Datasheet LT1612 Datasheet (Linear Technology)

FEATURES

Final Electrical Specifications

LT1612

Synchronous, Step-Down

800kHz PWM

DC/DC Converter

April 2000

U

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

U

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

U

(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

U

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.

UUU

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.

3

LT1612

BLOCK DIAGRA

V

3

IN

V

1

C

W

R

SENSE

0.08Ω

BOOST DIODE

6

BOOST

MODE

SHDN

2

FB

7

8

–

+

0.62V

SHUTDOWN

A1

+

–

0.7V

A3

ENABLE

OSCILLATOR

V/I

+

A2

U

OPERATIO

The LT1612 employs fixed frequency, current mode con­trol. This type of control uses two feedback loops. The
main control loop sets output voltage and operates as
follows: A load step causes V
perturbed slightly. The error amplifier responds to this
change in FB by driving the VC pin either higher or lower.
Because switch current is proportional to the VC pin
voltage, this change causes the switch current to be
adjusted until V

is once again satisfied. Loop compen-

OUT

sation is taken care of by an RC network from the VC pin
to ground.

Inside this main loop is another that sets current limit on
a cycle-by-cycle basis. This loop utilizes current compara­tor A2 to control peak current. The oscillator runs at
800kHz and issues a set pulse to the flip-flop at the
beginning of each cycle, turning the switch on. With the
switch now in the ON state the SW pin is effectively
connected to VIN. Current ramps up in the inductor linearly

and the FB voltage to be

OUT

–

FLIP-FLOP

RQ

at a rate of (VIN – V
pin voltage and when the voltage across R

S

SLOPE
COMPENSATION

SWITCH

SWITCH
DRIVER

RECTIFIER

DRIVE

)/L. Switch current is set by the V

OUT

RECTIFIER

SENSE

trips the

SW

5

GND

4

1612 BD

C

current comparator, a reset pulse will be generated and the
switch will be turned off. Since the inductor is now loaded
up with current, the SW pin will fly low and trigger the
rectifier to turn on. Current will flow through the rectifier
decreasing at a rate of V

/L until the oscillator issues a

OUT

new set pulse, causing the cycle to repeat.
If the load is light and VC decreases below A3’s trip point,

the device will enter the Burst Mode operation region (the
MODE pin must be at ground or floating). In this state the
oscillator and all other circuitry except the reference and
comparator A3 are switched on and off at low frequency.
This mode of operation increases efficiency at light loads
but introduces low frequency voltage ripple at the output.
For continuous switching and no low frequency output
voltage ripple, pull the MODE pin high. This will disable
comparator A3 which forces the oscillator to run
continuously.

4

OPERATIO

LT1612

U

Layout Hints

The LT1612 switches current at high speed, mandating
careful attention to layout for proper performance.

will not get advertised performance with careless layouts

You

.
Figure 2 shows recommended component placement for
a buck (step-down) converter. Follow this closely in your
PC layout. Note the direct path of the switching loops.
Input capacitor C1 must be placed close (< 5mm) to the IC
package. As little as 10mm of wire or PC trace from CIN to
VIN will cause problems such as inability to regulate or
oscillation.

The ground terminal of input capacitor C1 should tie close
to Pin 4 of the LT1612. Doing this reduces dI/dt in the
ground copper which keeps high frequency spikes to a
minimum. The DC/DC converter ground should tie to the
PC board ground plane at one place only, to avoid intro­ducing dI/dt in the ground plane.

C

R1

R2 R

V

IN

MULTIPLE

VIAs

C

C

SHDN

8

MODE

7

6

5

C3

L1

1612 F02

V

OUT

C1

GND

1

2

LT1612

3

4

C2

consumption but also adds low frequency voltage ripple to
the output. Figure 3 shows switching waveforms for a 5V
to 3.3V converter running in Burst Mode operation. Output
voltage ripple is approximately 20mV

. If the MODE pin

P-P

is pulled high, Burst Mode operation will be inhibited and
the oscillator runs continuously with no low frequency
ripple at the output. See Figures 4 and 5.

V

OUT

20mV/DIV

AC COUPLED

IL

200mA/DIV

5µs/DIV

Figure 3. Output Voltage Ripple is 20mV

P-P

the Circuit of Figure 1

V

OUT

200mV/DIV

AC COUPLED

I

L

200mA/DIV

I

10mA TO 310mA

LOAD

0.1ms/DIV

Figure 4. Transient Response for the Circuit of Figure 1
with the MODE Pin Tied to Ground or Floating

1612 F03

for

1612 F04

Figure 2. Recommended Component Placement. Traces
Carrying High Current are Direct. Trace Area at FB Pin and V

C

Pin Is Kept Low. Lead Length to Battery Should Be Kept Short

Burst Mode Operation Defeat

To maintain high efficiency at light loads, the LT1612 will
automatically shift into Burst Mode operation (MODE = 0V
or floating). In this mode of operation the oscillator and
switch drive circuitry is alternately turned on and off,
reducing quiescent current to 160µA. This reduces power

V

OUT

200mV/DIV

AC COUPLED

I

L

200mA/DIV

I

10mA TO 300mA

LOAD

0.1ms/DIV

Figure 5. With the MODE Pin Tied High, Low
Frequency Output Voltage Ripple Is No Longer Present

