LINEAR TECHNOLOGY LTC1474 Technical data

LOAD CURRENT (mA)

EFFICIENCY (%)

100

90

80

70

60

50

0.03 3 300

1474/75 TA01

0.3 30

VIN = 5V

VIN = 10V

VIN = 15V

L = 100µH

V

OUT

= 3.3V

R

SENSE

= 0Ω

FEATURES

■

High Efficiency: Over 92% Possible

■

Very Low Standby Current: 10µA Typ

■

Available in Space Saving 8-Lead MSOP Package

■

Internal 1.4Ω Power Switch (VIN = 10V)

■

Wide VIN Range: 3V to 18V Operation

■

Very Low Dropout Operation: 100% Duty Cycle

■

Low-Battery Detector Functional During Shutdown

■

Programmable Current Limit with Optional
Current Sense Resistor (10mA to 400mA Typ)

■

Short-Circuit Protection

■

Few External Components Required

■

Active Low Micropower Shutdown: IQ = 6µA Typ

■

Pushbutton On/Off (LTC1475 Only)

■

3.3V, 5V and Adjustable Output Versions

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APPLICATIONS

■

Cellular Telephones and Wireless Modems

■

4mA to 20mA Current Loop Step-Down Converter

■

Portable Instruments

■

Battery-Operated Digital Devices

■

Battery Chargers

■

Inverting Converters

■

Intrinsic Safety Applications

LTC1474/LTC1475

Low Quiescent Current

High Efficiency Step-Down

Converters

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DESCRIPTION

The LTC®1474/LTC1475 series are high efficiency step­down converters with internal P-channel MOSFET power
switches that draw only 10µA typical DC supply current at
no load while maintaining output voltage. The LTC1474
uses logic-controlled shutdown while the LTC1475 fea­tures pushbutton on/off.

The low supply current coupled with Burst ModeTM opera­tion enables the LTC1474/LTC1475 to maintain high effi­ciency over a wide range of loads. These features, along
with their capability of 100% duty cycle for low dropout
and wide input supply range, make the LTC1474/LTC1475
ideal for moderate current (up to 300mA) battery-powered
applications.

The peak switch current is user-programmable with an
optional sense resistor (defaults to 325mA minimum if not
used) providing a simple means for optimizing the design
for lower current applications. The peak current control
also provides short-circuit protection and excellent start­up behavior. A low-battery detector that remains functional
in shutdown is provided .

The LTC1474/LTC1475 series availability in 8-lead MSOP
and SO packages and need for few additional components
provide for a minimum area solution.

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

Burst Mode is a trademark of Linear Technology Corporation.

TYPICAL APPLICATION

V

IN

4V TO 18V

+

10µF
25V

LOW BATTERY IN

RUN SHDN

L1 = SUMIDA CDRH74-101

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LOW BATTERY OUT

0.1µF

6

SENSE

3

LBI

100k

8

RUN

Figure 1. High Efficiency Step-Down Converter

7

V

IN

LTC1474-3.3

GND

4

V

LBO

SW

1

FB

2

5

L1
100µH

D1
MBR0530

+

V

OUT

3.3V AT 250mA

100µF

6.3V

1474/75 F01

LTC1474 Efficiency

1

LTC1474/LTC1475

1
2
3
4

V

OUT/VFB

LBO

LBI

GND

8
7
6
5

RUN
V

IN

SENSE
SW

TOP VIEW

MS8 PACKAGE

8-LEAD PLASTIC MSOP

WW

W

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ABSOLUTE MAXIMUM RATINGS

Input Supply Voltage (VIN).........................–0.3V to 20V

Switch Current (SW, SENSE).............................. 750mA

Switch Voltage (SW) ............. (VIN – 20V) to (VIN + 0.3V)

VFB (Adjustable Versions) ..........................–0.3V to 12V

V

(Fixed Versions)................................ –0.3V to 20V

OUT

LBI, LBO ....................................................–0.3V to 20V

RUN, SENSE ..................................–0.3V to (VIN + 0.3V)

