LTC1261LIMS8-4.5#TRPBF

LTC1261L

Switched Capacitor

Regulated Voltage Inverter

FEATURES

DESCRIPTION

The LTC^®1261L is a switched-capacitor voltage inverter

designed to provide a regulated negative voltage from a

single positive supply. The LTC1261L operates from a

single 2.7V to 5.25V supply and provides an adjustable

output voltage from –1.23V to –5V. The LTC1261L-4/

LTC1261L-4.5 needs a single 4.5V to 5.25V supply and

provides a fixed output voltage of –4V to –4.5V respec-

tively.Threeexternalcapacitorsarerequired:a0.1µFflying

capacitor and an input and output bypass capacitors. An

optional compensation capacitor at ADJ (COMP) can be

used to reduce the output voltage ripple.

n

Regulated Negative Voltage from a

Single Positive Supply

n

REG Pin Indicates Output is in Regulation

n

Adjustable or Fixed Output Voltages

Output Regulation: 4.5%

Supply Current: 650µA Typ

n

Shutdown Mode Drops Supply Current to 5µA

n

Up to 20mA Output Current

n

Requires Only Three or Four External Capacitors

n

Available in MS8 and SO-8 Packages

Each version of the LTC1261L will supply up to 20mA

outputcurrentwithguaranteedoutputregulationof 4.5%.

The LTC1261L includes an open-drain REG output that

pulls low when the output is within 5% of the set value.

Quiescent current is typically 650µA when operating and

5µA in shutdown.

APPLICATIONS

n

GaAs FET Bias Generators

n

Negative Supply Generators

n

Battery-Powered Systems

n

Single Supply Applications

TheLTC1261Lisavailablein 8-pinMSOPandSOpackages.

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear

Technology Corporation. All other trademarks are the property of their respective owners.

TYPICAL APPLICATION

Waveforms for –4V Generator with Power Valid

–4V Generator with Power Valid

0V

5V

OUT

1

2

3

4

8

7

6

5

5V

V

SHDN

REG

CC

+

10k

–4V

C1

POWER VALID

C1

1µF

C2

0.1µF

5V

SHDN

0V

LTC1261L-4

V

= –4V

–

OUT

C1

OUT

AT 10mA

C4

3.3µF

+

GND

COMP

5V

POWER VALID

0V

C3*

100pF

1261L TA01

*OPTIONAL

1261L TA02

0.1ms/DIV

1261lfa

1

LTC1261L

ABSOLUTE MAXIMUM RATINGS ^{(Note 1)}

Supply Voltage (Note 2)...........................................5.5V

Output Short-Circuit Duration ......................... Indeﬁnite

Commercial Temperature Range (Note 4).... 0°C to 70°C

Industrial Temperature Range (Note 4)... –40°C to 85°C

Storage Temperature Range................... –65°C to 150°C

Lead Temperature (Soldering, 10 sec) .................. 300°C

Output Voltage (Note 3) ............................0.3V to –5.3V

Total Voltage, V to V

(Note 2)........................10.8V

OUT

CC

SHDN Pin......................................–0.3V to (V + 0.3V)

CC

REG Pin........................................................ –0.3V to 6V

ADJ Pin.............................(V

– 0.3V) to (V + 0.3V)

OUT

CC

PIN CONFIGURATION

TOP VIEW

V

1

2

3

4

8

7

6

5

SHDN

CC

+

V

C1

1

2

3

4

8 SHDN

7 REG

6 OUT

CC

+

C1

REG

–

OUT

5 ADJ (COMP)

GND

ADJ (COMP)

MS8 PACKAGE

8-LEAD PLASTIC MSOP

S8 PACKAGE

8-LEAD PLASTIC SO

T

= 150°C, θ = 200°C/W

JMAX

JA

T

= 150°C, θ = 135°C/W

JA

JMAX

ORDER INFORMATION

LEAD FREE FINISH

TAPE AND REEL

PART MARKING*

PACKAGE DESCRIPTION

8-Lead Plastic MSOP

8-Lead Plastic SO

TEMPERATURE RANGE

0°C to 70°C

LTC1261LCMS8#PBF

LTC1261LIMS8#PBF

LTC1261LCMS8-4#PBF

LTC1261LIMS8-4#PBF

LTC1261LCMS8#TRPBF

LTC1261LIMS8#TRPBF

LTC1261LCMS8-4#TRPBF

LTC1261LIMS8-4#TRPBF

LTFM

LTFN

–40°C to 85°C

0°C to 70°C

–40°C to 85°C

0°C to 70°C

LTC1261LCMS8-4.5#PBF LTC1261LCMS8-4.5#TRPBF LTFP

LTC1261LIMS8-4.5#PBF

LTC1261LCS8#PBF

LTC1261LIMS8-4.5#TRPBF LTFP

–40°C to 85°C

0°C to 70°C

LTC1261LCS8#TRPBF

LTC1261LIS8#TRPBF

1261L

LTC1261LIS8#PBF

1261L

8-Lead Plastic SO

–40°C to 85°C

0°C to 70°C

LTC1261LCS8-4#PBF

LTC1261LIS8-4#PBF

LTC1261LCS8-4.5#PBF

LTC1261LIS8-4.5#PBF

LTC1261LCS8-4#TRPBF

LTC1261LIS8-4#TRPBF

LTC1261LCS8-4.5#TRPBF

LTC1261LIS8-4.5#TRPBF

1261L4

261L45

8-Lead Plastic SO

–40°C to 85°C

0°C to 70°C

8-Lead Plastic SO

–40°C to 85°C

Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.

