+ Voltage: 5.8V
V+ and OUT Continuous Output Current 30 mA
Output Short-Circuit Duration to GND (Note 2) 1 sec.
Continuous Power
Dissipation (T
A
= 25˚C)(Note 3)
600 mW
T
JMax
(Note 3) 150˚C
θ
JA
(Note 3) 210˚C/W
Operating Junction
Temperature Range
−40˚ to 85˚C
Storage Temperature Range −65˚C to +150˚C
Lead Temp. (Soldering, 10 seconds) 300˚C
ESD Rating 2kV
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, C
1
=C
2
= 3.3 µF. (Note 4)
Symbol Parameter Condition Min Typ Max Units
V+ Supply Voltage 2.5 5.5 V
I
Q
Supply Current No Load 550 1000 µA
I
L
Output Current 20 mA
R
SW
Sum of the R
ds(on)
of the four
internal MOSFET switches I
L
=20mA 8 16 Ω
R
OUT
Output Resistance (Note 5) I
L
=20mA 15 40 Ω
f
OSC
Oscillator Frequency (Note 6) 80 160 kHz
f
SW
Switching Frequency (Note 6) 40 80 kHz
P
EFF
Power Efficiency R
L
(1.0k) between GND and
OUT 86 93 %
I
L
=20mAtoGND 90
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 3.3 µF, 0.3Ωmaximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce
output voltage and efficiency.
Note 5: Specified output resistance includes internal switch resistance and capacitor ESR. See the details in the application information for positive voltage doubler.
Note 6: The output switches operate at one half of the oscillator frequency, fOSC =2f
SW.
LM2681
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Test Circuit
Typical Performance Characteristics (Circuit of Figure 1, V+ = 5V unless otherwise specified)
Supply Current vs
Supply Voltage
Supply Current vs
Temperature
10096504 10096505
Output Source
Resistance vs Supply
Voltage
Output Source
Resistance vs
Temperature
10096506 10096507
10096503
FIGURE 1. LM2681 Test Circuit
LM2681
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Typical Performance Characteristics (Circuit of Figure 1, V+ = 5V unless otherwise specified)
(Continued)
Output Voltage Drop
vs Load Current Efficiency vs
Load Current
10096508 10096509
Oscillator Frequency vs
Supply Voltage
Oscillator Frequency vs
Temperature
10096510 10096511
Connection Diagram
6-Lead SOT (M6)
10096513
Top View With Package Marking
10096522
Actual Size
Ordering Information
Order Number Package Number Package Marking Supplied as
LM2681M6 MA06A S10A (Note 7) Tape and Reel (250 units/rail)
LM2681M6X MA06A S10A (Note 7) Tape and Reel (3000 units/rail)
Note 7: The first letter "S" identifies the part as a switched capacitor converter. The next two numbers are the device number. The fourth letter "A" indicates the
grade. Only one grade is available. Larger quantity reels are available upon request.
LM2681
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Pin Description
Pin Name
Function
Voltage Doubler Voltage Split
1 V+ Power supply positive voltage input Positive voltage output
2 GND Power supply ground input Same as doubler
3 CAP− Connect this pin to the negative terminal of the
charge-pump capacitor Same as doubler
4 GND Power supply ground input Same as doubler
5 OUT Positive voltage output Power supply positive voltage input
6 CAP+ Connect this pin to the positive terminal of the
charge-pump capacitor Same as doubler
Circuit Description
The LM2681 contains four large CMOS switches which are
switched in a sequence to double the input supply voltage.
Energy transfer and storage are provided by external capaci-
tors. Figure 2 illustrates the voltage conversion scheme.
When S
2
and S
4
are closed, C
1
charges to the supply
voltage V+. During this time interval, switches S
1
and S
3
are
open. In the next time interval, S
2
and S
4
are open; at the
same time, S
1
and S
3
are closed, the sum of the input
voltage V+ and the voltage across C
1
gives the 2V+ output
voltage when there is no load. The output voltage drop when
a load is added is determined by the parasitic resistance
(R
ds(on)
of the MOSFET switches and the ESR of the capaci-
tors) and the charge transfer loss between capacitors. De-
tails will be discussed in the following application information
section.
Application Information
POSITIVE VOLTAGE DOUBLER
The main application of the LM2681 is to double the input
voltage. The range of the input supply voltage is 2.5V to
5.5V.
