LTC1261CS_技术文档

LTC1261

Switched Capacitor

Regulated Voltage Inverter

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DESCRIPTIO

EATURE

Regulated Negative Voltage from a

Single Positive Supply

Can Provide Regulated –5V from a 3V Supply

REG Pin Indicates Output is in Regulation

Low Output Ripple: 5mV Typ

Supply Current: 600µA Typ

Shutdown Mode Drops Supply Current to 5µA

Up to 15mA Output Current

Adjustable or Fixed Output Voltages

Requires Only Three or Four External Capacitors

S

F

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

designed to provide a regulated negative voltage from a

single positive supply. The LTC1261CS operates from a

single 3V to 8V supply and provides an adjustable output

voltage from –1.25V to –8V. An on-chip resistor string

allows the LTC1261CS to be configured for output volt-

ages of –3.5V, –4V, –4.5V or –5V with no external

components. The LTC1261CS8 is optimized for applica-

tions which use a 5V or higher supply or which require

low output voltages. It requires a single external 0.1µF

capacitorandprovidesadjustableandfixedoutputvoltage

options in 8-pin SO packages. The LTC1261CS requires

one or two external 0.1µF capacitors, depending on input

voltage. Both versions require additional external input

and output bypass capacitors. An optional compensation

capacitor at ADJ/COMP can be used to reduce the output

voltage ripple.

■

^{Available in SO-8}O ^PaU ^ckages

PPLICATI

S

A

■

GaAs FET Bias Generators

Negative Supply Generators

Battery-Powered Systems

Single Supply Applications

Each version of the LTC1261 will supply up to 12mA

output current with guaranteed output regulation of 5%.

The LTC1261 includes an open-drain REG output which

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

Output ripple is typically as low as 5mV. Quiescent current

is typically 600µA when operating and 5µA in shutdown.

The LTC1261 is available in a 14-pin narrow body SO

package and an 8-pin SO package.

, LTC and LT are registered trademarks of Linear Technology Corporation.

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TYPICAL APPLICATION

Waveforms for –4V Generator with Power Valid

–4V Generator with Power Valid

0V

OUT

5V

1

2

3

4

8

7

6

5

5V

V

SHDN

REG

CC

10k

–4V

+

C1

POWER VALID

5V

C1

1µF

C2

0.1µF

LTC1261-4

SHDN

V

= –4V

–

OUT

C1

OUT

0V

AT 10mA

C4

3.3µF

5V

POWER VALID

0V

+

GND

COMP

C3*

100pF

*OPTIONAL

LTC1261 • TA01

0.2mS/DIV

LTC1261 • TAO2

1

LTC1261

W W W

U

ABSOLUTE AXI U RATI GS

/O

PACKAGE RDER I FOR ATIO

(Note 1)

ORDER PART

Supply Voltage (Note 2)............................................ 9V

Output Voltage (Note 5).............................. 0.3V to –9V

Total Voltage, V_CCto V_OUT(Note 2) ........................ 12V

Input Voltage

SHDN Pin ................................. –0.3V to V_CC+ 0.3V

REG Pin ............................................... –0.3V to 12V

ADJ, R_O,R1, R_ADJ............... V_OUT– 0.3V to V_CC+ 0.3V

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

Operating Temperature Range

Commercial ............................................ 0°C to 70°C

Extended Commercial (Note 7).......... –40°C to 85°C

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

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

TOP VIEW

NUMBER

V

1

2

3

4

8

7

6

5

SHDN

CC

+

LTC1261CS8

LTC1261CS8-4

LTC1261CS8-4.5

C1

REG

–

OUT

GND

ADJ (COMP*)

S8 PART MARKING

S8 PACKAGE

8-LEAD PLASTIC SO

*FOR FIXED VERSIONS

1261

12614

126145

T_JMAX= 150°C, θ_JA= 150°C/W

ORDER PART

NUMBER

TOP VIEW

NC

+

1

2

3

4

5

6

7

14

13

12

11

10

9

V

CC

C1

SHDN

REG

OUT

ADJ

LTC1261CS

–

+

–

C1

C2

GND

R0

R

ADJ

8

R1

S PACKAGE

14-LEAD PLASTIC SO

T

_JMAX= 150°C, θ_JA= 110°C/W

Consult factory for Industrial or Military grade parts.

V_CC= 3V to 6.5V, T_A= 25°C unless otherwise specified.

