LINEAR TECHNOLOGY LTC3448 Technical data

FEATURES

■

High Efficiency: Up to 96%

■

Very Low Quiescent Supply Current: 32µA During
Linear Regulator Operation

■

600mA Output Current (Buck Converter)

■

Optionally Operates as Linear Regulator Below
3mA—External or Automatic ON/OFF

■

2.5V to 5.5V Input Voltage Range

■

1.5MHz or 2.25MHz Constant Frequency Operation
or External Synchronization

■

No Schottky Diode Required

■

Low Dropout Operation: 100% Duty Cycle

■

0.6V Reference Allows Low Output Voltages

■

Shutdown Mode Draws <1µA Supply Current

■

Current Mode Operation for Excellent Line and
Load Transient Response

■

Overtemperature Protected

■

Low Profile (3mm × 3mm) 8-Lead DFN and 8-Lead
MSOP Packages

U

APPLICATIO S

■

Cellular Telephones

■

Personal Information Appliances

■

Wireless and DSL Modems

■

Digital Still Cameras

■

MP3 Players

■

Portable Instruments

, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 5481178, 6580258, 6304066, 6127815,
6498466, 6611131. Others pending.

LTC34 4 8

1.5MHz/2.25MHz, 600mA
Synchronous Step-Down

Regulator with LDO Mode

U

DESCRIPTIO

The LTC®3448 is a high efficiency, monolithic, synchronous
buck regulator using a constant frequency, current mode
architecture. Supply current during operation is only 32µA
(linear regulator mode) and drops to <1µA in shutdown. The

2.5V to 5.5V input voltage range makes the LTC3448 ide­ally suited for single Li-Ion battery-powered applications.
100% duty cycle provides low dropout operation, extend­ing battery life in portable systems. At moderate output load
levels, PWM pulse skipping mode operation provides very
low output ripple voltage for noise sensitive applications.

The LTC3448 automatically switches into linear regulator
operation at very low load currents to maintain <5mV
output voltage ripple. Supply current in this mode is
typically 32µA. The switch to linear regulator mode occurs
at a threshold of 3mA. Linear regulator operation can be set
to on, off or automatic turn on/off.

Switching frequency is selectable at either 1.5MHz or

2.25MHz, allowing the use of small surface mount induc­tors and capacitors.

The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode. Low
output voltages are easily supported with the 0.6V feed­back reference voltage. The LTC3448 is available in a low
profile 3mm × 3mm DFN package or thermally enhanced
8-lead MSOP.

P-P

TYPICAL APPLICATIO

1.5V High Efficiency Regulator with Automatic LDO Mode

V

2.5V TO 5.5V

IN

C

IN

4.7µF

V

IN

RUN

LTC3448

FREQ
SYNC

GND

SW

V

OUT

MODE

V

3448 TA01a

U

2.2µH

474k

22pF

FB

316k

C

OUT

4.7µF

V

1.5V

OUT

Efficiency and Power Loss vs Load Current

100

VIN = 3.6V

= 1.5V

V

90

OUT

= 25°C

T

A

80

70

EFFICIENCY

60

50

40

EFFICIENCY (%)

30

20

10

0

0.0001 0.01 0.1 1

0.001

POWER LOSS

LOAD CURRENT (A)

23448 TA01b

1

0.1
POWER LOSS (W)

0.01

0.001

0.0001

3448f

1

LTC34 4 8

TOP VIEW

9

DD PACKAGE

8-LEAD (3mm × 3mm) PLASTIC DFN

5

6

7

8

4

3

2

1V

FB

V

OUT

MODE

V

IN

RUN

SYNC

FREQ

SW

WWWU

ABSOLUTE AXI U RATI GS

Input Supply Voltage .................................. – 0.3V to 6V

RUN, SYNC Voltages ................... –0.3V to (V

MODE Voltage ............................. – 0.3V to (V

FREQ, V

SW Voltage .................................. – 0.3V to (V

V

OUT

P-Channel Switch Source Current (DC) ............. 800mA

N-Channel Switch Sink Current (DC) ................. 800mA

Voltages...................... – 0.3V to (VIN + 0.3V)

FB

Voltage................................ – 0.3V to (VIN + 0.3V)

+ 0.3V)

IN

+ 0.3V)

IN

+ 0.3V)

IN

(Note 1)

V

(LDO) Source Current .................................. 25mA

OUT

Peak SW Sink and Source Current ........................ 1.3A

Operating Temperature Range (Note 2) .. –40°C to 85°C

Junction Temperature (Notes 3, 7) ...................... 125°C

Storage Temperature Range ................ –65°C to 125°C

Lead Temperature (Soldering, 10 sec)

MSOP Only ...................................................... 300°C

UU

W

PACKAGE/ORDER I FOR ATIO

ORDER PART

NUMBER

LTC3448EDD

V

OUT

MODE

DD PART MARKING

T

= 125°C, θJA = 43°C/ W

JMAX

EXPOSED PAD (PIN 9) IS GND

MUST BE SOLDERED TO PCB

Consult LTC Marketing for parts specified with wider operating temperature ranges.

