Datasheet LT1308A, LT1308B Datasheet (LINEAR TECHNOLOGY)

FEATURES

LT1308A/LT1308B

High Current, Micropower

Single Cell, 600kHz

DC/DC Converters

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DESCRIPTIO

■

5V at 1A from a Single Li-Ion Cell

■

5V at 800mA in SEPIC Mode from Four NiCd Cells

■

Fixed Frequency Operation: 600kHz

■

Boost Converter Outputs up to 34V

■

Starts into Heavy Loads

■

Automatic Burst ModeTM Operation at
Light Load (LT1308A)

■

Continuous Switching at Light Loads (LT1308B)

■

Low V

■

Pin-for-Pin Upgrade Compatible with LT1308

■

Lower Quiescent Current in Shutdown: 1μA (Max)

■

Improved Accuracy Low-Battery Detector

Switch: 300mV at 2A

CESAT

Reference: 200mV ± 2%

■

Available in 8-Lead SO and 14-Lead TSSOP Packages

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APPLICATIO S

■

GSM/CDMA Phones

■

Digital Cameras

■

LCD Bias Supplies

■

Answer-Back Pagers

■

GPS Receivers

■

Battery Backup Supplies

■

Handheld Computers

The LT®1308A/LT1308B are micropower, fixed frequency
step-up DC/DC converters that operate over a 1V to 10V
input voltage range. They are improved versions of the
LT1308 and are recommended for use in new designs. The
LT1308A features automatic shifting to power saving
Burst Mode operation at light loads and consumes just
140μA at no load. The LT1308B features continuous
switching at light loads and operates at a quiescent current
of 2.5mA. Both devices consume less than 1μA in
shutdown.

Low-battery detector accuracy is significantly tighter than
the LT1308. The 200mV reference is specified at ± 2% at
room and ± 3% over temperature. The shutdown pin
enables the device when it is tied to a 1V or higher source
and does not need to be tied to VIN as on the LT1308. An
internal VC clamp results in improved transient response
and the switch voltage rating has been increased to 36V,
enabling higher output voltage applications.

The LT1308A/LT1308B are available in the 8-lead SO and
the 14-lead TSSOP packages.

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst
Mode is a registered trademark of Linear Technology Corporation. All other trademarks are
the property of their respective owners.

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TYPICAL APPLICATIO

L1

4.7μH

V

+

C1
47μF

Li-Ion
CELL

C1: AVX TAJC476M010
C2: AVX TPSD227M006
D1: IR 10BQ015

IN

LBI

LT1308B

SHDNSHUTDOWN

V

C

47k

100pF

L1: MURATA LQH6C4R7
*R1: 887k FOR V

Figure 1. LT1308B Single Li-Ion Cell to 5V/1A DC/DC Converter

SW

LBO

GND

OUT

FB

D1

= 12V

R1*
309k

R2
100k

5V
1A

+

C2
220μF

1308A/B F01a

95

90

85

80

75

70

EFFICIENCY (%)

65

60

55

50

1

Converter Efficiency

VIN = 3.6V

VIN = 1.5V

10 100 1000

LOAD CURRENT (mA)

VIN = 4.2V

VIN = 2.5V

1308A/B F01b

1308abfa

1

LT1308A/LT1308B

A

W

O

LUTEXI TIS

S

A

WUW

U

(Note 1)

ARB

G

VIN, SHDN, LBO Voltage ......................................... 10V

SW Voltage ............................................... –0.4V to 36V

FB Voltage ....................................................... VIN + 1V

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

LBI Voltage ................................................. –0.1V to 1V

Current into FB Pin .............................................. ±1mA

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PI CO FIGURATIO

TOP VIEW

LBO

8

LBI

7

V

6

IN

SW

5

SHDN

GND

V

C

FB

T

JMAX

1

2

3

4

S8 PACKAGE

8-LEAD PLASTIC SO

= 125°C, θJA = 190°C/W

Operating Temperature Range

Commercial ............................................ 0°C to 70°C

Extended Commerial (Note 2) ........... – 40°C to 85°C

Industrial ........................................... –40°C to 85°C

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

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

TOP VIEW

1

V

C

2

FB

3

SHDN

4

GND

5

GND

6

GND

7

GND

F PACKAGE

14-LEAD PLASTIC TSSOP

= 125°C, θJA = 80°C/W

T

JMAX

(Note 6)