ELECTRICAL CHARACTERISTICS

0

°

C

≤

T

≤

70

°C

–40°C ≤ T ≤ 85°C

A

(Note 7)

TYP MAX UNITS

SYMBOL PARAMETER

CONDITIONS

MIN

TYP

MAX

MIN

V

Reference Voltage

Supply Current

●

1.20

1.24

1.28

1.20

1.24

1.28

V

REF

I

No Load, SHDN Floating, Doubler Mode

No Load, SHDN Floating, Tripler Mode

●

600

900

5

1000

1500

20

600

900

5

1500

2000

20

µA

S

No Load, V

= V

CC

SHDN

f

Internal Oscillator Frequency

Power Efficiency

REG Output Low Voltage

REG Sink Current

550

65

0.1

550

65

0.1

kHz

%

V

OSC

P

V

EFF

OL

I

= 1mA

●

0.8

1

0.8

1

REG

I

V

= 0.8V, V = 3.3V

= 0.8V, V = 5.0V

●

5

8

15

0.01

5

8

15

0.01

mA

µA

V

REG

CC

I

Adjust Pin Current

= 1.24V

●

ADJ

V

I

t

SHDN Input High Voltage

SHDN Input Low Voltage

SHDN Input Current

Turn-On Time

2

IH

IL

0.8

20

0.8

25

V

µA

µs

V

= V

CC

5

IN

ON

SHDN

I

= 15mA

500

OUT

2

LTC1261

ELECTRICAL CHARACTERISTICS

Doubler Mode. V_CC= 5V ±10%, C1 = 0.1µF, C2 = 0 (Note 4), C_OUT= 3.3µF unless otherwise specified.

70

0

°C

≤

T

≤

°C

–40°C ≤ T ≤ 85°C

A

(Note 7)

TYP

SYMBOL PARAMETER

CONDITIONS (Note 2)

MIN

TYP

MAX

MIN

MAX

5

UNITS

∆V

Output Regulation

(Note 2)

–1.24V ≥ V

≥ –4V, 0 ≤ I

≤ 8mA

≤ 7mA

●

1

5

%

OUT

1

2

–4V ≥ V

≥ –5V, 0 ≤ I

≤ 8mA (Note 6)

2

60

5

OUT

I

Output Short-Circuit Current

Output Ripple Voltage

V

= 0V

●

125

60

5

125

mA

mV

SC

OUT

V

I

= 5mA, V

= –4V

OUT

RIP

LTC1261CS Only. Tripler Mode. V_CC= 2.7V, C1 = C2 = 0.1µF (Note 4), C_OUT= 3.3µF unless otherwise specified.

70 –40°C ≤ T ≤ 85°C

A

0

°C

≤

T

≤

°

C

A

(Note 7)

TYP

SYMBOL PARAMETER

CONDITIONS (Note 2)

–1.24V ≥ V ≥ –4V, 0 ≤ I ≤ 5mA

OUT

MIN

TYP

1

MAX

5

MIN

MAX

5

UNITS

%

∆V

Output Regulation

●

1

60

5

OUT

I

V

Output Short-Circuit Current

Output Ripple Voltage

V

I

= 0V

= 5mA, V

60

5

125

mA

mV

SC

OUT

= –4V

OUT

RIP

LTC1261CS Only. Tripler Mode. V_CC= 3.3V ±10%, C1 = C2 = 0.1µF (Note 4), C_OUT= 3.3µF unless otherwise specified.

70 –40°C ≤ T ≤ 85°C

A

0

°C

≤

T

≤

°

C

A

(Note 7)

TYP

SYMBOL PARAMETER

CONDITIONS (Note 2)

–1.24V ≥ V ≥ –4.5V, 0 ≤ I ≤ 6mA

OUT

MIN

TYP

MAX

MIN

MAX

UNITS

∆V

Output Regulation

(Note 2)

●

1

2

5

1

2

5

%

OUT

–4.5V ≥ V

≥ –5V, 0 ≤ I

≤ 3.5mA

OUT

I

Output Short-Circuit Current

Output Ripple Voltage

V

= 0V

●

35

5

75

35

5

75

mA

mV

SC

OUT

V

I

= 5mA, V

= –4V

OUT

RIP

LTC1261CS Only. Tripler Mode. V_CC= 5V ±10%, C1 = C2 = 0.1µF (Note 4), C_OUT= 3.3µF unless otherwise specified.

70 –40°C ≤ T ≤ 85°C

A

0

°C

≤

T

≤

°

C

A

(Note 7)

TYP

SYMBOL PARAMETER

CONDITIONS (Note 2)

–1.24V ≥ V ≥ –4V, 0 ≤ I ≤ 12mA

OUT

MIN

TYP

MAX

MIN

MAX

5

UNITS

∆V

Output Regulation

●

1

2

5

1

2

%

OUT

–4V ≥ V

≥ –5V, 0 ≤ I

≤ 10mA

OUT

I

V

Output Short-Circuit Current

Output Ripple Voltage

V

I

= 0V

= 5mA, V

●

35

5

75

35

5

75

mA

mV

SC

OUT

= –4V

OUT

RIP

–

+

to C2 with C1 and C2 floating. For the LTC1261CS8 in doubler mode,

The

● denotes specifications which apply over the full operating

+

–

C1 connects from C1 to C1 ; there are no C2 pins.

temperature range.

Note 5: Setting output to <–7V will exceed the absolute voltage maximum

rating with a 5V supply. With supplies higher than 5V, the output should

Note 1: The Absolute Maximum Ratings are those values beyond which

the life of a device may be impaired.

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.

never be set to exceed V – 12V.

