LM2767M5 - National Semiconductor

LM2767

LM2767 Switched Capacitor Voltage Converter

Literature Number: SNVS069B

LM2767

Switched Capacitor Voltage Converter

General Description

The LM2767 CMOS charge-pump voltage converter oper-

ates as a voltage doubler for an input voltage in the range of

+1.8V to +5.5V. Two low cost capacitors and a diode are

used in this circuit to provide at least 15 mA of output current.

The LM2767 operates at 11 kHz switching frequency to

avoid audio voice-band interference. With an operating cur-

rent of only 40 µA (operating efficiency greater than 90% with

most loads), the LM2767 provides ideal performance for

battery powered systems. The device is manufactured in a

SOT23-5 package.

Features

nDoubles Input Supply Voltage

nSOT23-5 Package

n20ΩTypical Output Impedance

n96% Typical Conversion Efficiency at 15mA

Applications

nCellular Phones

nPagers

nPDAs, Organizers

nOperational Amplifier Power Suppliers

nInterface Power Suppliers

nHandheld Instruments

Basic Application Circuit

Voltage Doubler

10127401

Ordering Information

Order Number Package Number Package Marking Supplied as

LM2767M5 MA05B S17B (Note 1) Tape and Reel (1000 units/reel)

LM2767M5X MA05B S17B (Note 1) Tape and Reel (3000 units/reel)

Note 1: The small physical size of the SOT-23 package does not allow for the full part number marking. Devices will be marked with the designation shown

in the column Package Marking.

February 2000

LM2767 Switched Capacitor Voltage Converter

Connection Diagram

5-Lead SOT (M5)

10127413

Top View With Package Marking

10127422

Actual Size

Pin Description

Pin Name Function

OUT

Positive voltage output.

2 GND Power supply ground input.

3 CAP− Connect this pin to the negative terminal of the

charge-pump capacitor.

4 V+ Power supply positive voltage input.

5 CAP+ Connect this pin to the positive terminal of the

charge-pump capacitor.

LM2767

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Absolute Maximum Ratings (Note 2)

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 V+ to V

OUT

) 5.8V

OUT

Continuous Output Current 30 mA

Output Short-Circuit Duration to GND (Note 3) 1 sec.

Continuous Power

Dissipation (T

= 25˚C)(Note 4)

400 mW

JMax

(Note 4) 150˚C

Operating Ratings

(Note 4) 210˚C/W

Junction Temperature Range −40˚C to 100˚C

Ambient Temperature Range −40˚C to 85˚C

Storage Temperature Range −65˚C to 150˚C

Lead Temp. (Soldering, 10 sec.) 240˚C

ESD Rating (Note 5)

Human Body Model

Machine Model

2kV

200V

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

= 10 µF. (Note 6)

Symbol Parameter Condition Min Typ Max Units

V+ Supply Voltage 1.8 5.5 V

Supply Current No Load 40 90 µA

Output Current 1.8V ≤V+ ≤5.5V 15 mA

OUT

Output Resistance (Note 7) I

=15mA 20 40 Ω

OSC

Oscillator Frequency (Note 8) 822 50 kHz

Switching Frequency (Note 8) 411 25 kHz

EFF

Power Efficiency R

(5.0k) between GND and

OUT 98 %

=15mAtoGND 96

OEFF

Voltage Conversion Efficiency No Load 99.96 %

Note 2: 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 3: VOUT may be shorted to GND for one second without damage. For temperatures above 85˚C, VOUT must not be shorted to GND or device may be

damaged.

Note 4: 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 5: The human body model is a 100pF capacitor discharged through a 1.5kΩresistor into each pin. The machine model is a 200pF capacitor discharged directly

into each pin.

Note 6: In the test circuit, capacitors C1and C2are 10 µF, 0.3Ωmaximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce

output voltage and efficiency.

Note 7: Specified output resistance includes internal switch resistance and capacitor ESR. See the details in the application information for positive voltage doubler.

Note 8: The output switches operate at one half of the oscillator frequency, fOSC =2f

SW.

