LINEAR TECHNOLOGY LTC1872 Technical data

FEATURES

■

High Efficiency: Over 90%

■

High Output Currents Easily Achieved

■

Wide VIN Range: 2.5V to 9.8V

■

V

Limited Only by External Components

OUT

■

Constant Frequency 550kHz Operation

■

Burst Mode

■

Current Mode Operation for Excellent Line and Load

TM

Operation at Light Load

Transient Response

■

Low Quiescent Current: 270µA

■

Shutdown Mode Draws Only 8µA Supply Current

■

±2.5% Reference Accuracy

■

Tiny 6-Lead SOT-23 Package

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APPLICATIO S

■

Lithium-Ion-Powered Applications

■

Cellular Telephones

■

Wireless Modems

■

Portable Computers

■

Scanners

LTC1872

Constant Frequency

Current Mode Step-Up

DC/DC Controller in SOT-23

U

DESCRIPTIO

The LTC®1872 is a constant frequency current mode step­up DC/DC controller providing excellent AC and DC load
and line regulation. The device incorporates an accurate
undervoltage lockout feature that shuts down the LTC1872
when the input voltage falls below 2.0V.

The LTC1872 boasts a ±2.5% output voltage accuracy and
consumes only 270µA of quiescent current. For applica-
tions where efficiency is a prime consideration, the LTC1872
is configured for Burst Mode operation, which enhances
efficiency at low output current.

In shutdown, the device draws a mere 8µA. The high
550kHz constant operating frequency allows the use of a
small external inductor.

The LTC1872 is available in a small footprint 6-lead
SOT-23.

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

Burst Mode is a trademark of Linear Technology Corporation.

TYPICAL APPLICATION

147k

220pF

80.6k

C1: TAIYO YUDEN CERAMIC EMK325BJ106MNT
C2: MURATA GRM42-2X5R226K6.3
D1: IR10BQ015
L1: MURATA LQN6C4R7M04
M1: IRLMS2002
R1: DALE 0.25W

1

2
3

ITH/RUN

LTC1872
GND
V

FB

422k

Figure 1. LTC1872 High Output Current 3.3V to 5V Boost Converter

SENSE

NGATE

5

V

IN

4

–

6

U

R1

0.03Ω

L1

4.7µH

M1

V

IN

C1
10µF
10V

+

D1

C2
2× 22µF

6.3V

3.3V

V
5V
1A

1872 TA01

OUT

EFFICIENCY (%)

Efficiency vs Load Current

100

VIN = 3.3V

= 5V

V

OUT

95

90

85

80

75

70

65

1

10 100 1000

LOAD CURRENT (mA)

1872 TA01b

1

LTC1872

ITH/RUN 1

GND 2

V

FB

3

6 NGATE
5 V

IN

4 SENSE

–

TOP VIEW

S6 PACKAGE

6-LEAD PLASTIC SOT-23

WW

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ABSOLUTE MAXIMUM RATINGS

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PACKAGE/ORDER INFORMATION

(Note 1)

Input Supply Voltage (VIN).........................–0.3V to 10V

SENSE–, NGATE Voltages ............ – 0.3V to (VIN + 0.3V)

VFB, ITH/RUN Voltages ..............................–0.3V to 2.4V

NGATE Peak Output Current (<10µs) ....................... 1A

ORDER PART

NUMBER

LTC1872ES6

Storage Ambient Temperature Range ... –65°C to 150°C
Operating Temperature Range (Note 2) .. –40°C to 85°C

S6 PART MARKING

Junction Temperature (Note 3)............................. 150°C

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

Consult factory for parts specified with wider operating temperature ranges.

ELECTRICAL CHARACTERISTICS

range, otherwise specifications are at TA = 25°C. VIN = 4.2V unless otherwise specified. (Note 2)

PARAMETER CONDITIONS MIN TYP MAX UNITS

Input DC Supply Current Typicals at VIN = 4.2V (Note 4)
Normal Operation 2.4V ≤ V
Sleep Mode 2.4V ≤ V
Shutdown 2.4V ≤ V
UVLO V

Undervoltage Lockout Threshold VIN Falling ● 1.55 2.00 2.35 V

V

IN

Shutdown Threshold (at ITH/RUN) ● 0.15 0.35 0.55 V
Start-Up Current Source V
Regulated Feedback Voltage 0°C to 70°C(Note 5) ● 0.780 0.800 0.820 V

VFB Input Current (Note 5) 10 50 nA
Oscillator Frequency VFB = 0.8V 500 550 650 kHz
Gate Drive Rise Time C
Gate Drive Fall Time C
Peak Current Sense Voltage (Note 6) 114 120 mV

ITH

–40°C to 85°C(Note 5)

LOAD

LOAD

The ● denotes specifications that apply over the full operating temperature

≤ 9.8V 270 420 µA

IN

≤ 9.8V 230 370 µA

IN

≤ 9.8V, V

IN

< UVLO Threshold 6 10 µA

IN

Rising 1.85 2.10 2.40 V

/RUN = 0V 0.25 0.5 0.85 µA

= 3000pF 40 ns
= 3000pF 40 ns

ITH

T

= 150°C, θJA = 230°C/W

JMAX

/RUN = 0V 8 22 µA

● 0.770 0.800 0.830 V

LTMK

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Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Note 2: The LTC1872E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.

