LM2681M6 NOPB - NATIONAL SEMICONDUCTOR

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 Half

10096502

January 2003

LM2681 Switched Capacitor Voltage Converter

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

= 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

= 3.3 µF. (Note 4)

Symbol Parameter Condition Min Typ Max Units

V+ Supply Voltage 2.5 5.5 V

Supply Current No Load 550 1000 µA

Output Current 20 mA

Sum of the R

ds(on)

of the four

internal MOSFET switches I

=20mA 8 16 Ω

OUT

Output Resistance (Note 5) I

=20mA 15 40 Ω

OSC

Oscillator Frequency (Note 6) 80 160 kHz

Switching Frequency (Note 6) 40 80 kHz

EFF

Power Efficiency R

(1.0k) between GND and

OUT 86 93 %

=20mAtoGND 90

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

and S

are closed, C

charges to the supply

voltage 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

voltage V+ and the voltage across C

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

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

and C

. Since the switching current charging and

discharging C

is approximately twice as the output current,

the effect of the ESR of the pumping capacitor C

will be

multiplied by four in the output resistance. The output ca-

pacitor C

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

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

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

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

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

/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

(V+) is the quiescent power loss of the IC device,

and I

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

), 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

, 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:

out

= 1.5R

out_1

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:

in_min

out_min

drop_max

(LP2980) + I

out_max

out-

_max

(LM2681)

in_max

out_max

drop_min

(LP2980) + I

out_min

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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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.