The output characteristics of this circuit can be approximated
by an ideal voltage source in series with a resistance. The
voltage source equals 2V+. The output resistance R
out
is a
function of the ON resistance of the internal MOSFET
switches, the oscillator frequency, the capacitance and ESR
of C
1
and C
2
. 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
will be
multiplied by four in the output resistance. The output ca-
pacitor C
2
is charging and discharging at a current approxi-
mately equal to the output current, therefore, its ESR only
counts once in the output resistance. A good approximation
of R
out
is:
where R
SW
is the sum of the ON resistance of the internal
MOSFET switches shown in Figure 2.
The peak-to-peak output voltage ripple is determined by the
oscillator frequency, the capacitance and ESR of the output
capacitor C
2
:
High capacitance, low ESR capacitors can reduce both the
output reslistance and the voltage ripple.
The Schottky diode D
1
is only needed for start-up. The
internal oscillator circuit uses the OUT pin and the GND pin.
Voltage across OUT and GND must be larger than 1.8V to
insure the operation of the oscillator. During start-up, D
1
is
used to charge up the voltage at the OUT 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 LM2681 as a precision voltage
divider. . This circuit can be derived from the voltage doubler
by switching the input and output connections. In the voltage
divider, the input voltage applies across the OUT pin and the
GND pin (which are the power rails for the internal oscillator),
therefore no start-up diode is needed. Also, since the off-
voltage across each switch equals V
in
/2, the input voltage
can be raised to +11V.
10096514
FIGURE 2. Voltage Doubling Principle
LM2681
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Application Information (Continued)
CAPACITOR SELECTION
As discussed in the Positive Voltage Doubler 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
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.
The selection of capacitors is based on the specifications of
the dropout voltage (which equals I
out
R
out
), the output volt-
age ripple, and the converter efficiency. Low ESR capacitors
(Table 1) are recommended to maximize efficiency, reduce
the output voltage drop and voltage ripple.
Low ESR Capacitor Manufacturers
Manufacturer Phone Capacitor Type
Nichicon Corp. (708)-843-7500 PL & PF series, through-hole aluminum electrolytic
AVX Corp. (803)-448-9411 TPS series, surface-mount tantalum
Sprague (207)-324-4140 593D, 594D, 595D series, surface-mount tantalum
Sanyo (619)-661-6835 OS-CON series, through-hole aluminum electrolytic
Murata (800)-831-9172 Ceramic chip capacitors
Taiyo Yuden (800)-348-2496 Ceramic chip capacitors
Tokin (408)-432-8020 Ceramic chip capacitors
Other Applications
PARALLELING DEVICES
Any number of LM2681s 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 3. The composite output resistance is:
CASCADING DEVICES
Cascading the LM2681s is an easy way to produce a greater
voltage (A two-stage cascade circuit is shown in Figure 4).
The effective output resistance is equal to the weighted sum
of each individual device:
R
out
= 1.5R
out_1
+R
out_2
Note that, the increasing of the number of cascading stages
is pracitically limited since it significantly reduces the effi-
ciency, increases the output resistnace and output voltage
ripple.
10096519
FIGURE 3. Lowering Output Resistance by Paralleling Devices
LM2681
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Other Applications (Continued)
REGULATING V
OUT
It is possible to regulate the output of the LM2681 by use of
a low dropout regulator (such as LP2980-5.0). The whole
converter is depicted in Figure 5.
A different output voltage is possible by use of LP2980-3.3,
LP2980-3.0, or LP2980-adj.
Note that, the following conditions must be satisfied simulta-
neously for worst case design:
2V
in_min
>V
out_min
+V
drop_max
(LP2980) + I
out_max
xR
out-
_max
(LM2681)
2V
in_max
<V
out_max
+V
drop_min
(LP2980) + I
out_min
xR
out-
_min
(LM2681)
10096520
FIGURE 4. Increasing Output Voltage by Cascading Devices
10096521
FIGURE 5. Generate a Regulated +5V from +3V Input Voltage
LM2681
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Physical Dimensions inches (millimeters) unless otherwise noted
6-Lead Small Outline Package (M6)
NS Package Number MA06A
For Order Numbers, refer to the table in the "Ordering Information" section of this document.
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www.national.com
LM2681 Switched Capacitor Voltage Converter
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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