The LM2681 CMOS charge-pump voltage converter operates asa 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 operating
current of only550 µA (operating efficiency greaterthan 90
with most loads) the LM2681 provides ideal performance for
battery powered systems. The device is in SOT-23-6 package.
Basic Application Circuits
Voltage Doubler
Splitting Vinin Half
Features
n Doubles or Splits Input Supply Voltage
n SOT23-6 Package
n 15Ω Typical Output Impedance
n 90%Typical Conversion Efficiency at 20 mA
Applications
n Cellular Phones
n Pagers
n PDAs
%
n Operational Amplifier Power Suppliers
n Interface Power Suppliers
n Handheld Instruments
If Military/Aerospace specified devices are required,
please contact theNational Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (V+ to GND, or GND to OUT)5.8V
V+ and OUT Continuous Output Current30 mA
Output Short-Circuit Duration to GND (Note 2)1 sec.
Continuous Power
Dissipation (T
=
A
25˚C)(Note 3)
600 mW
T
(Note 3)150˚C
JMax
θ
(Note 3)210˚C/W
JA
Operating Junction Temperature
−40˚ to 85˚C
Range
Storage Temperature Range−65˚C to +150˚C
Lead Temp. (Soldering, 10 seconds)300˚C
ESD Rating2kV
Electrical Characteristics
Limits in standard typeface are for T
less otherwise specified: V+=5V, C
SymbolParameterConditionMinTypMaxUnits
V+Supply Voltage2.55.5V
I
Q
I
L
R
SW
R
OUT
f
OSC
f
SW
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: OUTmay be shorted to GND for onesecond 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: Inthetestcircuit, capacitors C
voltage and efficiency.
Note 5: Specifiedoutputresistance includes internal switch resistance and capacitor ESR. See thedetails in the application information for positive voltage doubler.
Note 6: The output switches operate at one half of the oscillator frequency, f
Supply CurrentNo Load5501000µA
Output Current20mA
Sum of the R
internal MOSFET switches
ds(on)
Output Resistance (Note 5)I
Oscillator Frequency(Note 6)80160kHz
Switching Frequency(Note 6)4080kHz
Power EfficiencyRL(1.0k) between GND and
Voltage Conversion EfficiencyNo Load9999.96
is the junction-to-ambient thermal resistance of the specified package.
JA
=
25˚C, and limits in boldface type apply over the full operating temperature range. Un-
J
=
=
C
3.3 µF. (Note 4)
1
2
of the four
and C2are 3.3 µF, 0.3Ω maximum ESR capacitors. CapacitorswithhigherESRwillincreaseoutput resistance, reduce output
1
=
I
20 mA816Ω
L
=
20 mA1540Ω
L
OUT
=
I
20 mA to GND90
L
DMax
OSC
=
=
(T
JMax−TA
2f
SW
)/θJA, where T
.
8693
is the maximum junction temperature, TAis the
JMax
%
%
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Page 3
Test Circuit
FIGURE 1. LM2681 Test Circuit
Typical Performance Characteristics
DS100965-3
(Circuit of Figure 1, V+=5V unless otherwise specified)
Supply Current vs
Supply Voltage
Output Source
Resistance vs Supply
Voltage
DS100965-4
Supply Current vs
Temperature
DS100965-5
Output Source
Resistance vs
Temperature
DS100965-6
DS100965-7
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Page 4
Typical Performance Characteristics (Circuit of Figure 1, V+
specified) (Continued)
=
5V unless otherwise
Output Voltage Drop
vs Load Current
Oscillator Frequency vs
Supply Voltage
DS100965-8
Efficiency vs
Load Current
DS100965-9
Oscillator Frequency vs
Temperature
DS100965-10
DS100965-11
Connection Diagram
6-Lead SOT (M6)
DS100965-22
Actual Size
DS100965-13
Top View With Package Marking
Ordering Information
Order NumberPackage
Number
LM2681M6MA06AS10A (Note 7)Tape and Reel (250 units/rail)
LM2681M6XMA06AS10A (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.
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Package
Marking
Supplied as
Page 5
Pin Description
PinNameFunction
Voltage DoublerVoltage Split
1V+Power supply positive voltage inputPositive voltage output
2GNDPower supply ground inputSame as doubler
3CAP−Connect this pin to the negative terminal of the
charge-pump capacitor
4GNDPower supply ground inputSame as doubler
5OUTPositive voltage outputPower supply positive voltage
6CAP+Connect this pin to the positive terminal of the
charge-pump capacitor
Same as doubler
input
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 storageare provided byexternal capacitors. Figure 2 illustrates the voltage conversion scheme.