CC

Note 6: For output voltages below –4.5V the LTC1261 may reach 50%

duty cycle and fall out of regulation with heavy load or low input voltages.

Beyond this point, the output will follow the input with no regulation.

Note 3: All typicals are given at T = 25°C.

A

Note 7: C grade device specifications are guaranteed over the 0°C to 70°C

temperature range. In addition, C grade device specifications are assured

over the –40°C to 85°C temperature range by design or correlation, but

are not production tested.

Note 4: C1 = C2 = 0.1µF means the specifications apply to tripler mode

where V – V

= 3V (LTC1261CS only; the LTC1261CS8 cannot be

CC

OUT

CC

+

–

connected in tripler mode) with C1 connected between C1 and C1 and

C2 connected between C2 and C2 . C2 = 0 implies doubler mode where

+

–

+

V

CC

– V = 2V ; for the LTC1261CS this means C1 connects from C1

OUT CC

3

LTC1261

TYPICAL PERFORMANCE CHARACTERISTICS ^{(See Test Circuits)}

W

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Output Voltage

vs Output Current

Output Voltage (Doubler Mode)

vs Supply Voltage

Output Voltage (Tripler Mode)

vs Supply Voltage

–3.5

–3.6

–3.7

–3.8

–3.9

–4.0

–4.1

–4.2

–4.3

–4.4

–4.5

–3.5

–3.6

–3.7

–3.8

–3.9

–4.0

–4.1

–4.2

–4.3

–4.4

–4.5

–3.5

–3.6

–3.7

–3.8

–3.9

–4.0

–4.1

–4.2

–4.3

–4.4

–4.5

T

= 25°C

A

V

= 5V

CC

DOUBLER MODE

T

= 85°C

T

= 85°C

A

T = 25°C

A

T

= 25°C

V

= 3.3V

A

CC

TRIPLER MODE

T

= –40°C

A

T

= –40°C

A

3

5

6

5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0

SUPPLY VOLTAGE (V)

4

7

0

1

2

3

4

5

6

7

8

9

10

SUPPLY VOLTAGE (V)

OUTPUT CURRENT (mA)

LT1261 • TP02

LT1261 • TP01

LTC1261 • TPC03

Maximum Output Current

vs Supply Voltage

Supply Current

vs Supply Voltage

Supply Current

vs Temperature

1200

1000

900

800

700

600

500

1200

1000

900

50

40

30

20

V

A

= –4V ±5%

OUT

V

A

= –4V

V

= –4V

OUT

= 25°C

OUT

T

= 25°C

T

TRIPLER MODE

V

= 5V

CC

DOUBLER MODE

800

DOUBLER MODE

TRIPLER MODE

V

= 3.3V

700

600

500

CC

TRIPLER MODE

DOUBLER MODE

10

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

SUPPLY VOLTAGE (V)

40

80

100

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0

SUPPLY VOLTAGE (V)

–40 –20

0

20

60

TEMPERATURE (˚C)

LTC1261 • TPC04

LTC1261 • TPC05

LTC1261 • TPC06

TEST CIRCUITS

Tripler Mode

V

IN

= 3.3V

10µF

Doubler Mode

+

14

1

2

3

4

8

7

6

5

V

SHDN

REG

V

5V

CC

2

3

4

5

10

9

+

–

C1

ADJ

+

C1

0.1µF

+

LTC1261-4

10µF

0.1µF

C1

R

ADJ

–

V

= –4V ±5%

C1

OUT

LTC1261CS

8

+

C2

R1

3.3µF

+

GND

COMP

0.1µF

7

–

C2

R0

LTC1261 • TCO1

11

V

= –4V ±5%

OUT

GND

6

3.3µF

+

LTC1261 • TC02

4

LTC1261

U

PIN FUNCTIONS

Pin numbers are shown as (LTC1261CS/LTC1261CS8).

be used if it can provide the required output voltage.

Typically the resistor string should draw≥10µA from the

output to minimize errors due to the bias current at the

adjust pin. Fixed output 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 ripple on both adjustable and fixed

output voltage parts. See Applications Information sec-

tion for more information on compensation and output

ripple.

NC (Pin 1/NA): No Internal Connection.

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

capacitor between C1⁺and C1^–. With the LTC1261CS in

doubler mode, connect a 0.1µF capacitor from C1⁺to

C2^–.

C1^–(Pin 3/Pin 3): C1 Negative Input. Connect a 0.1µF

capacitor from C1⁺to C1^–. With the LTC1261CS in

doubler mode only, C1^–should float.

C2⁺(Pin 4/NA): C2 Positive Input. In tripler mode con-

nect a 0.1µF capacitor from C2⁺to C2^–. This pin is used

with the LTC1261CS in tripler mode only; in doubler

mode this pin should float.

C2^–(Pin 5/NA): C2 Negative Input. In tripler mode

connect a 0.1µF capacitor from C2⁺to C2^–. In doubler

mode connect a 0.1µF capacitor from C1⁺to C2^–.