Consult LTC Marketing for information on nonstandard lead based finish parts.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/

1261lfa

2

LTC1261L

ELECTRICAL CHARACTERISTICS

The ● denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at T_A= 25°C, C1 = 0.1µF, C_OUT= 3.3µF unless otherwise noted. (Notes 2, 4)

SYMBOL PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

V

Supply Voltage

CC

l

LTC1261LCMS8/LTC1261LCS8

LTC1261LCMS8-4/LTC1261LCS8-4

2.7

4.35

4.75

5.25

V

(Note 5)

LTC1261LCMS8-4.5/LTC1261LCS8-4.5 (Note 5)

V

Reference Voltage

1.23

V

REF

l

I

f

Supply Current

V

= 5.25V, No Load, SHDN Floating

650

5

1500

20

µA

CC

= 5.25V, No Load, V

= V

CC

SHDN

Internal Oscillator Frequency

REG Output Low Voltage

REG Sink Current

V

= 5V, V = –4V

OUT

650

0.1

kHz

V

OSC

CC

l

V

I

= 1mA, V = 5V, V = –4V

OUT

0.8

1

OL

REG

CC

l

I

V

= 0.8V, V = 3.3V

4

5

8

12

mA

REG

CC

= 0.8V, V = 5V

CC

l

I

Adjust Pin Current

V

= 1.23V

0.01

µA

V

ADJ

V

SHDN Input High Voltage

SHDN Input Low Voltage

SHDN Input Current

Turn-On Time

= 5V

2

IH

IL

CC

0.8

25

V

I

t

= V

CC

2.5

µA

IN

ON

SHDN

l

V

= 5V, I

= 10mA, –1.5V ≤ V ≤ –4V (LTC1261L)

OUT

250

1500

µs

CC

OUT

= 5mA, V

= –4.5V (LTC1261L)

OUT

= 10mA, V

= –4V (LTC1261L-4)

= 5mA, V

= –4.5V (LTC1261L-4.5)

OUT

l

V

Output Regulation (LTC1261L)

2.70V ≤ V ≤ 5.25V, 0mA ≤ I

≤ 10mA

≤ 20mA

–1.552 –1.5 –1.448

V

OUT

CC

OUT

3.25V ≤ V ≤ 5.25V, 0mA ≤ I

CC

l

2.70V ≤ V ≤ 5.25V, 0mA ≤ I

≤ 5mA

≤ 10mA

≤ 20mA

–2.070 –2.0 –1.930

–2.070 –2.0 –1.91

V

CC

OUT

2.95V ≤ V ≤ 5.25V, 0mA ≤ I

CC

3.50V ≤ V ≤ 5.25V, 0mA ≤ I

CC

l

2.95V ≤ V ≤ 5.25V, 0mA ≤ I

≤ 5mA

≤ 10mA

≤ 20mA

–2.587 –2.5 –2.413

–2.587 –2.5 –2.41

V

CC

OUT

3.30V ≤ V ≤ 5.25V, 0mA ≤ I

CC

3.85V ≤ V ≤ 5.25V, 0mA ≤ I

CC

l

3.40V ≤ V ≤ 5.25V, 0mA ≤ I

≤ 5mA

≤ 10mA

≤ 20mA

–3.105 –3.0 –2.895

–3.105 –3.0 –2.885

V

CC

OUT

3.70V ≤ V ≤ 5.25V, 0mA ≤ I

CC

4.25V ≤ V ≤ 5.25V, 0mA ≤ I

CC

l

3.85V ≤ V ≤ 5.25V, 0mA ≤ I

≤ 5mA

≤ 10mA

≤ 20mA

–3.622 –3.5 –3.378

–3.622 –3.5 –3.365

V

CC

OUT

4.10V ≤ V ≤ 5.25V, 0mA ≤ I

CC

4.60V ≤ V ≤ 5.25V, 0mA ≤ I

CC

l

Output Regulation

(LTC1261L/LTC1261L-4)

4.35V ≤ V ≤ 5.25V, 0mA ≤ I

≤ 5mA

≤ 10mA

≤ 20mA

–4.140 –4.0 –3.860

–4.140 –4.0 –3.83

V

CC

OUT

4.60V ≤ V ≤ 5.25V, 0mA ≤ I

CC

5.10V ≤ V ≤ 5.25V, 0mA ≤ I

CC

l

Output Regulation

(LTC1261L/LTC1261L-4.5)

4.75V ≤ V ≤ 5.25V, 0mA ≤ I

≤ 5mA

≤ 10mA

–4.657 –4.5 –4.343

V

CC

OUT

5.05V ≤ V ≤ 5.25V, 0mA ≤ I

CC

l

I

Output Short-Circuit Current

V

= 0V, V = 5.25V

100

220

mA

SC

OUT

CC

Note 1: Stresses beyond those listed under Absolute Maximum Ratings

may cause permanent damage to the device. Exposure to any Absolute

Maximum Rating condition for extended periods may affect device

reliability and lifetime.