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are TA = 25°C. VIN = 3.6V unless otherwise specified.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

VFB

V

FB

∆V

FB

∆V

OVL

∆V

OUT

I

PK

V

LOADREG

V

OUT(MAX)

V

IN

Feedback Current ● ±30 nA

Regulated Feedback Voltage TA = 25°C 0.5880 0.6 0.6120 V
(Note 4) 0°C ≤ T

Reference Voltage Line Regulation VIN = 2.5V to 5.5V (Note 4) ● 0.2 0.4 %/V
Output Overvoltage Lockout ∆V

Output Voltage Line Regulation VIN = 2.5V to 5.5V (LDO) 0.1 0.8 %/V

Peak Inductor Current VFB = 0.5V or V

Output Voltage Load Regulation LDO, 1mA to 10mA 0.5 %/V

Maximum Output Voltage (Note 9) VIN – 0.7 VIN – 0.3 V

Input Voltage Range ● 2.5 5.5 V

LBMJ

The ● denotes specifications which apply over the full operating

≤ 85°C 0.5865 0.6 0.6135 V

A

–40°C ≤ T

∆V

Duty Cycle < 35%

≤ 85°C ● 0.5850 0.6 0.6150 V

A

= V
= (V

– V

OVL

FB

– V

OVL

) • 100/V

OUT

= 90%, 0.7 1 1.3 A

OUT

OUT

OVL
OVL

TOP VIEW

V

1

FB

2
3
4

V

IN

MS8E PACKAGE

8-LEAD PLASTIC MSOP

T

= 125°C, θJA = 40°C/ W

JMAX

EXPOSED PAD (PIN 9) IS GND

MUST BE SOLDERED TO PCB

8
7

9

6
5

RUN
SYNC
FREQ
SW

15 35 55 mV

2.5 5.8 9.2 %

ORDER PART

NUMBER

LTC3448EMS8E

MS8 PART MARKING

LTBMK

2

3448f

LTC34 4 8

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating

temperature range, otherwise specifications are TA = 25°C. VIN = 3.6V unless otherwise specified.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

S

f

OSC

f

SYNC

V

TH(SYNC)

R

PFET

R

NFET

I

LSW

V

RUNH

V

RUNL

I

RUN

V

FREQH

V

FREQL

I

FREQ

V

MODEH

V

MODEL

I

MODE

I

SYNC

I

LDO(ON)

I

LDO(OFF)

Input DC Bias Current VIN = 3.6V (Note 5)
Active Mode (Pulse Skip, No LRO) V

Linear Regulator Operation (LRO) I

Shutdown V

= 0.5V or V

FB

= 0.5V or V

V

FB

≤ I

LOAD

LDO(ON)

= 0V, VIN = 5.5V 0.1 1 µA

RUN

= 90%, I

OUT

= 90%, I

OUT

= 0A, 1.5MHz 250 375 µA

LOAD

= 0A, 2.25MHz 275 400 µA

LOAD

32 43 µA

Oscillator Frequency FREQ = Low, VIN = 3.6V ● 1.2 1.5 1.8 MHz

FREQ = High

● 1.8 2.25 2.7 MHz

Synchronization Frequency (Note 6) 1.5 >4 MHz

SYNC Activation Input Threshold 1 1.3 V

R

of P-Channel FET ISW = 100mA 0.4 Ω

DS(ON)

R

of N-Channel FET ISW = –150mA 0.35 Ω

DS(ON)

SW Leakage V

= 0V, VSW = 0V or 5V, VIN = 5V ±0.01 ±1 µA

RUN

RUN Threshold High ● 1.5 V

RUN Threshold Low ● 0.3 V

RUN Leakage Current ● ±0.01 ±1 µA

FREQ Threshold High ● VIN – 1 V

FREQ Threshold Low ● 1V

FREQ Leakage Current ● ±0.01 ±1 µA

MODE Threshold High ● VIN – 0.15 V

MODE Threshold Low ● 0.12 V

MODE Leakage Current ● ±0.1 ±1 µA

SYNC Leakage Current ● ±0.01 ±1 µA

LRO ON Load Current Threshold 2.2mH Inductor (Note 8) 3 5 mA

LRO OFF Load Current Threhold 8 11 17 mA

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

Note 2: The LTC3448E 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

T

= TA + (PD)(43°C/W)

J

Note 4: The LTC3448 is tested in a proprietary test mode that connects

to the output of the error amplifier.