OUT (Pin 11/Pin 6): Negative Voltage Output. This pin

must be bypassed to ground with a 1µF or larger capaci-

tor; it must be at least 3.3µF to provide specified output

ripple. The size of the output capacitor has a strong effect

on output ripple. See the Applications Information sec-

tion for more details.

REG(Pin12/Pin7):Thisisanopendrainoutputthatpulls

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

It will sink 8mA to ground with a 5V supply. The external

circuitry must provide a pull-up or REG will not swing

high. The voltage at REG may exceed V_CCand can be

pulled up to 12V above ground without damage.

GND (Pin 6/Pin 4): Ground. Connect to a low impedance

ground. A ground plane will help to minimize regulation

errors.

R0 (Pin 7/NA): Internal Resistor String, 1st Tap. See

Table 2 in the Applications Information section for infor-

mation on internal resistor string pin connections vs

output voltage.

SHDN (Pin 13/Pin 8): Shutdown. When this pin is at

ground the LTC1261 operates normally. An internal 5µA

pull-down keeps SHDN low if it is left floating. When

SHDN is pulled high, the LTC1261 enters shutdown

mode. In shutdown the charge pump stops, the output

collapses to 0V and the quiescent current drops to 5µA

typically.

R1 (Pin 8/NA): Internal Resistor String, 2nd Tap.

R_ADJ(Pin 9/NA): Internal Resistor String Output. Con-

nect this pin to ADJ to use the internal resistor divider.

See Table 2 in the Applications Information section for

informationoninternalresistorstringpinconnectionsvs

output voltage.

V_CC(Pin 14/Pin 1): Power Supply. This requires an input

voltage between 3V and 6.5V. Certain combinations of

output voltage and operating mode may place additional

restrictions on the input voltage. V_CCmust be bypassed

to ground with at least a 0.1µF capacitor placed in close

proximity to the chip. See the Applications Information

section for details.

ADJ (COMP for fixed versions) (Pin 10/Pin 5): Output

Adjust/Compensation Pin. For adjustable parts this pin is

used to set the output voltage. The output voltage should

be divided down with a resistor divider and fed back to

this pin to set the regulated output voltage. The resistor

divider can be external or the internal divider string can

5

LTC1261

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APPLICATIONS INFORMATION

MODES OF OPERATION

however, with supply voltages above 4V in tripler mode

andabove6Vindoublermode. Astheinputsupplyvoltage

rises the allowable output voltage drops, finally reaching

–4V with an 8.5V supply. To avoid this problem use

doubler mode whenever possible with high input supply

voltages.

The LTC1261 uses a charge pump to generate a negative

output voltage that can be regulated to a value either

higher or lower than the original input voltage. It has two

modes of operation: a “doubler” inverting mode, which

can provide a negative output equal to or less than the

positive power supply and a “tripler” inverting mode,

whichcanprovidenegativeoutputvoltageseitherlargeror

smallerinmagnitudethantheoriginalpositivesupply.The

tripler offers greater versatility and wider input range but

requires four external capacitors and a 14-pin package.

The doubler offers the SO-8 package and requires only

three external capacitors.

1

2

3

4

5

6

7

14

13

12

11

10

9

1

2

3

4

5

6

7

14

13

12

11

10

9

+

–

+

–

+

C1

C2

C1

1

2

3

4

8

7

6

5

C2 LTC1261

+

–

C1

C2

C1

LTC1261

8

a.) LTC1261CS8

DOUBLER MODE

b.) LTC1261CS

DOUBLER MODE

c.) LTC1261CS

TRIPLER MODE

Doubler Mode

LTC1261 • F01

Figure 1. Flying Capacitor Connections

Doubler mode allows the LTC1261 to generate negative

outputvoltagemagnitudesuptothatofthesupplyvoltage,

creatingavoltagebetweenV_CCandOUTofuptotwotimes

V_S. In doubler mode the LT1261 uses a single flying

capacitor to invert the input supply voltage, and the output

voltage is stored on the output bypass capacitor between

switch cycles. The LTC1261CS8 is always configured in

doubler mode and has only one pair of flying capacitor

pins (Figure 1a). The LTC1261CS can be configured in

doubler mode by connecting a single flying capacitor

between the C1⁺and C2^–pins. C1^–and C2⁺should be left

floating (Figure 1b).

THEORY OF OPERATION

A block diagram of the LTC1261 is shown in Figure 2. The

heart of the LTC1261 is the charge pump core shown in

the dashed box. It generates a negative output voltage by

first charging the flying capacitors between V_CCand

ground. It then stacks the flying capacitors on top of each

other and connects the top of the stack to ground forcing

the bottom of the stack to a negative voltage. The charge

ontheflyingcapacitorsistransferredtotheoutputbypass

capacitor, leaving it charged to the negative output volt-

age. This process is driven by the internal clock.