Note 4: The LTC1261LC is guaranteed to meet specifications from 0°C

to 70°C and is designed, characterized and expected to meet industrial

temperature limits, but is not tested at –40°C and 85°C. The LTC1261LI

is guaranteed to meet specifications from –40°C and 85°C.

Note 2: All currents into device pins are positive; all currents out of device

pins are negative. All voltages are referenced to ground unless otherwise

specified.

Note 5: The LTC1261L-4 and LTC1261L-4.5 will operate with less than the

minimum V specified in the electrical characteristics table, but they are

CC

not guaranteed to meet the 4.5% V

specification.

OUT

Note 3: The output should never be set to exceed V – 10.8V.

CC

1261lfa

3

LTC1261L

TYPICAL PERFORMANCE CHARACTERISTICS ^{(See Test Circuits)}

Output Voltage vs Output Current

Output Voltage vs Supply Voltage

–3.0

–3.1

–3.2

–3.3

–3.4

–3.5

–3.6

–3.7

–3.8

–3.9

–4.0

–4.1

–4.2

0

–0.25

–0.50

–0.75

–1.00

–1.25

–1.50

–1.75

–2.00

–1.90

–1.95

–2.00

–2.05

–2.10

T

= 25°C

OUT

T

= 25°C

OUT

V

= –2V

A

OUT

V

= –4V

V

= –2V

V

= 4.5V

CC

V

= 2.7V

T

= 25°C

CC

A

T

= –40°C

A

T

= 85°C

A

= 5V

CC

V

= 3V

25

CC

–2.25

0

10

15

20

25

30

5

2.5

3.0

3.5

4.0

4.5

5.0

0

5

10

15

20

30

OUTPUT CURRENT (mA)

SUPPLY VOLTAGE (V)

1261L G02

1261L G03

1261L G01

Minimum Required V_CC

vs V_OUTand I_OUT

Maximum Output Current

vs Supply Voltage

Output Voltage vs Supply Voltage

5.6

5.2

4.8

4.4

4.0

3.6

3.2

2.8

2.4

2.0

80

70

60

50

40

30

20

10

0

–3.85

–3.90

–3.95

–4.00

–4.05

–4.10

–4.15

T

= 25°C

V

= –4V

A

OUT

V

= –2V

OUT

T

= 25°C

A

I

= 20mA

T

= –40°C

OUT

A

T

= 85°C

A

I

= 5mA

–3

I

= 10mA

–1

OUT

V

= –3V

V

= –4V

OUT

3.9 4.2

4.5 4.8 5.1 5.4

5.0 5.1

2.7 3.0 3.3 3.6

4.5 4.6 4.7 4.8 4.9

5.2 5.3

–5

–4

0

–2

SUPPLY VOLTAGE (V)

OUTPUT VOLTAGE (V)

1261L G06

1261L G04

1261L G05

Reference Voltage

vs Temperature

Supply Current vs Supply Voltage

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

3.0

2.5

2.0

1.5

1.0

0.5

0

1.25

1.24

1.23

1.22

V

I

= –2V

= 0

V

I

= –4.5V

= 0

V

= 5V

OUT

CC

ADJ = 0V

T

= 25°C

A

T

A

= 85°C

T

= 85°C

A

T

= 25°C

T

= –40°C

A

T

A

= –40°C

1.21

–55 –35 –15

5

25 45 65 85 105 125

4.5 4.6

5.1

5.2

2.5

3.0

3.5

SUPPLY VOLTAGE (V)

4.0

4.5

5.0

4.8 4.9

SUPPLY VOLTAGE (V)

5.3

4.7

5.0

TEMPERATURE (°C)

1261L G09

1261L G07

1261L G08

1261lfa

4

LTC1261L

TYPICAL PERFORMANCE CHARACTERISTICS

Oscillator Frequency

vs Temperature

Output Short-Circuit Current

vs Temperature

Start-Up Time vs Supply Voltage

725

700

675

650

625

600

575

550

525

160

140

120

100

80

0.7

0.6

T

I

= 25°C

OUT

T

V

= 25°C

A

= 10mA

= –4V

OUT

= 5V

CC

V

= 5.25V

= 5V

CC

0.5

V

= –4V

OUT

0.4

0.3

0.2

0.1

V

CC

60

V

= –2V

OUT

V

= 3V

40

V

= 2.7V

CC

20

0

20 35

TEMPERATURE (°C)

–40 –25 –10

5

50 65 80 95

20 35

TEMPERATURE (°C)

–40 –25 –10

5

50 65 80 95

2.5

3.5

4.5

3.0

4.0

5.0

SUPPLY VOLTAGE (V)

1261L G10

1261L G12

1261L G11

PIN FUNCTIONS

CC

V

(Pin 1): Power Supply. This requires an input voltage

ripple on both the adjustable and fixed output voltage

parts. See the Applications Information section for more

information on compensation and output ripple.

between2.7Vand5.25V. V mustbebypassedtoground

CC

withatleasta1µFcapacitorplacedincloseproximitytothe

chip. See the Applications Information section for details.