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PACKAGE/ORDER INFORMATION

TOP VIEW

8

V

1

OUT/VFB

LBO

2

LBI/OFF

3

GND

4

MS8 PACKAGE

8-LEAD PLASTIC MSOP

T

= 125°C, θJA = 150°C/W T

JMAX

= 125°C, θJA = 150°C/W

JMAX

7
6
5

ON
V

IN

SENSE
SW

Operating Ambient Temperature Range

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

Industrial ............................................ –40°C to 85°C

Junction Temperature (Note 1)............................ 125°C

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

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

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TOP VIEW

V

1

OUT/VFB

LBO

2

3

LBI

4

GND

S8 PACKAGE

8-LEAD PLASTIC SO

= 125°C, θJA = 110°C/W T

JMAX

8

7

6

5

RUN

V

IN

SENSE

SW

V

OUT/VFB

LBI/OFF

LBO

GND

TOP VIEW

1

2

3

4

S8 PACKAGE

8-LEAD PLASTIC SO

= 125°C, θJA = 110°C/WT

JMAX

8

7

6

5

ON

V

IN

SENSE

SW

ORDER PART NUMBER ORDER PART NUMBER ORDER PART NUMBERORDER PART NUMBER

LTC1474CMS8
LTC1474CMS8-3.3
LTC1474CMS8-5

MS8 PART MARKING

LTBW
LTCR
LTCS

Consult factory for Military grade parts.

LTC1475CMS8
LTC1475CMS8-3.3
LTC1475CMS8-5

MS8 PART MARKING S8 PART MARKING

LTBK
LTCP
LTCQ

LTC1474CS8
LTC1474IS8
LTC1474CS8-3.3
LTC1474CS8-5
LTC1474IS8-3.3
LTC1474IS8-5

1474
1474I
147433
14745
474I33
1474I5

LTC1475CS8
LTC1475IS8
LTC1475CS8-3.3
LTC1475CS8-5

S8 PART MARKING

1475
1475I
147533
14755

2

LTC1474/LTC1475

ELECTRICAL CHARACTERISTICS

TA = 25°C, VIN = 10V, V

= open, R

RUN

= 0, unless otherwise noted.

SENSE

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

FB

Feedback Voltage I

= 50mA ● 1.205 1.230 1.255 V

LOAD

LTC1474/LTC1475

V

OUT

I

FB

Regulated Output Voltage I

LTC1474-3.3/LTC1475-3.3
LTC1474-5/LTC1475-5

LOAD

= 50mA

● 3.234 3.300 3.366 V

● 4.900 5.000 5.100 V

Feedback Current ● 030 nA

LTC1474/LTC1475 Only

I

SUPPLY

∆V

OUT

I

Q

R

ON

I

PEAK

V

SENSE

V

HYST

t

OFF

V

LBI, TRIP

V

RUN

V

LBI, OFF

I

LBO, SINK

I

RUN, SOURCE

I

SW, LEAK

I

LBI, LEAK

I

LBO, LEAK

The ● denotes specifications which apply over the full operating
temperature range.

Note 1: T
dissipation P

LTC1474CS8/LTC1475CS8: TJ = TA + (PD • 110°C/W)
LTC1474CMS8/LTC1475CMS8: T

No Load Supply Current (Note 3) I
Output Voltage Line Regulation VIN = 7V to 12V, I
Output Voltage Load Regulation I
Output Ripple I

= 0 (Figure 1 Circuit) 10 µA

LOAD

= 50mA 5 20 mV

LOAD

= 0mA to 50mA 2 15 mV

LOAD

= 10mA 50 mV

LOAD

P-P

Input DC Supply Current (Note 2) (Exclusive of Driver Gate Charge Current)

Active Mode (Switch On) V
Sleep Mode (Note 3) V
Shutdown V

= 3V to 18V 100 175 µA

IN

= 3V to 18V 9 15 µA

IN

= 3V to 18V, V

IN

= 0V 6 12 µA

RUN

Switch Resistance ISW = 100mA 1.4 1.6 Ω
Current Comp Max Current Trip Threshold R

= 0Ω 325 400 mA

SENSE

= 1.1Ω 70 76 85 mA

R

SENSE

Current Comp Sense Voltage Trip Threshold ● 90 100 110 mV
Voltage Comparator Hysteresis 5mV
Switch Off-Time V

at Regulated Value 3.5 4.75 6.0 µs

OUT

V

= 0V 65 µs

OUT

Low Battery Comparator Threshold ● 1.16 1.23 1.27 V
Run/ON Pin Threshold 0.4 0.7 1.0 V
OFF Pin Threshold (LTC1475 Only) 0.4 0.7 1.0 V
Sink Current into Pin 2 V
Source Current from Pin 8 V
Switch Leakage Current VIN = 18V, VSW = 0V, V
Leakage Current into Pin 3 V
Leakage Current into Pin 2 V

= 0V, V

LBI

= 0V 0.4 0.8 1.2 µA

RUN

= 18V, VIN = 18V 0 0.1 µA

LBI

= 2V, V

LBI

= 0.4V 0.45 0.70 mA

LBO

= 0V 0.015 1 µA

RUN

= 5V 0 0.5 µA

LBO

Note 2: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.

is calculated from the ambient temperature TA and power

J

according to the following formulas:

D

Note 3: No load supply current consists of sleep mode DC current (9µA
typical) plus a small switching component (about 1µA for Figure 1 circuit)
necessary to overcome Schottky diode and feedback resistor leakage.