When S
age V+. During this time interval, switches S
open. In the next time interval, S
same time, S
age V+ and the voltage across C
age when there is no load. The output voltage drop when a
and S4are closed, C1charges to the supply volt-
2
and S4are open; at the
and S3are closed, the sum of the input volt-
1
2
gives the 2V+ output volt-
1
and S3are
1
load is added is determined by the parasitic resistance (R
of the MOSFET switchesand the ESR ofthe capacitors)
s(on)
and the charge transfer loss between capacitors. Details will
be discussed in the following application information section.
DS100965-14
FIGURE 2. Voltage Doubling Principle
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 ofthis circuit canbe approximated
by an ideal voltage source in series with a resistance. The
voltage source equals 2V+. The output resistance R
function of the ON resistance of the internal MOSFET
switches, the oscillator frequency, the capacitance and ESR
of C
and C2. Since the switching current charging and dis-
1
charging C
effect of the ESR of the pumping capacitor C
plied by four in the output resistance. The output capacitor
C
2
is approximately twice as the output current, the
1
1
is charging and discharging at a current approximately
is a
out
will be multi-
equal to the output current, therefore, its ESR only counts
once in the output resistance. A good approximation of R
is:
where RSWis 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
-
d
:
2
High capacitance, low ESR capacitors can reduce both the
output reslistance and the voltage ripple.
The Schottky diode D
nal oscillator circuit uses theOUT pin and the GNDpin. Volt-
is only needed for start-up. The inter-
1
age across OUT andGND must be largerthan 1.8V to insure
the operation of the oscillator. During start-up, D
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 Schottkydiode 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 LM2681 as a precision voltage divider. . This circuit can be derived from the voltage doubler
by switching the input andoutput connections. In the voltage
divider,the input voltage applies across the OUTpin and the
GND pin (which arethe power rails forthe internal oscillator),
therefore no start-up diode is needed. Also, since the
off-voltage across each switchequals V
can be raised to +11V.
is used to
1
/2, the input voltage
in
out
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Page 6
Application Information (Continued)
Capacitor Selection
As discussed in the
output resistance and ripple voltage are dependent on the
capacitance and ESR values of the external capacitors. The
output voltage dropis the loadcurrent times the output resistance, and the power efficiency is
Positive Voltage Doubler
section, the
Where IQ(V+) is the quiescent power loss of the IC device,
2
and I
R
is the conversion loss associated with the switch
L
out
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
age ripple, and the converter efficiency. Low ESR capacitors
outRout
(Table 1) are recommended to maximize efficiency, reduce
the output voltage drop and voltage ripple.
Any number ofLM2681s can beparalleled to reduce the output resistance. Each device must have its own pumping capacitor C
shown in Figure 3. The composite output resistance is:
, while only one outputcapacitor C
1
is needed as
out
), the output volt-
FIGURE 3. Lowering Output Resistance by Paralleling Devices
Cascading Devices
Cascading the LM2681sis an easyway 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
1.5R
out
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out_1+Rout_2
DS100965-19
Note that, the increasing of the number of cascading stages
is pracitically limited since it significantly reduces the efficiency, increases the output resistnace and output voltage
ripple.
Page 7
Other Applications (Continued)
FIGURE 4. Increasing Output Voltage by Cascading Devices
Regulating V
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.
OUT
2V
2V
in_min
in_max
>
V
out_min+Vdrop_max
<
V
out_max+Vdrop_min
Note that, the following conditions must be satisfied simultaneously for worst case design:
(LP2980) + I
(LP2980) + I
out_maxxRout_max
out_minxRout_min
(LM2681)
(LM2681)
DS100965-20
FIGURE 5. Generate a Regulated +5V from +3V Input Voltage
For Order Numbers, refer to the table in the ″Ordering Information″ section of this document.
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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into
the body, or(b) support orsustain life, and whose failure to perform when properly used in accordance
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 system,or to affect its safetyor effectiveness.
with instructions for use provided in the labeling, can
be reasonably expectedto result in a significantinjury
to the user.
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.