OUT (Pin 6): Negative Voltage Output. This pin must be

bypassed to ground with a 1µF or larger capacitor. The

value of the output capacitor and its ESR have a strong

effect on output ripple. See the Applications Information

section for more details.

+

C1 (Pin 2): C1 Positive Input. Connect a 0.1µF capacitor

+

–

between C1 and C1 .

–

C1 (Pin 3): C1 Negative Input. Connect a 0.1µF capacitor

+

–

from C1 to C1 .

REG (Pin 7): This is an open-drain output that pulls low

when the output voltage is within 5% of the set value. It

will sink 5mA to ground with a 5V supply. The external

circuitrymustprovideapull-uporREGwillnotswinghigh.

GND(Pin4):Ground.Connecttoalowimpedanceground.

A ground plane will help to minimize regulation errors.

ADJ (COMP for Fixed Versions) (Pin 5): Output Adjust/

Compensation Pin. For adjustable parts this pin is used to

set the output voltage. The output voltage is divided down

with an external resistor divider and fed back to this pin

to set the regulated output voltage. Typically the resistor

string should draw ≥10µA from the output to minimize

errorsduetothebiascurrentattheadjustpin.Fixedoutput

voltage parts have the internal resistor string connected

to this pin inside the package. The pin can be used to

trim the output voltage if desired. It can also be used as

an optional feedback compensation pin to reduce output

The voltage at REG may exceed V and can be pulled up

CC

to 6V above ground without damage.

SHDN (Pin 8): Shutdown. When this pin is at ground the

LTC1261L operates normally. An internal 5µA pull-down

keeps SHDN low if it is left floating. When SHDN is pulled

high, the LTC1261L enters shutdown mode. In shutdown,

the charge pump is disabled, the output collapses to 0V

and the quiescent current drops to 5µA typically.

1261lfa

5

LTC1261L

TEST CIRCUITS

Adjustable Output

Fixed Output

1

8

1

2

3

4

8

V

SHDN

V

SHDN

5V

CC

+

CC

+

2

3

4

7

6

5

+

10µF

C1

REG

LTC1261L-X

C1

REG

LTC1261L

0.1µF

6

5

–

V

= –4V (LTC1261L-4)

= –4.5V (LTC1261L-4.5)

–

OUT

C1

OUT

ADJ

V

OUT

3.3µF

+

GND

COMP

+

GND

1261L TCO1

1261L TCO2

APPLICATIONS INFORMATION

The LTC1261L uses an inverting charge pump to generate

a regulated negative output voltage that is either equal to

or less than the supply voltage. The LTC1261L needs only

three external capacitors and is available in the MSOP and

SO-8 packages

The next rising clock edge sets the RS latch, setting the

charge pump to transfer charge from the flying capacitor

to the output capacitor. As long as the output is below the

set point, COMP1 stays low, the latch stays set and the

charge pump runs at the full 50% duty cycle of the clock

gated through the AND gate. As the output approaches the

set voltage, COMP1 will trip whenever the divided signal

exceeds the internal 1.23V reference relative to OUT.

This resets the RS latch and truncates the clock pulses,

reducing the amount of charge transferred to the output

capacitor and regulating the output voltage. If the output

exceeds the set point, COMP1 stays high, inhibiting the

RS latch and disabling the charge pump.

THEORY OF OPERATION

A block diagram of the LTC1261L is shown in Figure 1.

TheheartoftheLTC1261Listhechargepumpcoreshown

in the dashed box. It generates a negative output voltage

by first charging the flying capacitor (C1) between V

CC

and ground. It then connects the top of the flying capaci-

tor to ground, forcing the bottom of the flying capacitor

to a negative voltage. The charge on the flying capacitor

is transferred to the output bypass capacitor, leaving it

charged to the negative output voltage. This process is

driven by the internal 650kHz clock.

COMP2 also monitors the divided signal at ADJ but it is

connected to a 1.17V reference, 5% below the main refer-

ence voltage. When the divided output exceeds this lower

reference voltage indicating that the output is within 5%

of the set value, COMP2 goes high turning on the REG

output transistor. This is an open-drain N-channel device

Figure 1 shows the charge pump configuration. With the

clock low, C1 is charged to V by S1 and S3. At the next

CC

capable of sinking 4mA with a 3.3V V and 5mA with a

CC

rising clock edge, S1 and S3 are open and S2 and S4

5V V . When in the “off” state (divided output is more

CC

+

–

closed. S2 connects C1 to ground, C1 is connected to

than 5% below V ) the drain can be pulled above V

REF

CC

the output by S4. The charge in C1 is transferred to C

setting it to a negative voltage.

