Datasheet LT1305 Datasheet (Linear Technology)

LT1305

Micropower High Power

DC/DC Converter with

Low-Battery Detector

EATU

F

■

5V at 400mA from 2V Input

■

Supply Voltage As Low As 1.8V

■

120µA Quiescent Current

■

Low-Battery Detector

■

Low V

■

Uses Inexpensive Surface Mount Inductors

■

8-Lead SO Package

PPLICATI

A

■

2-Cell and 3-Cell to 5V Conversion

■

EL Panel Drivers

■

Portable Instruments

S

RE

Switch: 310mV at 2A Typ

CESAT

U

O

S

A

PPLICATITYPICAL

O

U

DUESCRIPTIO

The LT®1305 is a micropower step-up DC/DC converter
that uses Burst ModeTM operation. Similar to the LT1303,
the LT1305 features a 2A internal low-loss switch and can
deliver up to four times the output power of the LT1303.

Quiescent current is only 120µA and the Shutdown pin
further reduces current to 10µA. A low-battery detector
provides an open-collector output that goes low when the
input voltage drops below a preset level. The LT1305 is
available in an 8-pin SO, easing board space requirements.

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

Burst Mode is a trademark of Linear Technology Corporation

2-Cell and 3-Cell to 5V/400mA DC/DC Converter

with Low-Battery Detect

316k
1%

2 TO 3
CELLS

C1, C2: AVX TPSE227010R0100
D1: MOTOROLA MBRS130LT3
L1: COILCRAFT D03316-103

+

C1
220µF

412k
1%

SHUTDOWN

L1

10µH

V

IN

LBI

LT1305

SHDN

GND PGND

SW

LBO

Efficiency

D1

LOW BATTERY

100k

301k

100k
1%

1%

C2

220µF

FB

GOES LOW AT

= 2.2V

V

BAT

V



OUT

5V
400mA

+

LT1305 • TA03

90

80

EFFICIENCY (%)

70

60

1

VIN = 4.00V

VIN = 3.00V

= 2.00V

V

IN

VIN = 2.50V

10 100 1000

LOAD CURRENT (mA)

LT1305 • TA02

1

LT1305

WU

U

PACKAGE

/

O

RDER I FOR ATIO

W

O

A

LUTEXI T

S

VIN Voltage .............................................................. 10V

SW1 Voltage ............................................................ 25V

FB Voltage ............................................................... 10V

Shutdown Voltage ................................................... 10V

LBO Voltage............................................................. 10V

LBI Voltage .............................................................. 10V

Maximum Power Dissipation ............................. 500mW

Operating Temperature Range ..................... 0°C to 70°C

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

A

WUW

ARB

U
G

I

S

GND

LBO

SHDN

1

2

3

FB

4

T

JMAX

TOP VIEW

S8 PACKAGE

8-LEAD PLASTIC SO

= 100°C, θJA = 80°C/ W

ORDER PART

NUMBER

PGND

8

SW

7

V



6

IN

LBI

5

LT1305CS8

S8 PART MARKING

1305

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

Consult factory for Industrial and Military grade parts.

LECTRICAL C CHARA TERIST

E

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

Q

VINInput Voltage Range 1.8 1.55 V

DC Maximum Duty Cycle ● 75 86 95 %
t

ON

V

CESAT

V

SHDNH

V

SHDNL

I

SHDN

Quiescent Current V

Feedback Voltage ● 1.22 1.24 1.26 V
Comparator Hysteresis ● 6 12.5 mV
Feedback Pin Bias Current VFB = 1V ● 720 nA
Oscillator Frequency Current Limit Not Asserted 120 155 185 kHz
Oscillator TC 0.2 %/°C

Switch On Time Current Limit Not Asserted 5.6 µs
Output Line Regulation 1.8V < VIN < 6V ● 0.06 0.15 %/V
Switch Saturation Voltage ISW = 1A ● 140 280 mV
Switch Leakage Current VSW = 5V, Switch Off ● 0.1 10 µA
Peak Switch Current VIN = 2V 1.35 2 2.35 A

LBI Trip Voltage (Note 2) ● 1.21 1.24 1.27 V
LBI Input Bias Current V
LBO Output Low I
LBO Leakage Current V
Shutdown Pin High ● 1.8 V
Shutdown Pin Low 0.5 V
Shutdown Pin Bias Current V

ICS

TA = 25°C, VIN = 2.0V, unless otherwise noted.

