Linear Technology LT1612 Datasheet

FEATURES

Final Electrical Specifications

LT1612

Synchronous, Step-Down

800kHz PWM

DC/DC Converter

April 2000

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DESCRIPTION

■

Operates from Input Voltage As Low As 2V

■

Internal 0.7A Synchronous Switches

■

Uses Ceramic Input and Output Capacitors

■

620mV Reference Voltage

■

800kHz Fixed Frequency Switching

■

Programmable Burst Mode Operation

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APPLICATIONS

■

Portable Devices

■

Lithium-Ion Step-Down Converters

■

5V to 3.3V Conversion

■

2-Cell Alkaline Step-Down Converters

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TYPICAL APPLICATION

The LT®1612 is an 800kHz, synchronous step-down DC/
DC converter that operates from an input voltage as low
as 2V. Internal 0.45Ω switches deliver output currents up
to 500mA, and the 800kHz switching frequency allows the
use of small, low value ceramic input and output capaci­tors. Input voltage ranges from 5.5V down to 2V and
output voltage can be set as low as the 620mV reference.
The device features Burst ModeTM operation, keeping
efficiency high at light loads. Burst Mode operation can be
defeated by pulling the MODE pin high, enabling constant
switching throughout the load range for low noise.

No-load quiescent current is 160µA and shutdown current
is less than 1µA. The device is available in 8-lead SO and
MSOP packages.

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

Burst Mode is a trademark of Linear Technology Corporation.

V

IN

2V

C1

10µF

C1: TAIYO-YUDEN JMK325BJ106MN
C2: PANASONIC EEFCDOF680R
L1: SUMIDA CD43-100

V

IN

SHDN

MODE

V

C

BOOST

SW

LT1612

GND

33k

330pF

Figure 1. 2V to 1.2V Converter

Efficiency for LT1612 vs Linear Regulator

0.1µF

L1

10µH

100pF

FB

R1

215k

R2

232k

1%

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 represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

1%

V

1.2V
500mA

C2
68µF

3.15V

OUT

1612 F01a

90

80

70

60

EFFICIENCY (%)

50

40

30

V

= 1.2V

OUT

VIN = 2V (LINEAR)

VIN = 3V (LINEAR)

10010 500

LOAD CURRENT (mA)

VIN = 2V

VIN = 3V

1612 • F01b

1

LT1612

TOP VIEW

SHDN
MODE
BOOST
SW

V

C

FB

V

IN

GND

S8 PACKAGE

8-LEAD PLASTIC SO

1

2

3

4

8

7

6

5

WW

W

ABSOLUTE MAXIMUM RATINGS

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(Note 1)

Supply Voltage (VIN)............................................... 5.5V

SW Pin Voltage....................................................... 5.5V

FB Pin Voltage ............................................... VIN + 0.3V

VC Pin Voltage ........................................................... 2V

SHDN Pin Voltage................................................... 5.5V

MODE Pin Voltage .................................................. 5.5V

U

W

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PACKAGE/ORDER INFORMATION

ORDER PART

NUMBER

TOP VIEW

8

V

1

C

2

FB

3

V

IN

4

GND

MS8 PACKAGE

8-LEAD PLASTIC MSOP

T

= 125°C, θJA = 120°C/ W

JMAX

7
6
5

SHDN
MODE
BOOST
SW

LT1612EMS8

MS8 PART MARKING

LTMS

BOOST Pin Voltage ....................................... VIN + 5.5V

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

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

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

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

ORDER PART

NUMBER

LT1612ES8

S8 PART MARKING

T

= 125°C, θJA = 120°C/W

JMAX

1612

Consult factory for Military grade parts.

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are TA = 25°C, V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

Q

V

FB

g

m

f

OSC

2

Quiescent Current MODE = 5V ● 12 mA

FB Voltage 0.605 0.62 0.635 V

FB Line Regulation ● 0.02 0.15 %/V
FB Pin Bias Current (Note 3) ● 750 nA
Error Amplifier Transconductance 250 µmhos
Min Input Voltage 2V

Max Input Voltage 5.5 V
Oscillator Frequency 700 800 900 kHz

f

Line Regulation 1%/V

OSC

Maximum Duty Cycle 85 90 %

Shutdown Threshold Minimum Voltage for Active

The ● denotes specifications which apply over the full operating

= V

IN

MODE = 0V, Not Switching
SHDN = 0V ● 1 µA

Maximum Voltage for Shutdown ● 0.2 V

SHDN

= 3V

● 160 220 µA

● 0.60 0.62 0.635 V

● 550 1100 kHz

● 80 %

● 2V

LT1612

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are TA = 25°C, V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

SHDN Pin Current SHDN = 2V

BOOST Pin Current BOOST = VIN + 2V 4 mA
Switch Current Limit (Note 4) Duty Cycle = 0% MODE = OV 600 710 900 mA

Burst Mode Operation Current Limit MODE = 0V 180 mA
Switch Voltage Drop ISW = 500mA 200 280 mV
Rectifier Voltage Drop I
SW Pin Leakage VSW = 5V ● 1 µA

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

Note 2: The LT1612E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating

The ● denotes specifications which apply over the full operating

= V

IN

SHDN = 5V ● 30 45 µA

= 500mA 300 400 mV

RECT

= 3V

SHDN

● 10 15 µA

MODE = 5V 550 650 900 mA

temperature range are assured by design, characterization and correlation
with statistical process controls.

Note 3: Bias current flows out of the FB pin.
Note 4: Duty cycle affects current limit due to slope compensation.

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PIN FUNCTIONS

VC (Pin 1): Compensation Pin. This is the current sink/
source output of the error amplifier. By connecting an RC
network from this pin to ground, frequency response can
be tuned for a wide range of circuit configurations. The
voltage at this pin also sets the current limit, and if
grounded, the switch will remain in the OFF state.

FB (Pin 2): Feedback Pin. This pin is the negative input to
the error amplifier. Connect the resistor divider tap to this
point which sets V

V

= 0.62V (1 + R1/R2)

OUT

according to:

OUT

VIN (Pin 3): Supply Pin. Bypass capacitor C1 must be right
next to this pin.

GND (Pin 4): Ground Pin. Connect directly to local ground
plane.

SW (Pin 5): Switch Pin. Connect inductor and boost
capacitor here. Minimize trace area at this pin to keep EMI
down.

BOOST (Pin 6): This is the supply pin for the switch driver
and must be above VIN by 1.5V for proper switch opera­tion. Connect the boost capacitor to this pin.

MODE (Pin 7): Burst Mode Operation Disable Pin. For
continuous switching operation (low noise), pull this pin
above 2V. For Burst Mode operation which gives better
light load efficiency, tie to ground. Output ripple voltage in
Burst Mode operation is typically 30mV

. See applica-

P-P

tions section for more information about this function.
SHDN (Pin 8): Shutdown Pin. Pull this pin low for shut-

down mode. Tie to a voltage between 2V and 5.5V for
normal operation.

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