Note 3: T
dissipation P

is calculated from the ambient temperature TA and power

J

according to the following formula:

D

TJ = TA + (PD • θJA°C/W)

2

Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.

Note 5: The LTC1872 is tested in a feedback loop that servos V
output of the error amplifier.

Note 6: Guaranteed by design at duty cycle = 30%. Peak current sense
voltage is V
increases due to slope compensation as shown in Figure 2.

/6.67 at duty cycle <40%, and decreases as duty cycle

REF

to the

FB

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TYPICAL PERFORMANCE CHARACTERISTICS

LTC1872

Reference Voltage
vs Temperature

825

VIN = 4.2V

820
815
810
805
800
795

VOLTAGE (mV)

FB

V

790
785
780
775

–35 5

–15

–55

85

45 125

25

TEMPERATURE (°C)

105

65

1872 G01

Maximum Current Sense Trip
Voltage vs Duty Cycle

130

120

110

100

– (mV)

90

SENSE

– V

80

IN

V

70

60

50

20 30

40 50

DUTY CYCLE (%)

60 70

Normalized Oscillator Frequency
vs Temperature

10

VIN = 4.2V

8
6
4
2

0
–2
–4
–6

NORMALIZED FREQUENCY (%)

–8

–10

–35 5

–55

VIN = 4.2V
T

= 25°C

A

80 90

–15

TEMPERATURE (°C)

100

1872 G04

45 125

65

25

85

105

1872 G02

Shutdown Threshold
vs Temperature

600

VIN = 4.2V

560
520
480
440
400
360

/RUN VOLTAGE (mV)

320

TH

I

280
240
200

–35 5

–15

–55

TEMPERATURE (

Undervoltage Lockout Trip
Voltage vs Temperature

2.24
VIN FALLING

2.20

2.16

2.12

2.08

2.04

2.00

1.96

UVLO TRIP VOLTAGE (V)

1.92

1.88

1.84

–35 5

–15

–55

45 125

65

25

°C)

25

TEMPERATURE (°C)

85

105

1872 G05

85

45 125

105

65

1872 G03

UUU

PIN FUNCTIONS

ITH/RUN (Pin 1): This pin performs two functions. It
serves as the error amplifier compensation point as well as
the run control input. Nominal voltage range for this pin is

0.7V to 1.9V. Forcing this pin below 0.35V causes the
device to be shut down. In shutdown all functions are
disabled and the NGATE pin is held low.

GND (Pin 2): Ground Pin.
VFB (Pin 3): Receives the feedback voltage from an exter-

nal resistive divider across the output.

SENSE– (Pin 4): The Negative Input to the Current Com­parator.

VIN (Pin 5): Supply Pin. Must be closely decoupled to GND
Pin 2.

NGATE (Pin 6): Gate Drive for the External N-Channel
MOSFET. This pin swings from 0V to VIN.

3

LTC1872

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FUNCTIONAL DIAGRA

–

SENSE

V

IN

5

4

4

+

ICMP

–

OSC

FREQ

FOLDBACK

V

IN

+

0.3V

GND

2

–

SLOPE
COMP

–

+

0.5µA

V

IN

VOLTAGE

REFERENCE

UNDERVOLTAGE

LOCKOUT

0.3V

0.15V

V

REF

0.8V

0.35V

V

RS

R

Q

S

+

–

I

/RUN

1

TH

+

–

BURST

CMP

SHDN

CMP

SWITCHING

LOGIC AND

BLANKING

CIRCUIT

SLEEP

SHDN

UV

OVP

EAMP

IN

NGATE

6

+

V

REF

+

–

60mV

V

REF

+

0.8V

–

1.2V

V

FB

3

V

IN

1872FD

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OPERATIO

Main Control Loop

The LTC1872 is a constant frequency current mode switch­ing regulator. During normal operation, the external
N-channel power MOSFET is turned on each cycle by the
oscillator and turned off when the current comparator
(ICMP) resets the RS latch. The peak inductor current at
which ICMP resets the RS latch is controlled by the voltage
on the ITH/RUN pin, which is the output of the error
amplifier EAMP. An external resistive divider connected
between V
output feedback voltage VFB. When the load current in­creases, it causes a slight decrease in VFB relative to the

and ground allows the EAMP to receive an

OUT

(Refer to Functional Diagram)

4

0.8V reference, which in turn causes the
ITH/RUN voltage to increase until the average inductor
current matches the new load current.

The main control loop is shut down by pulling the ITH/RUN
pin low. Releasing ITH/RUN allows an internal 0.5µA
current source to charge up the external compensation
network. When the ITH/RUN pin reaches 0.35V, the main
control loop is enabled with the ITH/RUN voltage then
pulled up to its zero current level of approximately 0.7V.
As the external compensation network continues to charge
up, the corre

sponding output current trip level follows,

allowing normal operation.