LM2765M6X - National Semiconductor

LM2765

LM2765 Switched Capacitor Voltage Converter

Literature Number: SNVS070B

LM2765

Switched Capacitor Voltage Converter

General Description

The LM2765 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 up to 20 mA of output current.

The LM2765 operates at 50 kHz switching frequency to

reduce output resistance and voltage ripple. With an operat-

ing current of only 130 µA (operating efficiency greater than

90% with most loads) and 0.1µA typical shutdown current,

the LM2765 provides ideal performance for battery powered

systems. The device is manufactured in a SOT-23-6 pack-

age.

Features

nDoubles Input Supply Voltage

nSOT23-6 Package

n20ΩTypical Output Impedance

n90% Typical Conversion Efficiency at 20 mA

n0.1µA Typical Shutdown Current

Applications

nCellular Phones

nPagers

nPDAs

nOperational Amplifier Power Supplies

nInterface Power Supplies

nHandheld Instruments

Basic Application Circuits

Voltage Doubler

10128101

Connection Diagram

6-Lead SOT (M6)

10128113

Top View With Package Marking

10128122

Actual Size

March 2000

LM2765 Switched Capacitor Voltage Converter

Ordering Information

Order Number Package Number Package Marking Supplied as

LM2765M6 MA06A S15B (Note 1) Tape and Reel (1000 units/reel)

LM2765M6X MA06A S15B (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.

Pin Description

Pin Name Function

1 V+ Power supply positive voltage input.

2 GND Power supply ground input.

3 CAP− Connect this pin to the negative terminal of the charge-pump

capacitor.

4 SD Shutdown control pin, tie this pin to ground in normal operation.

OUT

Positive voltage output.

6 CAP+ Connect this pin to the positive terminal of the charge-pump

capacitor.

LM2765

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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

SD (GND − 0.3V) to (V+ +

0.3V)

OUT

Continuous Output Current 40 mA

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

Continuous Power

Dissipation (T

= 25˚C)(Note 4)

600 mW

JMax

(Note 4) 150˚C

Operating Ratings

(Note 4) 210˚C/W

Junction Temperature Range −40˚ to 100˚C

Ambient Temperature Range −40˚ to 85˚C

Storage Temperature Range −65˚C to 150˚C

Lead Temp. (Soldering, 10

seconds) 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

= 3.3 µF. (Note 6)

Symbol Parameter Condition Min Typ Max Units

V+ Supply Voltage 1.8 5.5 V

Supply Current No Load 130 450 µA

Shutdown Supply Current 0.1 0.5 µA

= 85˚C 0.2

Shutdown Pin Input Voltage Shutdown Mode 2.0 V

Normal Operation 0.6

Output Current 2.5V ≤V

≤5.5V 20 mA

1.8V ≤V

<2.5V 10

OUT

Output Resistance (Note 7) I

=20mA 20 40 Ω

OSC

Oscillator Frequency (Note 8) 40 100 200 kHz

Switching Frequency (Note 8) 20 50 100 kHz

EFF

Power Efficiency I

=20mAtoGND 92 %

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. However, shorting VOUT 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 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 3.3 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.

LM2765

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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

10128104 10128105

Output Resistance vs

Supply Voltage

Output Resistance vs

Temperature

10128106 10128107

10128103

FIGURE 1. LM2765 Test Circuit

LM2765

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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

10128108 10128109

Switching Frequency vs

Supply Voltage

Switching Frequency vs

Temperature

10128110 10128111

Output Ripple vs

Load Current

10128112

LM2765

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Circuit Description

The LM2765 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 LM2765 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 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. R

is typically 8Ωfor

the LM2765.

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.

SHUTDOWN MODE

A shutdown (SD) pin is available to disable the device and

reduce the quiescent current to 0.1 µA. In normal operating

mode, the SD pin is connected to ground. The device can be

brought into the shutdown mode by applying to the SD pin a

voltage greater than 40% of the V+ pin voltage.

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.

10128114

FIGURE 2. Voltage Doubling Principle

LM2765

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Application Information (Continued)

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

Other Applications

PARALLELING DEVICES

Any number of LM2765s 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 LM2765s 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.

10128119

FIGURE 3. Lowering Output Resistance by Paralleling Devices

10128120

FIGURE 4. Increasing Output Voltage by Cascading Devices

LM2765

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Other Applications (Continued)

REGULATING V

OUT

It is possible to regulate the output of the LM2765 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

(LM2765)

in_max

out_max

drop_min

(LP2980) + I

out_min

out-

_min

(LM2765)

10128121

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

LM2765

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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.

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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(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

in a significant injury to the user.

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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LM2765 Switched Capacitor Voltage Converter

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