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 re-
duce 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 SOT-23-6 pack-
age.

Features

Doubles or Splits Input Supply Voltage

SOT23-6 Package

Typical Output Impedance

90% Typical Conversion Efficiency at 20 mA

Applications

Cellular Phones

Pagers

PDAs

Operational Amplifier Power Suppliers

Interface Power Suppliers

Handheld Instruments

Basic Application Circuits

Voltage Doubler

DS100965-1

Splitting V

in Half

DS100965-2

March 1999

LM2681

Switched

Capacitor

oltage

Converter

DS100965

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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)

5.8V

V+ and OUT Continuous Output Current

30 mA

Output Short-Circuit Duration to GND (Note 2)

1 sec.

Continuous Power
Dissipation (T

= 25°C)(Note 3)

600 mW

JMax

(Note 3)

150°C

(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

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

less otherwise specified: V+ = 5V, C

= C

= 3.3 µF. (Note 4)

Symbol

Parameter

Condition

Min

Typ

Max

Units

V+

Supply Voltage

2.5

5.5

Supply Current

No Load

550

1000

µA

Output Current

Sum of the R

ds(on)

of the four

internal MOSFET switches

= 20 mA

OUT

Output Resistance (Note 5)

= 20 mA

OSC

Oscillator Frequency

(Note 6)

160

kHz

Switching Frequency

(Note 6)

kHz

EFF

Power Efficiency

(1.0k) between GND and

OUT

= 20 mA to GND

OEFF

Voltage Conversion Efficiency

No Load

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 tem-
peratures 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

DMax

= (T

JMax

- T

, where T

JMax

is the maximum junction temperature, T

is the

ambient temperature, and

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

Note 4: In the test circuit, capacitors C

and C

are 3.3 µF, 0.3

maximum 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, f

OSC

= 2f

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Typical Performance Characteristics

(Circuit of Figure 1, V+ = 5V unless otherwise

specified) (Continued)

Connection Diagram

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

identifies the part as a switched capacitor converter. The next two numbers are the device number. The fourth letter

indicates the

grade. Only one grade is available. Larger quantity reels are available upon request.

Output Voltage Drop
vs Load Current

DS100965-8

Efficiency vs
Load Current

DS100965-9

Oscillator Frequency vs
Supply Voltage

DS100965-10

Oscillator Frequency vs
Temperature

DS100965-11

6-Lead SOT (M6)

DS100965-13

Top View With Package Marking

DS100965-22

Actual Size

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Pin Description

Pin

Name

Function

Voltage Doubler

Voltage Split

V+

Power supply positive voltage input

Positive voltage output

GND

Power supply ground input

Same as doubler

CAP-

Connect this pin to the negative terminal of the
charge-pump capacitor

Same as doubler

GND

Power supply ground input

Same as doubler

OUT

Positive voltage output

Power supply positive voltage
input

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

and S

are closed, C

charges to the supply volt-

age V+. During this time interval, switches S

and S

are

open. In the next time interval, S

and S

are open; at the

same time, S

and S

are closed, the sum of the input volt-

age V+ and the voltage across C

gives the 2V+ output volt-

age when there is no load. The output voltage drop when a
load is added is determined by the parasitic resistance (R

s(on)

of the MOSFET switches and the ESR of the capacitors)

and the charge transfer loss between capacitors. Details 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

and C

. Since the switching current charging and dis-

charging C

is approximately twice as the output current, the

effect of the ESR of the pumping capacitor C

will be multi-

plied by four in the output resistance. The output capacitor
C

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 R

out

is:

where R

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

High capacitance, low ESR capacitors can reduce both the
output reslistance and the voltage ripple.

The Schottky diode D

is only needed for start-up. The inter-

nal oscillator circuit uses the OUT pin and the GND pin. Volt-
age across OUT and GND must be larger than 1.8V to insure
the operation of the oscillator. During start-up, D

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 di-
ode D

should have enough current carrying capability to

charge the output capacitor at start-up, as well as a low for-
ward 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 di-
vider. . 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

/2, the input voltage

can be raised to +11V.

DS100965-14

FIGURE 2. Voltage Doubling Principle

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