= TA + (PD • 150°C/W)

J

3

LTC1474/LTC1475

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TYPICAL PERFORMANCE CHARACTERISTICS

Efficiency vs Input Voltage Line Regulation

100

95

90

85

EFFICIENCY (%)

80

75

70

0

I

LOAD

I

LOAD

4

8

INPUT VOLTAGE (V)

= 25mA

Current Trip Threshold vs
Temperature

500

VIN = 10V

400

300

200

100

CURRENT TRIP THRESHOLD (mA)

FIGURE 1 CIRCUIT
L: CDRH73-101

I

= 200mA

LOAD

= 1mA

12

1474/75 G01

R

= 0Ω

SENSE

R

= 1.1Ω

SENSE

16

40

FIGURE 1 CIRCUIT
I

LOAD

30

20

(mV)

10

OUT

∆V

0

–10

–20

0

Switch Resistance vs
Input Voltage

5

4

3

(Ω)

DS(ON)

2

R

1

= 100mA

4

8

INPUT VOLTAGE (V)

R

SENSE

T = 70°C

T = 25°C

R

12

= 0Ω

SENSE

= 0.33Ω

16

1474/75 G02

Load Regulation

40

FIGURE 1 CIRCUIT

30

20

10

(mV)

OUT

0

∆V

–10

–20

–30

0

50

150 200 300

100

LOAD CURRENT (mA)

Supply Current in Shutdown

10.0

7.5

5.0

SUPPLY CURRENT (µA)

2.5

VIN = 15V

VIN = 10V

VIN = 5V

250

1474/75 G03

0

0

20

TEMPERATURE (°C)

Switch Leakage Current vs
Temperature

1.0
VIN = 18V

0.8

0.6

0.4

LEAKAGE CURRENT (µA)

0.2

0

20

0

40 60 80 100

TEMPERATURE (°C)

40

60

80

1474/75 G04

0

0

5

10

INPUT VOLTAGE (V)

15

20

1474/75 G05

VIN DC Supply Current

120

ACTIVE MODE

SLEEP MODE

4

8

INPUT VOLTAGE (V)

16 20

12

1474/75 G08

1474/75 G07

100

80

60

40

SUPPLY CURRENT (µA)

20

0

0

0

0

5

10

INPUT VOLTAGE (V)

Off-Time vs Output Voltage

80

60

40

OFF-TIME (µs)

20

0

0

% OF REGULATED OUTPUT VOLTAGE (%)

40

20

15

VIN = 10V

60 80

20

1474/75 G06

100

1474/75 G09

4

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PIN FUNCTIONS

LTC1474/LTC1475

V

OUT/VFB

(Pin 1): Feedback of Output Voltage. In the fixed

versions, an internal resistive divider divides the output
voltage down for comparison to the internal 1.23V refer­ence. In the adjustable versions, this divider must be
implemented externally.

LBO (Pin 2): Open Drain Output of the Low Battery
Comparator. This pin will sink current when Pin 3 is below

1.23V.
LBI/OFF (Pin 3): Input to Low Battery Comparator. This

input is compared to the internal 1.23V reference. For the
LTC1475, a momentary ground on this pin puts regulator
in shutdown mode.

GND (Pin 4): Ground Pin.

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FUNCTIONAL DIAGRA

LBI/OFF

100mV

–

+

–

V

+

1.23V

1.23V

REFERENCE

LTC1474: RUN

LTC1475: ON

LBO

1µA

×

8

1-SHOT

TRIGGER OUT

STRETCH

2

CONNECTION NOT PRESENT IN LTC1474 SERIES

×

CONNECTION PRESENT IN LTC1474 SERIES ONLY

WAKEUP

LB

4.75µs

+

–

3

LTC1474: LBI
LTC1475: LBI/OFF

READY

ON

ON

C

SW (Pin 5): Drain of Internal PMOS Power Switch. Cath­ode of Schottky diode must be closely connected to this
pin.

SENSE (Pin 6): Current Sense Input for Monitoring Switch
Current and Source of Internal PMOS Power Switch.
Maximum switch current is programmed with a resistor
between SENSE and VIN pins.

V

(Pin 7): Main Supply Pin.

