Linear Technology LT1611 Datasheet

FEATURES

■

Very Low Noise: 1mV

■

–5V at 150mA from a 5V Input

■

Better Regulation Than a Charge Pump

■

Effective Output Impedance: 0.14Ω

■

Uses Tiny Capacitors and Inductors

■

Internally Compensated

■

Fixed Frequency 1.4MHz Operation

■

Low Shutdown Current: <1µA

■

Low V

■

Tiny 5-Lead SOT-23 Package

Switch: 300mV at 300mA

CESAT

Output Ripple

P–P

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APPLICATIO S

■

MR Head Bias

■

Digital Camera CCD Bias

■

LCD Bias

■

GaAs FET Bias

■

Positive-to-Negative Conversion

LT1611

Inverting 1.4MHz Switching

Regulator in SOT-23

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DESCRIPTIO

The LT®1611 is the industry’s first inverting 5-lead SOT-23
current mode DC/DC converter. Intended for use in small,
low power applications, it operates from an input voltage
as low as 1.1V and switches at 1.4MHz, allowing the use
of tiny, low cost capacitors and inductors 2mm or less in
height. Its small size and high switching frequency enable
the complete DC/DC converter function to consume less
than 0.25 square inches of PC board area. Capable of
generating –5V at 150mA from a 5V supply or –5V at
100mA from a 3V supply, the LT1611 replaces nonregulated
“charge pump” solutions in many applications.

The LT1611 operates in a dual inductor inverting topology
which filters the input side as well as the output side of the
DC/DC converter. Fixed frequency switching ensures a
clean output free from low frequency noise typically present
with charge pump solutions. No load quiescent current of
the LT1611 is 3mA, while in shutdown quiescent current
drops to 0.5µA. The 36V switch allows VIN to V
differential of up to 33V.

OUT

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TYPICAL APPLICATIO

R1

29.4k

R2
10k

C2

1µF

L1B

22µH

D1

1200pF

C3
22µF

L1A

V

IN

SHDN

22µH

SW

LT1611

NFB

GND

V

IN

5V

+

C1
22µF

C1: AVX TAJB226M010
C2: TAIYO YUDEN LMK212BJ105MG
C3: TAIYO YUDEN JMK325BJ226MM (1210 SIZE)
D1: MBR0520
L1: SUMIDA CLS62-220 OR 2× MURATA LQH3C220 (UNCOUPLED)

Figure 1. 5V to –5V, 150mA Low Noise Inverting DC/DC Converter

The LT1611 is available in the 5-lead SOT-23 package.

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

Transient Response

V

OUT

–5V
150mA

LOAD CURRENT

1611 TA01

V

OUT

20mV/DIV

AC COUPLED

150mA

50mA

100µs/DIV

1611 F10

1

LT1611

WW

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ABSOLUTE MAXIMUM RATINGS

(Note 1)

VIN Voltage .............................................................. 10V

SW Voltage ................................................–0.4V to 36V

NFB Voltage .............................................................–3V

Current into NFB Pin ............................................. ±1mA

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PACKAGE/ORDER INFORMATION

ORDER PART

SW 1

GND 2

NFB 3

TOP VIEW

5 V

IN

4 SHDN

NUMBER

LT1611CS5

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SHDN Voltage .......................................................... 10V

Maximum Junction Temperature .......................... 125°C

S5 PACKAGE

5-LEAD PLASTIC SOT-23

S5 PART MARKING

Operating Temperature Range

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

T

= 125°C, θJA = 256°C/W

JMAX

LTES

Extended Commercial (Note 2)........... – 40°C to 85°C

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

Consult factory for Industrial and Military grade parts.

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

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 1.5V, V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Operating Voltage 0.9 1.1 V
Maximum Operating Voltage 10 V
NFB Pin Bias Current V
Feedback Voltage ● –1.205 –1.23 –1.255 V
Quiescent Current V
Quiescent Current in Shutdown V

Reference Line Regulation 1.5V ≤ VIN ≤ 10V 0.02 0.2 %/V
Switching Frequency ● 1.0 1.4 1.8 MHz
Maximum Duty Cycle ● 82 86 %
Switch Current Limit (Note 3) 550 800 mA
Switch V

CESAT

Switch Leakage Current VSW = 5V 0.01 1 µA
SHDN Input Voltage High 1V
SHDN Input Voltage Low 0.3 V
SHDN Pin Bias Current V

= –1.23V ● –2.7 –4.7 –6.7 µA

NFB

= 1.5V, Not Switching 3 4.5 mA

SHDN

= 0V, VIN = 2V 0.01 0.5 µA

SHDN

= 0V, VIN = 5V 0.01 1.0 µA

V

SHDN

ISW = 300mA 300 350 mV

= 3V 25 50 µA

SHDN

V

= 0V 0 0.1 µA

SHDN

The ● denotes the specifications which apply over the full operating

= VIN unless otherwise noted.

SHDN

Note 1: Absolute Maximum Ratings are those values beyond which the life

of a device may be impaired.
Note 2: 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.

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Note 3: Current limit guaranteed by design and/or correlation to static test.
Slope compensation reduces current limit at higher duty cycle.