V

FB

Note 5: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. LRO is “linear regulator operation.”

Note 6: 4MHz operation is guaranteed by design but is not production
tested and is subject to duty cycle limitations.

Note 7: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature is active. Continu­ous operation above the specified maximum operating junction tempera­ture may impair device reliability.

Note 8: The load current below which the switching regulator turns off and
the LDO turns on is, to first order, inversely proportional to the value of
the inductor. This effect is covered in more detail in the Operation section.
This parameter is not production tested but is guaranteed by design.

Note 9: For 2.5V < V

< 2.7V the output voltage is limited to VIN – 0.7V

IN

to ensure regulation in linear regulator mode. This parameter is not
production tested but is guaranteed by design.

3448f

3

LTC34 4 8

LOAD CURRENT (A)

30

EFFICIENCY (%)

90

100

20

10

80

50

70

60

40

0.0001 0.01 0.1 1

23448 G03

0

0.001

V

OUT

= 1.5V

T

A

= 25°C

VIN = 2.7V
V

IN

= 3.6V

V

IN

= 4.2V

TEMPERATURE (°C)

–50

FREQUENCY (MHz)

1.65

25

3448 G06

1.50

1.40

–25 0 50

1.35

1.30

1.70

1.60

1.55

1.45

75 100 125

VIN = 3.6V

UW

TYPICAL PERFOR A CE CHARACTERISTICS

(From Figure1a Except for the Resistive Divider Resistor Values)

Efficiency vs Input Voltage

100

95

90

I

= 30mA

OUT

85

80

75

70

EFFICIENCY (%)

65

60

55

50

2

I

= 100mA

OUT

I

= 600mA

OUT

3

INPUT VOLTAGE (V)

4

V

OUT

T

A

5

= 25°C

Efficiency vs Load Current
(Switcher Only)

100

VIN = 2.7V

90

= 2.5V

V

OUT

= 25°C

T

A

80

70

60

50

40

EFFICIENCY (%)

30

20

10

0

0.0001 0.01 0.1 1

0.001
LOAD CURRENT (A)

= 1.8V

3448 G01

23448 G04

6

0.615

0.610

0.605

0.600

0.595

REFERENCE VOLTAGE (V)

0.590

0.585

Efficiency vs Load Current

100

V

= 1.2V

OUT

90

= 25°C

T

A

80

70

60

50

40

EFFICIENCY (%)

30

20

10

0

0.0001 0.01 0.1 1

0.001
LOAD CURRENT (A)

VIN = 2.7V

= 3.6V

V

IN

= 4.2V

V

IN

23448 G02

Reference Voltage
vs Temperature

VIN = 3.6V

–50

–25 0

TEMPERATURE (°C)

50 100 125

25 75

3448 G05

Efficiency vs Load Current

Oscillator Frequency
vs Temperature

Oscillator Frequency
vs Supply Voltage

1.8
= 25°C

T

A

1.7

1.6

1.5

1.4

FREQUENCY (MHz)

1.3

1.2

4

2

34 56

SUPPLY VOLTAGE (V)

3448 G07

Output Voltage vs Load Current

1.525
VIN = 3.6V

1.520

= 25°C

T

A

1.515

1.510

1.505

1.500

1.495

1.490

OUTPUT VOLTAGE (V)

1.485

1.480

1.475

0.0001

0.001

0.01

LOAD CURRENT (A)

0.1

3448 G08

R

vs Input Voltage

DS(ON)

0.40

0.38

0.36

0.34

0.32

(Ω)

0.30

0.28

0.26

0.24

0.22

0.20

SYNCHRONOUS

2

DS(ON)

R

1

MAIN
SWITCH

SWITCH

3

4

INPUT VOLTAGE (V)

TA = 25°C

5

6

3448 G09

3448f

UW

TEMPERATURE (°C)

–50

DYNAMIC SUPPLY CURRENT (µA)

280

300

320

25 75

3448 G12

260

240

–25 0

50 100 125

220

200

VIN = 3.6V
I

LOAD

= 0A

2.25MHz

1.5MHz

TYPICAL PERFOR A CE CHARACTERISTICS

(From Figure1a Except for the Resistive Divider Resistor Values)

LTC34 4 8

R

vs Temperature

DS(ON)

0.6

0.5

0.4

(Ω)

0.3

DS(ON)

R

0.2

0.1

0

–50

MAIN SWITCH

–25 0

SYNCH SWITCH

2.5V

3.6V

4.2V

25 75

TEMPERATURE (°C)