,

OUT

without damage up to a maximum of 6V above ground.

NotethattheREGoutputonlyindicatesifthemagnitudeof

the output is below the magnitude of the set point by 5%

The output voltage is monitored by COMP1 which com-

pares a divided replica of the output at ADJ (COMP for

fixed output voltage parts) to the internal reference. At the

beginning of a cycle the clock is low, forcing the output

of the AND gate low and charging the flying capacitor.

(i.e., V

> –4.75V for a –5V set point). If the magnitude

OUT

of the output is forced higher than the magnitude of the

set point (i.e., to –5.25V when the output is set for –5V)

the REG output will stay low.

1261lfa

6

LTC1261L

APPLICATIONS INFORMATION

V

CC

CLK

650kHz

S1

OUT

C

OUT

+

R2

R1

C1

S

Q

C1

S4

–

S2

C1

R

INTERNALLY

CONNECTED FOR

FIXED OUTPUT

VOLTAGE PARTS

S3

ADJ (COMP)

REG

+

COMP1

+

–

COMP2

60mV

1.17V

V

REF

= 1.23V

1261L F01

V

OUT

Figure 1. Block Diagram

OUTPUT RIPPLE

fixed output voltage versions) through an internal or

external resistor divider from the OUT pin to ground. As

the flying capacitor is first connected to the output, the

output voltage begins to change quite rapidly. As soon as

it exceeds the set point COMP1 trips, switching the state

of the charge pump and stopping the charge transfer.

Because the RC time constant of the capacitors and the

switches is quite short, the ADJ pin must have a wide AC

bandwidth to be able to respond to the output in time.

External parasitic capacitance at the ADJ pin can reduce

the bandwidth to the point where the comparator cannot

respond by the time the clock pulse finishes. When this

happens the comparator will allow a few complete pulses

through, then overcorrect and disable the charge pump

until the output drops below the set point. Under these

conditions the output will remain in regulation but the

output ripple will increase as the comparator “hunts” for

the correct value.

OutputrippleintheLTC1261Lispresentfromtwosources;

voltage droop at the output capacitor between clocks and

frequency response of the regulation loop. Voltage droop

is easy to calculate. With a typical clock frequency of

650kHz, the charge on the output capacitor is refreshed

once every 1.54µs. With a 15mA load and a 3.3µF output

capacitor, the output will droop by:







∆t

_OUT

 1.54µs

I_LOAD

=15mA

= 7mV











3.3µF

C



Thiscanbeasignificantripplecomponentwhentheoutput

isheavilyloaded,especiallyiftheoutputcapacitorissmall.

If absolute minimum output ripple is required, a 10µF or

greater output capacitor should be used.

Regulation loop frequency response is the other major

contributor to output ripple. The LTC1261L regulates the

output voltage by limiting the amount of charge trans-

ferred to the output capacitor on a cycle-by-cycle basis.

The output voltage is sensed at the ADJ pin (COMP for

To prevent this from happening, an external capacitor can

be connected from ADJ (or COMP for fixed output voltage

parts)togroundtocompensateforexternalparasiticsand

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7

LTC1261L

APPLICATIONS INFORMATION

5V

increase the regulation loop bandwidth (Figure 2). This

soundscounterintuitiveuntilwerememberthattheinternal

referenceisgeneratedwithrespecttoOUT,notground.The

feedback loop actually sees ground as its “output,” thus

the compensation capacitor should be connected across

the “top” of the resistor divider, from ADJ (or COMP) to

ground. By the same token, avoid adding capacitance

1µF

V

CC

10Ω

2

3

6

5

+

V

= – 4V

C1

OUT

0.1µF

LTC1261L-4

–

3.3µF

+

C1

COMP

GND

4

100pF

between ADJ (or COMP) and V . This will slow down

OUT

the feedback loop and increase output ripple. A 100pF

capacitor from ADJ or COMP to ground will compensate

the loop properly under most conditions for fixed voltage

versions of the LTC1261L. For the adjustable LTC1261L,

the capacitor value will be dependent upon the values of

the external resistors in the divider network.

1261L F03

Figure 3. Output Filter Cuts Ripple Below 3mV

CAPACITOR SELECTION

Capacitor Sizing

TO CHARGE

PUMP

The performance of the LTC1261L is affected by the

capacitors to which it connects. The LTC1261L requires

bypass capacitors to ground for both the V and OUT

RESISTORS ARE

INTERNAL FOR FIXED

OUTPUT VOLTAGE PARTS

CC

pins. The input capacitor provides most of LTC1261L’s

supply current while it is charging the flying capacitors.