Tripler Mode

The LTC1261CS can be used in a tripler mode which can

generate negative output voltages up to twice the supply

voltage. The total voltage between the V_CCand OUT pins

can be up to three times V_S. For example, tripler mode can

be used to generate –5V from a single positive 3.3V

supply. Tripler mode requires two external flying capaci-

tors. The first connects between C1⁺and C1^–and the

second between C2⁺and C2^–(Figure 1c). Because of the

relativelyhighvoltagesthatcanbegeneratedinthismode,

care must be taken to ensure that the total input-to-output

voltage never exceeds 12V or the LTC1261 may be dam-

aged. In most applications the output voltage will be kept

in check by the regulation loop. Damage is possible

Figure 2 shows the charge pump configured in tripler

mode. With the clock low, C1 and C2 are charged to V_CC

byS1, S3, S5andS7. Atthenextrisingclockedge, S1, S3,

S5 and S7 open and S2, S4 and S6 close, stacking C1 and

C2 on top of each other. S2 connects C1⁺to ground, S4

connects C1^–to C2⁺and C2^–is connected to the output

by S6. The charge in C1 and C2 is transferred to C_OUT

,

setting it to a negative voltage. Doubler mode works the

same way except that the single flying capacitor (C1) is

connected between C1⁺and C2^–. S3, S4 and S5 don’t do

anything useful in doubler mode. C1 is charged initially by

S1 and S7 and connected to the output by S2 and S6.

6

LTC1261

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APPLICATIONS INFORMATION

V

CC

CLK

550kHz

S1

S5

OUT

+

C

OUT

+

124k

C1

C2

S

R *

ADJ

Q

C1

C2

S4

S6

–

226k

100k

50k

R

S2

C1

C2

R1*

R0*

INTERNALLY

CONNECTED FOR

FIXED OUTPUT

VOLTAGE PARTS

S3

S7

ADJ/COMP

+

COMP 1

REG

+

–

COMP 2

60mV

1.18V

*LTC1261CS14 ONLY

V

= 1.24V

REF

LTC1261 • F02

V

OUT

Figure 2. Block Diagram

The output voltage is monitored by COMP1 which com-

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

fixed output parts) to the internal reference. At the begin-

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

AND gate low and charging the flying capacitors. The next

rising clock edge sets the RS latch, setting the charge

pump to transfer charge from the flying capacitors 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 1.24V reference relative to OUT. This

resets the RS latch and truncates the clock pulses, reduc-

ing the amount of charge transferred to the output capaci-

torandregulatingtheoutputvoltage. Iftheoutputexceeds

the set point, COMP1 stays high, inhibiting the RS latch

and disabling the charge pump.

COMP2 also monitors the divided signal at ADJ but it is

connected to a 1.18V reference, 5% below the main

reference 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

capable of sinking 5mA with a 3.3V V_CCand 8mA with a 5V

V_CC. When in the “off” state (divided output more than 5%

below V_REF) the drain can be pulled above V_CCwithout

damage up to a maximum of 12V above ground. Note that

the REG output only indicates if the magnitude of the

output is below the magnitude of the set point by 5% (i.e.,

V_OUT> –4.75Vfora –5Vsetpoint). Ifthemagnitudeofthe

outputisforced higher thanthemagnitudeofthesetpoint

( i.e., to –6V when the output is set for –5V) the REG

output will stay low.

7

LTC1261

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APPLICATIONS INFORMATION

OUTPUT RIPPLE

To prevent this from happening, an external capacitor can

be connected from ADJ (or COMP for fixed output parts)

to ground to compensate for external parasitics and in-

crease the regulation loop bandwidth (Figure 3). This

sounds counterintuitive until we remember that the inter-

nalreferenceisgeneratedwithrespecttoOUT,notground.

Output ripple in the LTC1261 comes from two sources;

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

550kHz, the charge on the output capacitor is refreshed

once every 1.8µs. With a 15mA load and a 3.3µF output

capacitor, the output will droop by:

TO CHARGE

PUMP

RESISTORS ARE

INTERNAL FOR

FIXED OUTPUT PARTS

1.8µs

3.3µF

∆t

OUT

I

×

= 15mA ×

= 8.2mV

LOAD

)

C

)

COMP 1

This can be a significant ripple component when the

output is heavily loaded, especially if the output capacitor

is small. If absolute minimum output ripple is required, a

10µF or greater output capacitor should be used.

C

R1

R2

100pF

ADJ/COMP

+

–

REF

1.24V

V

OUT

Regulation loop frequency response is the other major

contributor to output ripple. The LTC1261 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

fixed output versions) through an internal or external

resistor divider from the OUT pin to ground. As the flying

capacitors are 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. Be-

cause 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.

LTC1261 • F03

Figure 3. Regulator Loop Compensation

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 between ADJ (or COMP) and V_OUT. This will

slow down the feedback loop and increase output ripple.

A 100pF capacitor from ADJ or COMP to ground will

compensate the loop properly under most conditions.

OUTPUT FILTERING

If extremely low output ripple (<5mV) is required, addi-

tional output filtering is required. Because the LTC1261

uses a high 550kHz 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

3.3µFcapacitorwillcutoutputrippletobelow3mV(Figure

4). Further reductions can be obtained with larger filter

capacitors or by using an LC output filter.