2.5V

3.6V

4.2V

50 100 125

Switch Leakage vs Temperature

350

VIN = 5.5V
RUN = 0V

300

250

200

150

100

SWITCH LEAKAGE (nA)

50

0

–50

–25 0

SYNCHRONOUS

SWITCH

50 100 125

25 75

TEMPERATURE (°C)

MAIN

SWITCH

3448 G10

3448 G13

Dynamic Supply Current
vs Supply Voltage

340

I

= 0A

LOAD

= 25°C

T

A

320

300

280

260

240

DYNAMIC SUPPLY CURRENT (µA)

220

200

2

2.25MHz

1.5MHz

34

SUPPLY VOLTAGE (V)

Switch Leakage vs Input Voltage

10

RUN = 0V

= 25°C

T

A

1

0.1

SWITCH LEAKAGE (nA)

0.01

0.001
0

SYNCHRONOUS

1234

SWITCH

INPUT VOLTAGE (V)

SWITCH

5

MAIN

56

3448 G11

3448 G14

6

500mA/DIV

Dynamic Supply Current
vs Temperature

Start-Up from Shutdown

RUN

5V/DIV

V

OUT

1V/DIV

I

L

= 3.6V

V

IN

= 1.5V

V

OUT

= 600mA

I

LOAD

40µs/DIV

3448 G15

Load Step

V

OUT

200mV/DIV

AC COUPLED

I

LOAD

100mA/DIV

500mA/DIV

I

L

= 3.6V

V

IN

= 1.5V

V

OUT

I

LOAD

= 10µF

C

OUT

10µs/DIV

= 100µA TO 200mA

3448 G16

100mV/DIV

AC COUPLED

250mA/DIV

500mA/DIV

V

I

LOAD

OUT

I

Load Step

L

= 3.6V

V

IN

= 1.5V

V

OUT

= 50mA TO 600mA

I

LOAD

= 10µF

C

OUT

10µs/DIV

3448 G17

3448f

5

LTC34 4 8

UW

TYPICAL PERFOR A CE CHARACTERISTICS

(From Figure 1a Except for the Resistive Divider Resistor Values)

Load Step

V

V

OUT

100mV/DIV

AC COUPLED

I

LOAD

250mA/DIV

500mA/DIV

I

L

OUT

20mV/DIV

AC COUPLED

MODE PIN

2V/DIV

External Mode Control (Constant
1mA Load)

SWITCHER SWITCHER

LDO

3448 G18

U

V

= 3.6V

IN

= 1.5V

V

OUT

= 100mA TO 600mA

I

LOAD

10µs/DIV

UU

PI FU CTIO S

VFB (Pin 1): Feedback Pin. This pin receives the feedback
voltage from an external resistive divider across the
output.

V

(Pin 2): Output Pin. This pin connects to an external

OUT

resistor divider and the linear regulator output. Connect
externally to the inductor and the output capacitor. The
internal linear regulator will supply current up to the
I

LDO(OFF)

the buck regulator. Internal circuitry automatically enables
the buck switching regulator at load currents higher than
the I
pin is 2µF.

MODE (Pin 3): Linear Regulator Control. Grounding this
pin turns off the linear regulator. Setting this pin to V
turns on the linear regulator regardless of the load current.
Tying this pin midrange (i.e., to V
regulator in auto mode, where turn on/off is a function of
the load current. In applications where MODE is externally
driven high or low, this pin should be held low for 50µs
after the RUN pin is pulled high.

current. Load currents above that are supplied by

LDO(OFF)

. The minimum required capacitance on this

) will place the linear

OUT

IN

= 1.5V

V

OUT

= 25°C

T

A

V

(Pin 4): Main Supply Pin. This pin must be closely

IN

200µs/DIV

3448 G19

decoupled to GND with a 2.2µF or greater ceramic
capacitor.

SW (Pin 5): Switch Node Connection to Inductor. This pin
connects to the drains of the internal main and synchro­nous power MOSFET switches.

FREQ (Pin 6): Frequency Select. Switching frequency is
set to 1.5MHz when FREQ = 0V and to 2.25MHz when
FREQ = VIN. Do not float this pin.

SYNC (Pin 7): External Synchronization Pin. The oscilla­tion frequency can be synchronized to an external oscilla­tor applied to this pin. For external frequencies above

2.2MHz, pull FREQ high.

RUN (Pin 8): Run Control Input. Forcing this pin above

1.5V enables the part. Forcing this pin below 0.3V shuts
down the device. In shutdown, all functions are disabled
drawing <1µA supply current. Do not leave RUN floating.

Exposed Pad (Pin 9): Ground. This pin must be soldered
to PCB.

6

3448f