This capacitor should be mounted as close to the package

as possible and its value should be at least ten times larger

than the flying capacitor. Ceramic capacitors generally

provide adequate performance. Avoid using a tantalum

capacitor as the input bypass unless there is at least a

0.1µFceramiccapacitorinparallelwithit.Thechargepump

capacitor is somewhat less critical since its peak current

is limited by the switches inside the LTC1261L. Most ap-

plications should use a 0.1µF as the flying capacitor value.

Conveniently, ceramic capacitors are the most common

type of 0.1µF capacitor and they work well here. Usually

the easiest solution is to use the same capacitor type for

both the input bypass capacitor and the flying capacitor.

COMP1

C

R1

R2

100pF

ADJ/COMP

+

–

REF

1.23V

V

OUT

1261L F02

Figure 2. Regulator Loop Compensation

OUTPUT FILTERING

Ifextremelylowoutputripple(<5mV)isrequired,additional

output filtering is required. Because the LTC1261L uses

a high 650kHz switching frequency, fairly low value RC

or LC networks can be used at the output to effectively

filter the output ripple. A 10Ω series output resistor and

a 3.3µF capacitor will cut output ripple to below 3mV

(Figure 3). Further reductions can be obtained with larger

filter capacitors or by using an LC output filter.

In applications where the maximum load current is well-

defined and output ripple is critical or input peak currents

need to be minimized, the flying capacitor value can be

1261lfa

8

LTC1261L

APPLICATIONS INFORMATION

tailored to the application. Reducing the value of the flying

capacitor reduces the amount of charge transferred with

each clock cycle. This limits maximum output current, but

alsocutsthesizeofthevoltagestepattheoutputwitheach

clock cycle. The smaller capacitor draws smaller pulses

oscillation under such conditions but it also creates an

output error as the feedback loop regulates based on

the top of the spike, not the average value of the output

(Figure 4). The resulting output voltage behaves as if a

resistor of value C

× (I /I )Ω was placed in series

ESR

PK AVE

of current out of V as well, limiting peak currents and

with the output. To avoid this nasty sequence of events,

connect a 0.1µF ceramic capacitor in parallel with the

larger output capacitor. The ceramic capacitor will “eat”

the high frequency spike, preventing it from fooling the

feedback loop, while the larger but slower tantalum or

aluminum output capacitor supplies output current to the

load between charge cycles.

CC

reducing the demands on the input supply. Table 1 shows

recommended values of flying capacitor vs maximum

load capacity.

Table 1. Typical Max Load (mA) vs Flying Capacitor Value at

T_A= 25°C, V_OUT= –4V

MAX LOAD (mA)

FLYING CAPACITOR VALUE (µF)

V

= 5V

CC

0.1

20

CLOCK

0.047

0.033

0.022

0.01

15

10

5

V

SET

OUT

AVERAGE

LOW ESR

V

OUT

OUTPUT CAP

COMP1

OUTPUT

1

V

SET

The output capacitor performs two functions: it provides

output current to the load during half of the charge pump

cycle and its value helps to set the output ripple voltage.

For applications that are insensitive to output ripple, the

output bypass capacitor can be as small as 1µF. Larger

output capacitors will reduce output ripple further at the

expense of turn-on time.

OUT

AVERAGE

HIGH ESR

OUTPUT CAP

OUT

COMP1

OUTPUT

1261L F04

Figure 4. Output Ripple with Low and High ESR Capacitors

NotethatESRintheflyingcapacitorwillnotcausethesame

condition; in fact, it may actually improve the situation by

cutting the peak current and lowering the amplitude of the

spike. However, more flying capacitor ESR is not neces-

sarily better. As soon as the RC time constant approaches

halfofaclockperiod(thetimethecapacitorshavetoshare

chargeatfulldutycycle)theoutputcurrentcapabilityofthe

LTC1261L starts to diminish. For a 0.1µF flying capacitor,

this gives a maximum total series resistance of:

Capacitor ESR

Output capacitor Equivalent Series Resistance (ESR) is

another factor to consider. Excessive ESR in the output

capacitor can fool the regulation loop into keeping the

output artificially low by prematurely terminating the

charging cycle. As the charge pump switches to recharge

the output a brief surge of current flows from the flying

capacitors to the output capacitor. This current surge can

be as high as 100mA under full load conditions. A typical

3.3µF tantalum capacitor has 1Ω or 2Ω of ESR; 100mA

× 2Ω = 200mV. If the output is within 200mV of the set

point this additional 200mV surge will trip the feedback

comparator and terminate the charging cycle. The pulse

dissipates quickly and the comparator returns to the

correct state, but the RS latch will not allow the charge

pump to respond until the next clock edge. This prevents

the charge pump from going into very high frequency







1

t_CLK

2 C

1







=

/0.1µF= 7.7Ω









2 650kHz



_FLY

Most of this resistance is already provided by the internal

switches in the LTC1261L. More than 1Ω or 2Ω of ESR

on the flying capacitors will start to affect the regulation

at maximum load.