IN

RUN/ON (Pin 8): On LTC1474, voltage level on this pin

controls shutdown/run mode (ground = shutdown, open/
high = run). On LTC1475, a momentary ground on this pin
puts regulator in run mode. A 100k series resistor must be
used between Pin 8 and the switch or control voltage.

V

IN

7

R

SENSE

(OPTIONAL)

V

CC

6

1M

20×

3M

SENSE

SW

5

V

OUT/VFB

1

OUTPUT DIVIDER IS
IMPLEMENTED EXTERNALLY IN
ADJUSTABLE VERSIONS

5Ω

1×

(5V VERSION)

1.68M

(3.3V VERSION)

GND

4

V

IN

+

V

OUT

+

1474/75 FD

5

LTC1474/LTC1475

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OPERATIO

(Refer to Functional Diagram)

The LTC1474/LTC1475 are step-down converters with
internal power switches that use Burst Mode operation to
keep the output capacitor charged to the proper output
voltage while minimizing the quiescent current. Burst
Mode operation functions by using short “burst” cycles to
ramp the inductor current through the internal power
switch and external Schottky diode, followed by a sleep
cycle where the power switch is off and the load current is
supplied by the output capacitor. During sleep mode, the
LTC1474/LTC1475 draw only 9µA typical supply current.
At light loads, the burst cycles are a small percentage of
the total cycle time; thus the average supply current is very
low, greatly enhancing efficiency.

Burst Mode Operation

At the beginning of the burst cycle, the switch is turned on
and the inductor current ramps up. At this time, the internal
current comparator is also turned on to monitor the switch
current by measuring the voltage across the internal and
optional external current sense resistors. When this volt­age reaches 100mV, the current comparator trips and
pulses the 1-shot timer to start a 4.75µs off-time during
which the switch is turned off and the inductor current
ramps down. At the end of the off-time, if the output
voltage is less than the voltage comparator threshold, the
switch is turned back on and another cycle commences. To
minimize supply current, the current comparator is turned
on only during the switch-on period when it is needed to
monitor switch current. Likewise, the 1-shot timer will only
be on during the 4.75µs off-time period.

The average inductor current during a burst cycle will
normally be greater than the load current, and thus the
output voltage will slowly increase until the internal volt­age comparator trips. At this time, the LTC1474/LTC1475
go into sleep mode, during which the power switch is off
and only the minimum required circuitry is left on: the
voltage comparator, reference and low battery compara­tor. During sleep mode, with the output capacitor supply­ing the load current, the VFB voltage will slowly decrease
until it reaches the lower threshold of the voltage com­parator (about 5mV below the upper threshold). The
voltage comparator then trips again, signaling the LTC1474/
LTC1475 to turn on the circuitry necessary to begin a new
burst cycle.

Peak Inductor Current Programming

The current comparator provides a means for program­ming the maximum inductor/switch current for each switch
cycle. The 1X sense MOSFET, a portion of the main power
MOSFET, is used to divert a sample (about 5%) of the
switch current through the internal 5Ω sense resistor. The
current comparator monitors the voltage drop across the
series combination of the internal and external sense
resistors and trips when the voltage exceeds 100mV. If the
external sense resistor is not used (Pins 6 and 7 shorted),
the current threshold defaults to the 400mA maximum due
to the internal sense resistor.

Off-Time

The off-time duration is 4.75µs when the feedback voltage
is close to the reference; however, as the feedback voltage
drops, the off-time lengthens and reaches a maximum
value of about 65µs when this voltage is zero. This ensures
that the inductor current has enough time to decay when
the reverse voltage across the inductor is low such as
during short circuit.

Shutdown Mode

Both LTC1474 and LTC1475 provide a shutdown mode
that turns off the power switch and all circuitry except for
the low battery comparator and 1.23V reference, further
reducing DC supply current to 6µA typical. The LTC1474’s
run/shutdown mode is controlled by a voltage level at the
RUN pin (ground = shutdown, open/high = run). The
LTC1475’s run/shutdown mode, on the other hand, is
controlled by an internal S/R flip-flop to provide pushbutton
on/off control. The flip-flop is set (run mode) by a momen­tary ground at the ON pin and reset (shutdown mode) by
a momentary ground at the LBI/OFF pin.

Low Battery Comparator

The low battery comparator compares the voltage on the
LBI pin to the internal reference and has an open drain
N-channel MOSFET at its output. If LBI is above the
reference, the output FET is off and the LBO output is high
impedance. If LBI is below the reference, the output FET is
on and sinks current. The comparator is still active in
shutdown.

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