1612 F05

5

LT1612

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TYPICAL APPLICATIONS

2.7V TO 4.2V

CERAMIC

V

IN

10µF

Single Li-Ion to 2V Converter

0.1µF

V

IN

SHDN

MODEBURSTMODE
V

C

30k

LT1612

680pF

BOOST

SW

FB

GND

453k

1%

L1

10µH

20pF

1M
1%

V

OUT

2V
500mA

22µF
CERAMIC

V

50mV/DIV

OUT

200mA/DIV

I

L

LOAD STEP

125mA TO 300mA

VIN = 4V
V

= 2V

OUT

MODE = HIGH

Transient Response

100µs/DIV

Burst Mode Operation

C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100

1612 TA03

1612 TA02

Li-Ion to 2V Converter Efficiency

85

80

VIN = 2.8V

75

70

65

EFFICIENCY (%)

60

55

50

1

VIN = 3.5V

10 100 1000

LOAD CURRENT (mA)

Inrush Current at Start-Up

VIN = 4.2V

1612 TA04

6

V

20mV/DIV

OUT

100mA/DIV

I

L

MODE = LOW

5µs/DIV

1612 TA05

V

OUT

INRUSH

CURRENT

200mA/DIV

V

SHDN

2V/DIV

5V/DIV

0.2ms/DIV

1612 TA06

U

TYPICAL APPLICATIONS

LT1612

5V to 2.5V Converter

V

IN

5V

C1

10µF

CERAMIC

V

IN

SHDN

LT1612

MODEBURSTMODE

V

C

30k

680pF

C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100

BOOST

SW

GND

0.1µF

L1

10µH

20pF

FB

1M

332k

V

OUT

2.5V
500mA

C2
22µF
CERAMIC

1612 TA07

U

PACKAGE DESCRIPTION

0.007
(0.18)

0.021

± 0.006

(0.53 ± 0.015)

* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH,

PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.

INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

° – 6° TYP

0

Dimension in inches (millimeters) unless otherwise noted.

MS8 Package

8-Lead Plastic MSOP

(LTC DWG # 05-08-1660)

0.040 ± 0.006
(1.02 ± 0.15)

SEATING

PLANE

0.012

(0.30)

0.0256

REF

(0.65)

TYP

0.034 ± 0.004
(0.86 ± 0.102)

0.006 ± 0.004
(0.15 ± 0.102)

5V to 2.5V Converter Efficiency

85

80

75

70

65

EFFICIENCY (%)

60

55

50

1

0.118 ± 0.004*

10 100 1000

LOAD CURRENT (mA)

(3.00 ± 0.102)

8

0.192

± 0.004

(4.88 ± 0.10)

12

7

1612 TA08

6

5

0.118 ± 0.004**

4

3

(3.00 ± 0.102)

MSOP (MS8) 1197

8-Lead Plastic Small Outline (Narrow 0.150)

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

*

DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

**

DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

× 45°

0.016 – 0.050

(0.406 – 1.270)

0.053 – 0.069

(1.346 – 1.752)

0°– 8° TYP

0.014 – 0.019

(0.355 – 0.483)

S8 Package

(LTC DWG # 05-08-1610)

0.050

(1.270)

TYP

0.004 – 0.010

(0.101 – 0.254)

0.228 – 0.244

(5.791 – 6.197)

0.189 – 0.197*
(4.801 – 5.004)

7

8

1

2

5

6

0.150 – 0.157**
(3.810 – 3.988)

3

4

SO8 0996

7

LT1612

U

TYPICAL APPLICATIONS

2V to 0.9V Converter Efficiency for LT1612 vs Linear Regulator.

V

IN

2V

10µF

C1

V

IN

SHDN

LT1612

MODE

V

C

33k

330pF

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

BOOST

SW

GND

0.1µF

10µH

100pF

FB

R2

232k

L1

R1

105k

V

0.9V
500mA

C2
68µF

3.15V

OUT

1612 TA09

80

70

60

50

40

EFFICIENCY (%)

30

20

10

1

V

= 0.9V.

OUT

VIN = 2V

VIN = 3V

V

= 2V (LINEAR)

IN

VIN = 3V (LINEAR)

10 100 1000

LOAD CURRENT (mA)

1612 TA10

5V to 3.3V Converter

V

IN

5V

10µF

C1

V

IN

SHDN

LT1612

MODE

V

C

R3
33k

C4
680pF

C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100

BOOST

SW

GND

C3

0.1µF

L1

10µH

20pF

FB

R2

232k

1%

R1
1M
1%

C2
22µF

V

OUT

3.3V
500mA

1612 TA01a

85

VIN = 5V
V

OUT

80

75

70

65

EFFICIENCY (%)

60

55

50

1

Efficiency

= 3.3V

10 100 1000

LOAD CURRENT (mA)

1612 TA01b

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

®

LTC

1474 Low IQ Step-Down Switching Regulator 10µA IQ, VIN from 3V to 18V, MSOP Package up to 300mA

LTC1701 SOT-23 Step-Down Switching Regulator 500mA in SOT-23 Package, 1MHz Switching Frequency
LTC1707 Monolithic Synchronous Step-Down Switching Regulator 500mA, VIN from 2.65V to 8.5V

8

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear-tech.com

1612i LT/TP 0400 4K • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1999