= 0.5V, VFB = 2V ● 120 200 µA

SHDN

V

= 1.8V ● 715 µA

SHDN

● 2.0 V

● 1.20 2.50 A

VIN = 5V 1.15 2.15 A

= 1V ● 720 nA

LBI

= 100µA ● 0.11 0.4 V

LOAD

= 1.3V, V

LBI

= 5V ● 8.0 20 µA

SHDN

= 2V ● 3.0 µA

V

SHDN

= 0V ● 0.1 1 µA

V

SHDN

= 5V ● 0.1 5 µA

LBO

The ● denotes specifications which apply over the 0°C to 70°C operating
temperature range.

Note 1: Hysteresis specified is DC. Output ripple may be higher if output
capacitance is insufficient or capacitor ESR is excessive.

2

Note 2: Low-battery detector comparator is inoperative when device is in
shutdown.

W

TEMPERATURE (˚C)

–50



1.8

2.0

2.2

25 75

LT1305 • G06

1.6

1.4

–25 0

50 100

1.2

1.0

PEAK SWITCH CURRENT (A)

2.4

TEMPERATURE (°C)

–50

1.200

FEEDBACK VOLTAGE (V)

1.210

1.215

1.220

1.225

1.230

1.235

–25

0

25 50

LT1305 • G09

75

1.240

1.245

1.250

1.205

100

U

TYPICAL PERFORMANCE CHARACTERISTICS

LT1305

Switch On Time

8

7

6

5

ON TIME (µs)

4

3

2

–50

–25

02550

TEMPERATURE (°C)

75 100

LT1305 • G01

Oscillator Frequency

200
190
180
170

160
150
140

FREQUENCY (kHz)

130
120
110
100

–50

–25

Quiescent Current Quiescent Current

QUIESCENT CURRENT (µA)

500

400

300

200

100

0

TA = 25°C
SWITCH OFF

0

2

200
190
180
170

160
150
140
130

QUIESCENT CURRENT (µA)

120
110
100

SWITCH OFF
V

IN

–50

= 2V

–25

TEMPERATURE (°C)

0

25 50

75

LT1305 • G04

100

25 50

0

TEMPERATURE (°C)

6

4

INPUT VOLTAGE (V)

75

LT1305 • G02

8

LT1305 • G05

100

10

Maximum Duty Cycle

100

95
90
85

80
75
70

DUTY CYCLE (%)

65
60
55
50

–50

–25

TEMPERATURE (°C)

Current Limit

0

25 50

75

LT1305 • G03

100

LBI Pin Bias Current FB Pin Bias Current

20
18
16
14

12
10

8

BIAS CURRENT (nA)

6
4
2
0

–50

–25

25 50

0

TEMPERATURE (°C)

75

LT1305 • G07

100

20
18
16
14

12
10

8

BIAS CURRENT (nA)

6
4
2
0

–50

–25

25 50

0

TEMPERATURE (°C)

75

LT1305 • G08

FB Voltage

100

3

LT1305

TEMPERATURE (°C)

–50

0

SWITCH SATURATION VOLTAGE (mV)

50

100

150

200

300

–25

02550

LT1305 • G13

75 100

250

ISW = 1A

W

U

TYPICAL PERFORMANCE CHARACTERISTICS

Low-Battery Detect Trip Point

1.250

1.245

1.240

1.235

1.230

1.225

1.220

LBI VOLTAGE (V)

1.215

1.210

1.205

1.200
–50

–25

25 50

0

TEMPERATURE (°C)

U

100

75

LT1305 • G10



UU

Switch Saturation Voltage

400

TA = 25°C

350

300

250

200

150

100

50

SWITCH SATURATION VOLTAGE (mV)

0

0.4

0.2

0

0.6

SWITCH CURRENT (A)

PI FU CTIO S

GND (Pin 1): Signal Ground. Tie to PGND under the
package.

LBO (Pin 2): Open-Collector Output of Comparator C3.
Can sink 100µA. High impedance when device is in shut-
down.

Switch Saturation Voltage

1.2

1.0

1.4

1.6

2.0

0.8

1.8

LT1305 • F12

LBI (Pin 5): Low-Battery Comparator Input. When voltage
on this pin is below 1.24V, LBO is low.

VIN (Pin 6): Supply Pin. Must be bypassed with a large
value capacitor to gound. Keep bypass within 0.2" of the
device.

SHDN (Pin 3): Shutdown. Pull high to shut down the
LT1305. Ground for normal operation.

FB (Pin 4): Feedback Input. Connects to main comparator
C1 input.