LM2767

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Test Circuit

Typical Performance Characteristics

(Circuit of Figure 1, V

= 5V, T

= 25˚C unless otherwise

specified)

Supply Current vs

Supply Voltage

Output Resistance vs

Capacitance

10127404 10127405

Output Resistance vs

Supply Voltage

Output Resistance vs

Temperature

10127406 10127407

10127403

FIGURE 1. LM2767 Test Circuit

LM2767

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Typical Performance Characteristics (Circuit of Figure 1, V

= 5V, T

= 25˚C unless otherwise

specified) (Continued)

Output Voltage vs

Load Current Efficiency vs

Load Current

10127408 10127409

Switching Frequency vs

Supply Voltage

Switching Frequency vs

Temperature

10127410 10127411

Output Ripple vs

Load Current

10127423

LM2767

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Circuit Description

The LM2767 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 LM2767 is to double the input

voltage. The range of the input supply voltage is 1.8V 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, and the capacitance and

ESR of C

and C

. Since the switching current charging and

discharging C

is approximately twice 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

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 resistances of the internal

MOSFET switches shown in Figure 2. R

is typically 4.5Ω

for the LM2767.

The peak-to-peak output voltage ripple is determined by the

oscillator frequency as well as the capacitance and ESR of

the output capacitor C

High capacitance, low ESR capacitors can reduce both the

output resistance and the voltage ripple.

The Schottky diode D

is only needed to protect the device

from turning-on its own parasitic diode and potentially

latching-up. During start-up, D

will also quickly charge up

the output capacitor to V

minus the diode drop thereby

decreasing the start-up time. Therefore, the Schottky diode

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.

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 allowable voltage

droop (which equals I

out

), and the desired output volt-

age ripple. Low ESR capacitors (Table 1) are recommended

to maximize efficiency, reduce the output voltage drop and

voltage ripple.

TABLE 1. Low ESR Capacitor Manufacturers

Manufacturer Phone Website Capacitor Type

Nichicon Corp. (847)-843-7500 www.nichicon.com PL & PF series, through-hole aluminum electrolytic

AVX Corp. (843)-448-9411 www.avxcorp.com TPS series, surface-mount tantalum

Sprague (207)-324-4140 www.vishay.com 593D, 594D, 595D series, surface-mount tantalum

Sanyo (619)-661-6835 www.sanyovideo.com OS-CON series, through-hole aluminum electrolytic

Murata (800)-831-9172 www.murata.com Ceramic chip capacitors

Taiyo Yuden (800)-348-2496 www.t-yuden.com Ceramic chip capacitors

Tokin (408)-432-8020 www.tokin.com Ceramic chip capacitors

10127414

FIGURE 2. Voltage Doubling Principle

LM2767

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Other Applications

PARALLELING DEVICES

Any number of LM2767s can be paralleled to reduce the

output resistance. Since there is no closed loop feedback, as

found in regulated circuits, stable operation is assured. 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 LM2767s 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 increasing the number of cascading stages is

pracitically limited since it significantly reduces the efficiency,

increases the output resistance and output voltage ripple.

REGULATING VOUT

It is possible to regulate the output of the LM2767 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:

2Vin_min >V

out_min

drop_max

(LP2980) + I

out_max

(LM2767)

in_max

out_max

drop_min

(LP2980) + I

out_min

(LM2767)

10127419

FIGURE 3. Lowering Output Resistance by Paralleling Devices

10127420

FIGURE 4. Increasing Output Voltage by Cascading Devices

LM2767

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Other Applications (Continued)

10127421

FIGURE 5. Generate a Regulated +5V from +3V Input Voltage

LM2767

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Physical Dimensions inches (millimeters) unless otherwise noted

5-Lead Small Outline Package (M5)

NS Package Number MA05B

For Order Numbers, refer to the table in the "Ordering Information" section of this document.

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.

For the most current product information visit us at www.national.com.

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WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR

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(b) support or sustain life, and whose failure to perform when

properly used in accordance with instructions for use

provided in the labeling, can be reasonably expected to result

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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

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LM2767 Switched Capacitor Voltage Converter

Tel: 81-3-5639-7560www.national.com

LM2767 Switched Capacitor Voltage Converter

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