1261lfa

9

LTC1261L

APPLICATIONS INFORMATION

RESISTOR SELECTION

The LTC1261L can be internally configured for other fixed

output voltages. Contact the Linear Technology Marketing

department for details.

Resistor selection is easy with the fixed output voltage

versions of the LTC1261L—no resistors are needed! Se-

lecting the right resistors for the adjustable parts is only

a little more difficult. A resistor divider should be used to

divide the signal at the output to give 1.23V at the ADJ

4

GND

R1

R2

LTC1261L

ADJ

5

6

pin with respect to V

(Figure 5). The LTC1261L uses

OUT

a positive reference with respect to V , not a negative

OUT

R1 + R2

R2

reference with respect to ground (Figure 1 shows the

reference connection). Be sure to keep this in mind when

connecting the resistors! If the initial output is not what

you expected, try swapping the two resistors.

V

= –1.23V

OUT

(

)

1261L F05

Figure 5. External Resistor Connections

TYPICAL APPLICATIONS

5V Input, –4V Output GaAs FET Bias Generator

P-CHANNEL

POWER SWITCH

V

BAT

SHUTDOWN

10k

1

2

8

7

6

5

5V

V

SHDN

REG

CC

+

C1

1µF

0.1µF

LTC1261L-4

3

4

–4V BIAS

3.3µF

–

C2

OUT

GaAs

TRANSMITTER

GND

COMP

+

100pF

1261 TA03

1mV Ripple, 5V Input, –4V Output GaAs FET Bias Generator

P-CHANNEL

POWER SWITCH

V

BAT

SHUTDOWN

10k

1

2

8

7

6

5

5V

V

SHDN

REG

CC

+

C1

1µF

0.1µF

LTC1261L-4

100µH

3

4

–4V BIAS

10µF

–

C2

OUT

GaAs

TRANSMITTER

10µF

GND

COMP

+

100pF

1261 TA04

1261lfa

10

LTC1261L

TYPICAL APPLICATIONS

5V Input, –0.5V Output GaAs FET Bias Generator

P-CHANNEL

POWER SWITCH

V

BAT

SHUTDOWN

5V

43.2k

10k

1%

1

2

3

4

8

7

6

5

V

SHDN

REG

CC

+

C1

LTC1261L

1µF

0.1µF

–0.5V BIAS

3.3µF

–

C2

OUT

GaAs

TRANSMITTER

12.4k

1%

GND

ADJ

+

100pF

1261 TA05

PACKAGE DESCRIPTION

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

MS8 Package

8-Lead Plastic MSOP

(Reference LTC DWG # 05-08-ꢀꢂꢂ0 Rev F)

0.889 0.ꢀꢁ7

(.035 .005)

5.ꢁ3

3.ꢁ0 – 3.45

(.ꢁ0ꢂ)

(.ꢀꢁꢂ – .ꢀ3ꢂ)

MIN

3.00 0.ꢀ0ꢁ

(.ꢀꢀ8 .004)

(NOTE 3)

0.5ꢁ

(.0ꢁ05)

REF

0.ꢂ5

(.0ꢁ5ꢂ)

BSC

0.4ꢁ 0.038

(.0ꢀꢂ5 .00ꢀ5)

TYP

8

7 ꢂ

5

RECOMMENDED SOLDER PAD LAYOUT

3.00 0.ꢀ0ꢁ

(.ꢀꢀ8 .004)

(NOTE 4)

4.90 0.ꢀ5ꢁ

(.ꢀ93 .00ꢂ)

DETAIL “A”

0.ꢁ54

(.0ꢀ0)

0° – ꢂ° TYP

GAUGE PLANE

ꢀ

ꢁ

3

4

0.53 0.ꢀ5ꢁ

(.0ꢁꢀ .00ꢂ)

ꢀ.ꢀ0

(.043)

MAX

0.8ꢂ

(.034)

REF

DETAIL “A”

0.ꢀ8

(.007)

SEATING

PLANE

0.ꢁꢁ – 0.38

0.ꢀ0ꢀꢂ 0.0508

(.009 – .0ꢀ5)

(.004 .00ꢁ)

0.ꢂ5

(.0ꢁ5ꢂ)

BSC

TYP

MSOP (MS8) 0307 REV F

NOTE:

ꢀ. DIMENSIONS IN MILLIMETER/(INCH)

ꢁ. DRAWING NOT TO SCALE

3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.

MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.ꢀ5ꢁmm (.00ꢂ") PER SIDE

4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.

INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.ꢀ5ꢁmm (.00ꢂ") PER SIDE

5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.ꢀ0ꢁmm (.004") MAX

1261lfa

11

LTC1261L

PACKAGE DESCRIPTION

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

S8 Package

8-Lead Plastic Small Outline (Narrow .150 Inch)

(Reference LTC DWG # 05-08-1610 Rev G)

.189 – .197

(4.801 – 5.004)

.045 ±.005

NOTE 3

.050 BSC

7

5

8

6

.245

MIN

.160 ±.005

.150 – .157

(3.810 – 3.988)

NOTE 3

.228 – .244

(5.791 – 6.197)

.030 ±.005

TYP

1

3

4

2

RECOMMENDED SOLDER PAD LAYOUT

.010 – .020

(0.254 – 0.508)

× 45°

.053 – .069

(1.346 – 1.752)

.004 – .010

(0.101 – 0.254)

.008 – .010

(0.203 – 0.254)

0°– 8° TYP

.016 – .050

(0.406 – 1.270)

.050

(1.270)

BSC

.014 – .019

(0.355 – 0.483)

TYP

NOTE:

INCHES

1. DIMENSIONS IN

(MILLIMETERS)

2. DRAWING NOT TO SCALE

3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.

MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)

4. PIN 1 CAN BE BEVEL EDGE OR A DIMPLE

SO8 REV G 0212

1261lfa

12

LTC1261L

REVISION HISTORY

REV

DATE

DESCRIPTION

PAGE NUMBER

A

9/12

Added I-grade option

Throughout

Modified Output Regulation specifications

Modified Package/Order Information tables

Modified Notes 4 and 5

1, 3

2

4

1261lfa

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

13

LTC1261L

TYPICAL APPLICATIONS

Low Output Voltage Generator

Minimum Parts Count –4.5V Generator

5V

1

2

3

4

8

7

6

5

5V

V

SHDN

REG

CC

+

1µF

1

C1

100pF

R

S

V

LTC1261L-4.5

C1

1µF

0.1µF

CC

5

6

V

= – 4.5V

–

OUT

ADJ

OUT

AT 5mA

2

3

+

C1

3.3µF

GND

COMP

+

0.1µF

LTC1261L

–

124k

C1

1261L TA07

V

= V – 9.92µA (R + 124k)

CC S

OUT

= –0.5V (R = 432k)

GND

4

S

3.3µF

1N5817

+

= –1V (R = 487k)

S

1261L TA06

RELATED PARTS

PART NUMBER

DESCRIPTION

COMMENTS

LT1121

Micropower Low Dropout Regulator with Shutdown

0.4V Dropout Voltage at 150mA, Low Noise, Switched

Capacitor Regulated Voltage Inverter

LTC1429

Clock Synchronized Switched Capacitor Regulated Voltage Inverter GaAs FET Bias

Fixed 1.8V or 2V Output from 2.4V to 6V Input,

= 100mA

LTC1503-1.8/LTC1503-2 High Efficiency Inductorless Step-Down DC/DC Converter

I

OUT

LTC1514/LTC1515

Step-Up/Step-Down Switched Capacitors DC/DC Converters

V : 2V to 10V, V

is Fixed or Adjustable, I

Up to

OUT

IN

OUT

50mA

LTC1516

Micropower, Regulated 5V Charge Pump DC/DC Converter

Low Noise Switched Capacitor Regulated Voltage Inverter

Sim Power Supply and Level Translator

I

= 20mA (V ≥ 2V), I

= 50mA (V ≥ 3V)

OUT IN

OUT

IN

LTC1517-5

LTC1522 without Shutdown and Packaged in SOT-23

= 10mA (V ≥ 2.7V), I = 20mA (V ≥ 3V)

LTC1522

I

OUT

IN

OUT

IN

LTC1550L/LTC1551L

LTC1555/LTC1556

GaAs FET Bias with Linear Regulator, <1mV Ripple, MSOP

Step-Up/Step-Down Sim Power Supply and Level

Translators

LT1611

1.4MHz Inverting Mode Switching Regulator

–5V at 150mA from a 5V Input, 5-lead SOT-23

–5V at 200mA from 5V Input in MSOP

LT1614

Inverting 600kHz Switching Regulator with Low Battery Detector

Micropower Inverting DC/DC Converters

LT1617/LT1617-1

–15V at 12mA from 2.5V Input, 5-lead SOT-23

LTC1682/LTC1682-3.3/ Low Noise Doubler Charge Pumps

LTC1682-5

Output Noise = 60µV

, 2.5V to 5.5V Output

RMS

LTC1754-5

Micropower, Regulated 5V Charge Pump with Shutdown in SOT-23 I = 13µA, I

= 50mA (V ≥ 3V), I

= 25mA

OUT

CC

OUT

IN

(V ≥ 2.7V)

IN

1261lfa

LT 0912 REV A • PRINTED IN USA

LinearTechnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

14

●

 LINEAR TECHNOLOGY CORPORATION 1999

(408) 432-1900 FAX: (408) 434-0507 www.linear.com


型号：	LTC1261LIMS8-4.5#TRPBF
厂家：	Linear
描述：	LTC1261L - Switched Capacitor Regulated Voltage Inverter; Package: MSOP; Pins: 8; Temperature Range: -40°C to 85°C 光电二极管
文件：	总14页 (文件大小：2381K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LTC1261LIMS8-4.5#TRPBF [Linear]

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