4

SW (Pin 7): Switch Pin. Connect inductor and diode here.
Keep layout short and direct to minimize radio frequency
interference.

PGND (Pin 8): Power Ground. Tie to signal ground (pin 1)
under the package. Bypass capacitor from VIN should be
tied directly to PGND within 0.2" of the device.

BLOCK DIAGRAM

LT1305

W

SW

Q1
160×

D1

++

+

–

L1

36mV

R1
3Ω

Q2
1×

V

IN

C5 C4

R1

FB

4

R2

V

IN

6 7

CURRENT 

COMPARATOR

C2

HYSTERETIC

COMPARATOR

OFF

–

A3

DRIVER

REFERENCE

1.24V

C1

OSCILLATOR

+

–

C3

+

LBOLBIGND

1

25

SHUTDOWN

3

PGND

8

LT1305 • F01

Figure 1. LT1305 Block Diagram

U

OPERATION

Operation of the LT1305 is best understood by referring to
the Block Diagram in Figure 1. When C1’s negative input,
related to the output voltage by the appropriate resistor­divider ratio, is higher than the 1.24V reference voltage,
C1’s output is low. C2, A3 and the oscillator are turned off,
drawing no current. Only the reference and C1 consume
current, typically 120µA. When C1’s negative input drops
below 1.24V and overcomes C1’s 6mV hysteresis, C1’s
output goes high, enabling the oscillator, current compara­tor C2 and driver A3. Quiescent current increases to 2mA
as the device goes into active switching mode. Q1 then
turns on in controlled saturation for nominally 6µ s or until
current comparator C2 trips, whichever comes first. The
switch then turns off for approximately 1.5µ s, then turns on
again. The LT1305’s switching causes current to alter­nately build up in L1 and dump into output capacitor C4 via
D1, increasing the output voltage. When the output is high
enough to cause C1’s output to go high, switching action
ceases. Capacitor C4 is left to supply current to the load

until V

decreases enough to force C1’s output high, and

OUT

the entire cycle repeats. Figure 2 details relevant wave­forms. C1’s cycling causes low-to-mid-frequency ripple
voltage on the output. Ripple can be reduced by making the
output capacitor large. The 220µ F unit specified results in
ripple of 50mV to 100mV on the 5V output. Paralleling two
capacitors will decrease ripple by approximately 50%.

V

OUT

100mV/DIV

AC COUPLED

V

SW

5V/DIV

I

L

1A/DIV

50µs/DIV

Figure 2. Burst Mode Operation

LT1305 • F02

5

LT1305

U

OPERATION

If switch current reaches 2A, causing C2 to trip, switch on
time is reduced and off time increases slightly. This allows
continuous operation during bursts. C2 monitors the
voltage across 3Ω resistor R1 which is directly related to
the switch current. Q2’s collector current is set by the
emitter-area ratio to 0.6% of Q1’s collector current. When
R1’s voltage drop exceeds 36mV, corresponding to 2A
switch current, C2’s output goes high, truncating the on
time portion of the oscillator cycle and increasing off time
to about 2µs. Response time of C2, which determines
minimum on time, is approximately 300ns.

Low-Battery Detector

The low-battery detector is enabled when SHDN is low and
disabled when SHDN is high. The comparator has no
hysteresis built in, but hysteresis can be added by
connecting a high-value resistor from LBI to LBO as
shown in Figure 3. The internal reference can be accessed
via the comparator as shown in Figure 4.

V

BAT

R1

R2

49.9k
1%

R1 = (V
HYSTERESIS ≈ 30mV

Figure 3. R3 Adds Hysteresis to Low-Battery Detector

–1.24V) (43.5k)

TRIP

1.24V

LBI

V

–

+

IN

LT1305

R3
2M

LBO

5V

R4
47k

LT1305 • F03

Inductor Selection

Inductors used with the LT1305 must fulfill two require­ments. First, the inductor must be able to handle current
of 2A to 2.5A without runaway saturation. Rod or drum
core units usually saturate gradually and it is acceptable to
exceed manufacturer’s published saturation current by
20% or so. Second, the unit must have low DCR, under

0.05Ω so that copper loss is kept low and excess heating
is avoided. Inductance value is not critical. Generally, for
low voltage inputs below 3V a 10µH inductor is recom-
mended (such as Coilcraft DO3316-103). For inputs above
4V to 5V use a 22µ H unit (such as Coilcraft DO3316-223).
Switching frequency can reach up to 300kHz so the core
material should be able to operate at high frequency
without excessive core loss. Ferrite or molypermalloy
cores are a better choice than powdered iron. If EMI is a
concern, a toroidal inductor is suggested, such as
Coiltronics CTX20-4.