8

LTC1261

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APPLICATIONS INFORMATION

output with each clock cycle. The smaller capacitors

draw smaller pulses of current out of V_CCas well, limiting

peak currents and reducing the demands on the input

supply. Table 1 shows recommended values of flying

capacitor vs maximum load capacity.

5V

1µF

V

CC

10Ω

2

3

6

5

+

V

OUT

= –4V

C1

OUT

0.1µF

LTC1261CS8-4

–

3.3µF

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

T_A= 25°C, V_OUT= –4V

+

C1

COMP

GND

100pF

FLYING

CAPACITOR

4

MAX LOAD (mA)

LTC1261 • F04

VALUE (µF)

V

CC

= 5V DOUBLER MODE V = 3.3V TRIPLER MODE

CC

0.1

22

16

8

20

15

11

5

Figure 4. Output Filter Cuts Ripple Below 3mV

0.047

0.033

0.022

0.01

CAPACITOR SELECTION

Capacitor Sizing

4

1

3

The performance of the LTC1261 can be affected by the

capacitors it is connected to. The LTC1261 requires by-

pass capacitors to ground for both the V_CCand OUT pins.

The input capacitor provides most of LTC1261’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 five times larger

than the flying capacitor. Ceramic capacitors generally

provide adequate performance but avoid using a tantalum

capacitor as the input bypass unless there is at least a

0.1µF ceramic capacitor in parallel with it. The charge

pump capacitors are somewhat less critical since their

peak currents are limited by the switches inside the

LTC1261. Most applications should use 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 and the

flying capacitors.

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

outputbypasscapacitorcanbeassmallas1µF.Toachieve

specified output ripple with 0.1µF flying capacitors, the

output capacitor should be at least 3.3µF. Larger output

capacitors will reduce output ripple further at the expense

of turn-on time.

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

outputartificiallylowbyprematurelyterminatingthecharg-

ing 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 com-

parator and terminate the charging cycle. The pulse dissi-

pates quickly and the comparator returns to the correct

state, but the RS latch will not allow the charge pump to

responduntilthenextclockedge.Thispreventsthecharge

In applications where the maximum load current is well-

definedandoutputrippleiscriticalorinputpeakcurrents

need to be minimized, the flying capacitor values can be

tailored to the application. Reducing the value of the

flying capacitors reduces the amount of charge trans-

ferredwitheachclockcycle. Thislimitsmaximumoutput

current, but also cuts the size of the voltage step at the

9

LTC1261

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APPLICATIONS INFORMATION

Most of this resistance is already provided by the internal

switchesintheLTC1261(especiallyintriplermode).More

than 1Ω or 2Ω of ESR on the flying capacitors will start to

affect the regulation at maximum load.

pump from going into very high frequency 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 5). The

resulting output voltage behaves as if a resistor of value

C

_ESR×(I_PK/I_AVE)Ωwasplacedinserieswiththeoutput.To

RESISTOR SELECTION

avoid this nasty sequence of events connect a 0.1µF

ceramic capacitor in parallel with the larger output capaci-

tor. The ceramic capacitor will “eat” the high frequency

spike, preventing it from fooling the feedback loop, while

thelargerbutslowertantalumoraluminumoutputcapaci-

tor supplies output current to the load between charge

cycles.

Resistor selection is easy with the fixed output versions of

the LTC1261—no resistors are needed! Selecting 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 1.24V at the ADJ pin with

respect to V_OUT(Figure 6). The LTC1261 uses a positive

reference with respect to V_OUT, not a negative reference

with respect to ground (Figure 2 shows the reference

connection). Besuretokeepthisinmindwhenconnecting

the resistors! If the initial output is not what you expected,

try swapping the two resistors.

CLOCK

V

SET

OUT

AVERAGE

LOW ESR

V

OUT

OUTPUT CAP

COMP1

OUTPUT

6 (4*)

GND

V

SET

R1

R2

LTC1261

ADJ

10 (5*)

OUT

AVERAGE

HIGH ESR

V

OUTPUT CAP

11 (6*)

R1 + R2

R2

V

= –1.24V

OUT

(

)

COMP1

OUTPUT

LTC1261 • F05

*LTC1261CS8

LTC1261 • F06

Figure 6. External Resistor Connections

Figure 5. Output Ripple with Low and High ESR Capacitors

Note that ESR in the flying capacitors will not cause the

same condition; in fact, it may actually improve the situa-

tion by cutting the peak current and lowering the ampli-

tude of the spike. However, more flying capacitor ESR is

not necessarily better. As soon as the RC time constant

approaches half of a clock period (the time the capacitors

have to share charge at full duty cycle) the output current

capability of the LTC1261 will begin to diminish. For 0.1µF

flying capacitors, this gives a maximum total series resis-

tance of:

The 14-pin adjustable parts include a built-in resistor

stringwhichcanprovideanassortmentofoutputvoltages

by using different pin-strapping options at the R0, R1, and

R_ADJpins (Table 2). The internal resistors are roughly

124k, 226k, 100k, and 50k (see Figure 2) giving output

options of –3.5V, –4V, –4.5V, and –5V. The resistors are

carefully matched to provide accurate divider ratios, but

the absolute values can vary substantially from part to

part. It is not a good idea to create a divider using an

external resistor and one of the internal resistors unless

the output voltage accuracy is not critical.

t

1

2

1

2

1

CLK

=

/ 0.1µF = 9.1Ω

)

C

)

550kHz

FLY

10

LTC1261

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APPLICATIONS INFORMATION

Table 2. Output Voltages Using the Internal Resistor Divider

able. The output voltage can be trimmed, if desired, by

connecting external resistance from the COMP pin to OUT

or ground to alter the divider ratio. As in the adjustable

parts, the absolute value of the internal resistors may vary

significantly from unit to unit. As a result, the further the

trim shifts the output voltage the less accurate the output

voltage will be. If a precise output voltage other than one

of the available fixed voltages is required, it is better to use

an adjustable LTC1261 and use precision external resis-

tors. The internal reference is trimmed at the factory to

within 3.5% of 1.24V; with 1% external resistors the

output will be within 5.5% of the nominal value, even

under worst case conditions.

PIN CONNECTIONS

ADJ to R

OUTPUT VOLTAGE

–5V

ADJ

ADJ to R , R0 to GND

–4.5V

ADJ

ADJ to R , R1 to R0

–4V

ADJ

ADJ to R , R1 to GND

–3.5V

ADJ

ADJ to R1

ADJ to R0

ADJ to GND

–1.77V

–1.38V

–1.24V

There are some oddball output voltages available by

connecting ADJ to R0 or R1 and shorting out some of the

internal resistors. If one of these combinations gives you

the output voltage you want, by all means use it!

The LTC1261 can be internally configured with nonstand-

ard fixed output voltages. Contact the Linear Technology

Marketing Department for details.

The internal resistor values are the same for the fixed

output versions of the LTC1261 as they are for the adjust-

U

TYPICAL APPLICATIONS N

3.3V Input, –4.5V Output GaAs FET Bias Generator

P-CHANNEL

POWER SWITCH

V

BAT

3.3V

1µF

SHUTDOWN

10k

14

V

CC

SHDN

2

3

4

5

8

13

12

11

10

9

+

C1

C2

R1

0.1µF

–

REG

LTC1261

–4.5V BIAS

+

OUT

GaAs

TRANSMITTER

3.3µF

+

–

ADJ

NC

R

ADJ

LTC1261 • TA03

R0

GND

100pF

7

6

11

LTC1261

TYPICAL APPLICATIONS N

U

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

LTC1261-4

1µF

0.1µF

3

4

–4V BIAS

–

C2

OUT

GaAs

TRANSMITTER

3.3µF

GND

COMP

+

100pF

LTC1261 • TA04

7 Cells to –1.24V Output GaAs FET Bias Generator

P-CHANNEL

POWER SWITCH

V

= 8.4V

BAT

(7 NiCd CELLS)

SHUTDOWN

10k

1

2

3

4

8

7

6

5

V

SHDN

REG

CC

+

C1

C2

LTC1261

1µF

0.1µF

–1.24V BIAS

–

OUT

ADJ

GaAs

TRANSMITTER

3.3µF

GND

+

LTC1261 • TA05

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

LTC1261-4

1µF

0.1µF

100µH

3

4

–4V BIAS

–

C2

OUT

GaAs

TRANSMITTER

10µF

GND

COMP

+

100pF

LTC1261 • TA06

12

LTC1261

U

TYPICAL APPLICATIONS N

High Supply Voltage, –5V Output GaAs FET Bias Generator

P-CHANNEL

POWER SWITCH

8V ≤ V

≤ 12V

BAT

1N4733A

5.1V

1µF

10k

SHUTDOWN

14

V

CC

SHDN

2

3

4

5

8

13

+

C1

C2

R1

0.1µF

12

11

10

9

–

REG

LTC1261

–5V BIAS

+

OUT

GaAs

TRANSMITTER

0.1µF

3.3µF

–

+

ADJ

NC

R

ADJ

LTC1261 • TA07

R0

GND

100pF

7

6

Low Output Voltage Generator

–5V Supply Generator

3V ≤ V ≤ 7V

5V

CC

1µF

1

14

100pF

R

S

V

CC

V

5

6

CC

2

3

4

5

10

9

100pF

ADJ

+

–

C1

C2

ADJ

2

3

+

C1

0.1µF

LTC1261

124k

R

ADJ

–

C1

LTC1261

8

+

R1

R0

NC

V

OUT

= V – 10µA (R + 124k)

CC S

OUT

= –0.5V (R = 426k)

GND

4

S

7

–

3.3µF

1N5817

+

= –1V (R = 476k)

