Datasheet LM2681M6X, LM2681M6 Datasheet (NSC)

LM2681
Switched Capacitor Voltage Converter

LM2681 Switched Capacitor Voltage Converter

March 1999

General Description

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

DS100965-1

DS100965-2

© 1999 National Semiconductor Corporation DS100965 www.national.com

Absolute Maximum Ratings (Note 1)

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 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

(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 Rating 2kV

Electrical Characteristics

Limits in standard typeface are for T
less otherwise specified: V+=5V, C

Symbol Parameter Condition Min Typ Max Units

V+ Supply Voltage 2.5 5.5 V
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 Current No Load 550 1000 µA
Output Current 20 mA
Sum of the R

internal MOSFET switches

ds(on)

Output Resistance (Note 5) I
Oscillator Frequency (Note 6) 80 160 kHz
Switching Frequency (Note 6) 40 80 kHz
Power Efficiency RL(1.0k) between GND and

Voltage Conversion Efficiency No Load 99 99.96

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 mA 8 16 Ω

L

=

20 mA 15 40 Ω

L

OUT

=

I

20 mA to GND 90

L

DMax

OSC

=

=

(T

JMax−TA

2f

SW

)/θJA, where T

.

86 93

is the maximum junction temperature, TAis the

JMax

%

%

www.national.com 2

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

www.national.com3

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 Number Package

Number

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.

www.national.com 4

Package

Marking

Supplied as

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
4 GND Power supply ground input Same as doubler
5 OUT Positive voltage output Power supply positive voltage

6 CAP+ 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

www.national.com5

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.

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 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

www.national.com 6

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.

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

DS100965-21

www.national.com7

Physical Dimensions inches (millimeters) unless otherwise noted

LM2681 Switched Capacitor Voltage Converter

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.

LIFE SUPPORT POLICY

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 Semiconductor
Corporation

Americas
Tel: [phone]
Fax: [phone]
Email: [e-mail]

www.national.com

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.

National Semiconductor
Europe

Fax: [phone]

Email: [e-mail]
Deutsch Tel: [phone]
English Tel: [phone]
Français Tel: [phone]
Italiano Tel: [phone]

National Semiconductor
Asia Pacific Customer
Response Group

Tel: [phone]
Fax: [phone]
Email: [e-mail]

National Semiconductor
Japan Ltd.

Tel: [phone]
Fax: [phone]