Capacitor Selection

Output and input capacitors should have low ESR for best
performance. Inexpensive aluminum electrolytics some­times have ESR above 1Ω, even for relatively large values
such as 100µF, 16V units. Since the LT1305 has a 2A
current limit, 2V of ripple voltage would result with such a
capacitor at the output. Keep ESR below 0.05Ω to 0.1Ω for
reasonable ripple voltage. Tantalum capacitors such as
AVX TPS series or Sprague 593D have low ESR and are
surface mount components. For lowest ESR, use Sanyo
OS-CON units (OS-CON is also available from Vishay).
These capacitors have superior ESR, small size and per­form well at cold temperatures.

6

100k

2N3906

V

REF

OUTPUT

= 1.24V 1 + 

V

REF

VIN ≥ V

REF

R1 + R2 ≈ 33k

Figure 4. Accessing Internal Reference

R2

+

2.2µF

R2

()

R1

+ 200mV

V

IN

LBO

LT1305

LBI

GND

R1

LT1305 • F04

Diode Selection

A 2A Schottky diode such as Motorola MBRS130LT3 is a
good choice for the rectifier diode. A 1N5821 or
MBRS130T3 are suitable as well. Do not use “general
purpose” diodes such as 1N4001. They are much too slow
for use in switching regulator applications.

U

V

IN

3.5V to 6.5V

SHDN

100k

4 CELLS

100µF

150µF**

22µH*

100µF**

SHUTDOWN

LT1305

GND PGND

FB

SW

LT1305 • TA05

L1*
22µH

309k

+

COILCRAFT DO3316-223 OR
SUMIDA CD105-220

SANYO OS-CON

*

**



5V
400mA

+

MBRS130LT3

+

TYPICAL APPLICATIONS N

LT1305

4-Cell-to-5V ConverterSetting Output Voltage

SW

1N5817

V

OUT

R2

+

FB

220µF

R1

LT1305 • TA03

V

IN

+

100µF

V

= 1.24V 1 +

OUT

L1

V

IN

LT1305

GND PGND

R2

()

R1

5V Step-Up Converter with Reference Output

MBRS130LT3

SW

FB

309k

100k

5V
400mA

+

220µF

V

REF

OUTPUT

1.24V

1.8V TO 4.5V
INPUT

2N3906



+

2.2µF

33k

100k

+

100µF

10µH*

V

IN

LBO

LT1305

LBI

GND PGND

1.5V TO 8V



V

IN

100Hz TO 1000Hz

+

47µF

SQUARE WAVE

DRIVE

*COILCRAFT DO3316-103

LT1305 • TA06

EL Panel Driver

T1**

1:15

•

4, 5 6

•

SW

FBSHDN

OUT

83V

10

R1†
25k

1N5818

RMS

3.3M

3.3M

51k

TO 115V

10Ω

1, 2

V

††

0.1µF
CERAMIC

SHUTDOWN

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.

IN

LT1305

GND PGND

ADD C1 FOR OPEN-PANEL PROTECTION

*

DALE LPE5047-A132 1:15 TURNS RATIO 

**

10µH PRIMARY INDUCTANCE [phone]



†

R1 ADJUSTS V

††

AVX TPS OR SANYO OS-CON MUST HAVE ESR ≤0.15Ω

3.3M

1N4148

RMS

MUR160

10k

4.7µF

160V

+

1k
1/2W

ZETEX
FZT658

LT1305 • TA04

C1*
50pF

EL PANEL

C

PANEL

≤ 100nF

7

LT1305

PACKAGE DESCRIPTION

U

Dimensions in inches (millimeters) unless otherwise noted.

S8 Package

8-Lead Plastic SOIC

0.189 – 0.197*
(4.801 – 5.004)

7

8

5

6

4

0.050

(1.270)

BSC

0.150 – 0.157*
(3.810 – 3.988)

0.004 – 0.010

(0.101 – 0.254)

SO8 0294

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).

0.010 – 0.020

0.016 – 0.050

0.406 – 1.270

× 45°

0°– 8° TYP

0.228 – 0.244

(5.791 – 6.197)

0.053 – 0.069

(1.346 – 1.752)

0.014 – 0.019

(0.355 – 0.483)

1

3

2

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8

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

[phone]

●

FAX

: [phone]

●

TELEX

: 499-3977

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