S

11

–5V ±5%

AT 10mA

LTC1261 • TA10

OUT

GND

6

3.3µF

+

LTC1261 • TA09

Minimum Parts Count –4V Generator

1

2

3

4

8

7

6

5

5V

V

SHDN

REG

CC

+

C1

LTC1261-4

1µF

0.1µF

V

= –4V

–

OUT

C1

OUT

at 10mA

3.3µF

GND

COMP

+

LTC1261 • TA12

13

LTC1261

U

TYPICAL APPLICATIONS N

This circuit uses the LTC1261CS8 to generate a –1.24V

output at 20mA. Attached to this output is a 312Ω resistor

to make the current/voltage conversion. 4mA through

312Ω generates 1.24V, giving a net 0V output. 20mA

through312Ωgives6.24Vacrosstheresistor, givinganet

5V output. If the 4mA to 20mA source requires an operat-

ing voltage greater than 8V, it should be powered from a

separate supply; the LTC1261 can then be powered from

any convenient supply, 3V ≤ V_S≤ 8V. The Schottky diode

preventstheexternalvoltagefromdamagingtheLTC1261

in shutdown or under fault conditions. The LTC1261’s

reference is trimmed to 3.5% and the resistor adds 1%

uncertainty, giving 4.5% total output error.

–1.24V Generator for 4mA-20mA to 0V-5V Conversion

OPTIONAL

INPUT

PROTECTION

DIODES

8V

+

–

4mA

TO 20mA

SENSOR

1µF

0V TO 5V

±5%

1

312Ω

1%

V

CC

6

2

3

+

–

–1.24V

OUT

C1

3.3µF

0.1µF

LTC1261

+

5

1N5817

ADJ

C1

GND

4

LTC1261 • TA11

14

LTC1261

U

Dimensions in inches (millimeters) unless otherwise noted.

S8 Package

PACKAGE DESCRIPTION

8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

0.189 – 0.197*

(4.801 – 5.004)

7

5

8

6

0.150 – 0.157**

(3.810 – 3.988)

0.228 – 0.244

(5.791 – 6.197)

1

0.053 – 0.069

3

4

2

0.010 – 0.020

(0.254 – 0.508)

× 45°

(1.346 – 1.752)

0.004 – 0.010

(0.101 – 0.254)

0.008 – 0.010

(0.203 – 0.254)

0°– 8° TYP

0.016 – 0.050

0.406 – 1.270

0.050

(1.270)

TYP

0.014 – 0.019

(0.355 – 0.483)

*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

SO8 0996

S Package

14-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

0.337 – 0.344*

(8.560 – 8.738)

13

12

11

10

9

8

14

0.228 – 0.244

(5.791 – 6.197)

0.150 – 0.157**

(3.810 – 3.988)

1

2

3

4

5

6

7

0.010 – 0.020

(0.254 – 0.508)

× 45°

0.053 – 0.069

(1.346 – 1.752)

0.004 – 0.010

(0.101 – 0.254)

0.008 – 0.010

(0.203 – 0.254)

0° – 8° TYP

0.050

(1.270)

TYP

0.014 – 0.019

(0.355 – 0.483)

0.016 – 0.050

0.406 – 1.270

S14 0695

*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

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 represen-

tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.

15

LTC1261

TYPICAL APPLICATION

U

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

P-CHANNEL

POWER SWITCH

V

BAT

SHUTDOWN

5V

42.2k

10k

1

2

3

4

8

7

6

5

V

SHDN

REG

CC

+

C1

C2

–0.5V BIAS

LTC1261

1µF

0.1µF

±5.5%

–

OUT

ADJ

GaAs

12.4k

TRANSMITTER

GND

3.3µF

+

100pF

LTC1261 • TA08

RELATED PARTS

PART NUMBER

LTC1550/LTC1551

LTC1429

DESCRIPTION

Low Noise Switched Capacitor Regulated Voltage Inverter

Clock Synchronized Switched Capacitor Regulated Voltage Inverter GaAs FET Bias

Micropower Low Dropout Regulators with Shutdown 0.4V Dropout Voltage at 150mA, Low Noise,

Switched Capacitor Regulated Voltage Inverter

COMMENTS

GaAs FET Bias with Linear Regulator 1mV Ripple

LT1121

1261fa LT/TP 0198 REV A 4K • PRINTED IN USA

Linear Technology Corporation

●

1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900

16

●

FAX: (408) 434-0507 TELEX: 499-3977 www.linear-tech.com

LINEAR TECHNOLOGY CORPORATION 1994


型号：	LTC1261CS
厂家：	Linear
描述：	Switched Capacitor Regulated Voltage Inverter 开关电容稳压电压型逆变器开关
文件：	总16页 (文件大小：313K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LTC1261CS [Linear]

相关型号：

LTC1261CS#PBF

LTC1261CS8

LTC1261CS8#TR

LTC1261CS8#TRPBF

LTC1261CS8-4

LTC1261CS8-4#TRPBF

LTC1261CS8-4.5

LTC1261CS8-4.5#PBF

LTC1261CS8-4.5#TRPBF

LTC1261CS84.5

LTC1261IS8

LTC1261IS8#TR