LM2681
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SNVS042B – MARCH 1999 – REVISED MAY 2013
LM2681 Switched Capacitor Voltage Converter
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FEATURES
DESCRIPTION
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The LM2681 CMOS charge-pump voltage converter
operates 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.
1
2
Doubles or Splits Input Supply Voltage
SOT-23 6-Lead Package
15Ω Typical Output Impedance
90% Typical Conversion Efficiency at 20 mA
APPLICATIONS
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Cellular Phones
Pagers
PDAs
Operational Amplifier Power Suppliers
Interface Power Suppliers
Handheld Instruments
The LM2681 operates at 160 kHz oscillator frequency
to reduce output resistance and voltage ripple. With
an operating current of only 550 µA (operating
efficiency greater than 90% with most loads) the
LM2681 provides ideal performance for battery
powered systems. The device is in a SOT-23, 6-lead
package.
Basic Application Circuits
Voltage Doubler
Splitting Vin in Half
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 1999–2013, Texas Instruments Incorporated
LM2681
SNVS042B – MARCH 1999 – REVISED MAY 2013
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings (1) (2)
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 (3)
1 sec.
Continuous Power Dissipation (TA = 25°C) (4)
600 mW
TJMax (4)
150°C
θJA (4)
210°C/W
−40° to 85°C
Operating Junction Temperature Range
−65°C to +150°C
Storage Temperature Range
Lead Temp. (Soldering, 10 seconds)
300°C
ESD Rating
(1)
2kV
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.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
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.
The maximum allowable power dissipation is calculated by using PDMax = (TJMax − TA)/θJA, where TJMax is the maximum junction
temperature, TA is the ambient temperature, and θJA is the junction-to-ambient thermal resistance of the specified package.
(2)
(3)
(4)
Electrical Characteristics
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range.
Unless otherwise specified: V+ = 5V, C1 = C2 = 3.3 μF. (1)
Symbol
Parameter
V+
Supply Voltage
IQ
Supply Current
IL
Output Current
RSW
Sum of the Rds(on)of the four internal
MOSFET switches
ROUT
Output Resistance
Condition
Min
Typ
Max
5.5
V
550
1000
µA
2.5
No Load
20
(2)
Units
mA
IL = 20 mA
8
16
Ω
IL = 20 mA
15
40
Ω
(3)
fOSC
Oscillator Frequency
See
80
160
kHz
fSW
Switching Frequency
See (3)
40
80
kHz
PEFF
Power Efficiency
RL (1.0k) between GND and OUT
86
93
IL = 20 mA to GND
VOEFF
(1)
(2)
(3)
2
Voltage Conversion Efficiency
No Load
90
99
99.96
%
%
In the test circuit, capacitors C1 and C2 are 3.3 µF, 0.3Ω maximum ESR capacitors. Capacitors with higher ESR will increase output
resistance, reduce output voltage and efficiency.
Specified output resistance includes internal switch resistance and capacitor ESR. See POSITIVE VOLTAGE DOUBLER
The output switches operate at one half of the oscillator frequency, fOSC = 2fSW.
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Test Circuit
Figure 1. LM2681 Test Circuit
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Typical Performance Characteristics
(Circuit of Figure 1, V+ = 5V unless otherwise specified)
4
Supply Current vs
Supply Voltage
Supply Current vs
Temperature
Figure 2.
Figure 3.
Output Source
Resistance
vs
Supply
Voltage
Output Source
Resistance
vs
Temperature
Figure 4.
Figure 5.
Output Voltage Drop
vs Load Current
Efficiency
vs
Load Current
Figure 6.
Figure 7.
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Typical Performance Characteristics (continued)
(Circuit of Figure 1, V+ = 5V unless otherwise specified)
Oscillator Frequency vs
Supply Voltage
Oscillator Frequency vs
Temperature
Figure 8.
Figure 9.
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LM2681
SNVS042B – MARCH 1999 – REVISED MAY 2013
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CONNECTION DIAGRAM
Figure 10. SOT-23, 6-Lead Package – Top View
See Package Number DBV
Figure 11. Actual Size
PIN DESCRIPTION
Pin
Name
1
V+
2
Function
Voltage Doubler
Voltage Split
Power supply positive voltage input
Positive voltage output
GND
Power supply ground input
Same as doubler
3
CAP−
Connect this pin to the negative terminal of the chargepump 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 capacitors. Figure 12 illustrates the voltage
conversion scheme. When S2 and S4 are closed, C1 charges to the supply voltage V+. During this time interval,
switches S1 and S3 are open. In the next time interval, S2 and S4 are open; at the same time, S1 and S3 are
closed, the sum of the input voltage V+ and the voltage across C1 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 (Rds(on) of the
MOSFET switches and the ESR of the capacitors) and the charge transfer loss between capacitors. Details will
be discussed in the APPLICATION INFORMATION section.
Figure 12. Voltage Doubling Principle
6
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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 Rout is a function of the ON resistance of the
internal MOSFET switches, the oscillator frequency, the capacitance and ESR of C1 and C2. Since the switching
current charging and discharging C1 is approximately twice as the output current, the effect of the ESR of the
pumping capacitor C1 will be multiplied by four in the output resistance. The output capacitor C2 is charging and
discharging at a current approximately equal to the output current, therefore, its ESR only counts once in the
output resistance. A good approximation of Rout is:
(1)
where RSW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 12.
The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the
output capacitor C2:
(2)
High capacitance, low ESR capacitors can reduce both the output reslistance and the voltage ripple.
The Schottky diode D1 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 startup, D1 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 D1 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 Application 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 Vin/2, the input voltage can be raised to +11V.
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
(3)
IL2Rout
Where IQ(V+) is the quiescent power loss of the IC device, and
switch on-resistance, the two external capacitors and their ESRs.
is the conversion loss associated with the
The selection of capacitors is based on the specifications of the dropout voltage (which equals Iout Rout), the
output voltage ripple, and the converter efficiency. Low ESR capacitors are recommended to maximize efficiency,
reduce the output voltage drop and voltage ripple.
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LM2681
SNVS042B – MARCH 1999 – REVISED MAY 2013
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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 C1, while only one output capacitor Cout is needed as shown in Figure 13. The composite
output resistance is:
(4)
Figure 13. Lowering Output Resistance by Paralleling Devices
CASCADING DEVICES
Cascading the LM2681s is an easy way to produce a greater voltage (A two-stage cascade circuit is shown in
Figure 14).
The effective output resistance is equal to the weighted sum of each individual device:
Rout = 1.5Rout_1 + Rout_2
(5)
Note that, the increasing of the number of cascading stages is practically limited since it significantly reduces the
efficiency, increases the output resistance and output voltage ripple.
Figure 14. Increasing Output Voltage by Cascading Devices
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SNVS042B – MARCH 1999 – REVISED MAY 2013
REGULATING VOUT
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 15.
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 simultaneously for worst case design:
2Vin_min >Vout_min +Vdrop_max (LP2980) + Iout_max × Rout_max (LM2681)
2Vin_max < Vout_max +Vdrop_min (LP2980) + Iout_min × Rout_min (LM2681)
(6)
(7)
Figure 15. Generate a Regulated +5V from +3V Input Voltage
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LM2681
SNVS042B – MARCH 1999 – REVISED MAY 2013
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REVISION HISTORY
Changes from Revision A (May 2013) to Revision B
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Page
Changed layout of National Data Sheet to TI format ............................................................................................................ 9
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM2681M6/NOPB
ACTIVE
SOT-23
DBV
6
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 85
S10A
LM2681M6X/NOPB
ACTIVE
SOT-23
DBV
6
3000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 85
S10A
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