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TYPICAL PERFOR A CE CHARACTERISTICS

Efficiency, V

85

OUT

= –5V

–1.245

vs Temperature

NFB

LT1611

NFB Pin Bias Current vs
TemperatureV

6

80

75

70

65

EFFICIENCY (%)

60

55

50

0

Switch V

700

TA = 25°C

600

500

400

(mV)

300

CESAT

V

200

100

0

0 100 200 300 400 500 600 700

50 75 100 125 150

25

LOAD CURRENT (mA)

CESAT

SWITCH CURRENT (mA)

VIN = 5V

VIN = 3V

vs Switch Current Switch Current Limit vs Duty CycleSHDN Pin Bias Current vs V

Oscillator Frequency vs
Temperature

2.00

1.75

1.50

1.25

1.00

0.75

0.50

SWITCHING FREQUENCY (MHz)

0.25

VIN = 5V

VIN = 1.5V

0

–50 –25 0 25 50 75 100

TEMPERATURE (°C)

1611 G01

1611 G04

1611 G07

–1.240

–1.235

–1.230

(V)

NFB

V

–1.225

–1.220

–1.215

–1.210

–50 0 50–25 25 75 100

50

40

30

20

10

SHDN PIN BIAS CURRENT (µA)

0

012345

TEMPERATURE (°C)

SHDN PIN VOLTAGE (V)

No-Load Operating Quiescent
Current vs Temperature*

6.0

5.5

5.0

4.5

4.0

3.5

3.0

OPERATING CURRENT (mA)

2.5

2.0
–50 0 50–25 25 75 100

TEMPERATURE (°C)

1611 G02

SHDN

1611 G05

1611 G08

5

4

3

2

NFB PIN BIAS CURRENT (µA)

1

0

–50 0 50–25 25 75 100

900

TA = 25°C

800

700

600

500

400

300

200

SWITCH CURRENT LIMIT (mA)

100

0

10

TEMPERATURE (°C)

20 30 40 50 60 70 80

DUTY CYCLE (%)

Switch Current Limit vs
Temperature (Duty Cycle = 30%)

900

800

700

600

500

400

300

200

SWITCH CURRENT LIMIT (mA)

100

0

–25 0 25 50 75 100

–50

TEMPERATURE (°C)

1611 G03

1611 G06

1611 G09

* Includes bias current through R1, R2 and Schottky leakage current at T ≥ 75°C

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LT1611

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PIN FUNCTIONS

SW (Pin 1): Switch Pin. Minimize trace area at this pin to
keep EMI down.

GND (Pin 2): Ground. Tie directly to local ground plane.
NFB (Pin 3): Negative Feedback Pin. Minimize trace area.

Reference voltage is –1.23V. Connect resistive divider tap
here. The suggested value for R2 is 10k. Set R1 and R2
according to:

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BLOCK DIAGRAM

V

IN

R6
40k

+

A1
g

m

–

Q2
x10

R3
30k

R4
140k

3

NFB

R

C

C

C

C

(OPTIONAL)

V

5

IN

R5
40k

Q1

V

OUT

PL

R1
(EXTERNAL)

NFB

R2
(EXTERNAL)

V

R

1

=

123

.

R

2

OUT



+



−

45

.•

123

.

10

6

−




SHDN (Pin 4): Shutdown Pin. Tie to 1V or more to enable
device. Ground to shut the device down.

VIN (Pin 5): Input Supply Pin. Must be locally bypassed.

1

SW

Q3

+

0.15Ω

–

2

GND

1611 BD

RAMP

GENERATOR

1.4MHz

OSCILLATOR

SHDN

4

Σ

COMPARATOR

–

A2

+

SHUTDOWN

FF

RQ

S

A = 3

DRIVER

Figure 2

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OPERATIO

The LT1611 combines a current mode, fixed frequency
PWM architecture with a –1.23V reference to directly
regulate negative outputs. Operation can be best under­stood by referring to the block diagram of Figure 2. Q1 and
Q2 form a bandgap reference core whose loop is closed
around the output of the converter. The driven reference
point is the lower end of resistor R4, which normally sits
at a voltage of –1.23V. As the load current changes, the
NFB pin voltage also changes slightly, driving the output
of gm amplifier A1. Switch current is regulated directly on
a cycle-to-cycle basis by A1’s output. The flip-flop is set at
the beginning of each cycle, turning on the switch. When
the summation of a signal representing switch current and
a ramp generator (introduced to avoid subharmonic oscil­lations at duty factors greater than 50%) exceeds the V
signal, comparator A2 changes stage, resetting the flip-

C

flop and turning off the switch. Output voltage decreases
(the magnitude increases) as switch current is increased.
The output, attenuated by external resistor divider R1 and
R2, appears at the NFB pin, closing the overall loop.
Frequency compensation is provided internally by RC and
CC. Transient response can be optimized by the addition of
a phase lead capacitor, CPL, in parallel with R1 in applica­tions where large value or low ESR output capacitors are
used.

As load current is decreased, the switch turns on for a
shorter period each cycle. If the load current is further
decreased, the converter will skip cycles to maintain
output voltage regulation.

The LT1611 can work in either of two topologies. The
simpler topology appends a capacitive level shift to a

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