LBO

14

LBI

13

V

12

IN

V

11

IN

SW

10

SW

9

SW

8

NOT RECOMMENDED FOR NEW DESIGNS

Contact Linear Technology for Potential Replacement

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ORDER I FOR ATIO

LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE

LT1308ACS8#PBF LT1308ACS8#TRPBF 1308A 8-Lead Plastic SO 0°C to 70°C

LT1308AIS8#PBF LT1308AIS8#TRPBF 1308AI 8-Lead Plastic SO –40°C to 85°C

LT1308BCS8#PBF LT1308BCS8#TRPBF 1308B 8-Lead Plastic SO 0°C to 70°C

LT1308BIS8#PBF LT1308BIS8#TRPBF 1308BI 8-Lead Plastic SO –40°C to 85°C

LT1308ACF#PBF LT1308ACF#TRPBF LT1308ACF 14-Lead Plastic TSSOP 0°C to 70°C

LT1308BCF#PBF LT1308BCF#TRPBF LT1308BCF 14-Lead Plastic TSSOP 0°C to 70°C

LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE

LT1308ACS8 LT1308ACS8#TR 1308A 8-Lead Plastic SO 0°C to 70°C

LT1308AIS8 LT1308AIS8#TR 1308AI 8-Lead Plastic SO –40°C to 85°C

LT1308BCS8 LT1308BCS8#TR 1308B 8-Lead Plastic SO 0°C to 70°C

LT1308BIS8 LT1308BIS8#TR 1308BI 8-Lead Plastic SO –40°C to 85°C

LT1308ACF LT1308ACF#TR LT1308ACF 14-Lead Plastic TSSOP 0°C to 70°C

LT1308BCF LT1308BCF#TR LT1308BCF 14-Lead Plastic TSSOP 0°C to 70°C

Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/

2

1308abfa

LT1308A/LT1308B

ELECTRICAL CHARACTERISTICS

range, otherwise specifications are TA = 25°C. Commercial Grade 0°C to 70°C. VIN = 1.1V, V

The ● denotes specifications which apply over the full operating temperature

= VIN, unless otherwise noted.

SHDN

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

Q

Quiescent Current Not Switching, LT1308A 140 240 μA

Switching, LT1308B 2.5 4 mA

= 0V (LT1308A/LT1308B) 0.01 1 μA

V

SHDN

V

FB

I

B

Feedback Voltage ● 1.20 1.22 1.24 V

FB Pin Bias Current (Note 3) ● 27 80 nA

Reference Line Regulation 1.1V ≤ VIN ≤ 2V ● 0.03 0.4 %/V

2V ≤ V

≤ 10V 0.01 0.2 %/V

IN

Minimum Input Voltage 0.92 1 V

g

A

f

m

V

OSC

Error Amp Transconductance ΔI = 5μA60μmhos

Error Amp Voltage Gain 100 V/V

Switching Frequency VIN = 1.2V ● 500 600 700 kHz

Maximum Duty Cycle ● 82 90 %

Switch Current Limit Duty Cyle = 30% (Note 4) 2 3 4.5 A

Switch V

CESAT

ISW = 2A (25°C, 0°C), VIN = 1.5V 290 350 mV
I

= 2A (70°C), VIN = 1.5V 330 400 mV

SW

Burst Mode Operation Switch Current Limit VIN = 2.5V, Circuit of Figure 1 400 mA
(LT1308A)

Shutdown Pin Current V

= 1.1V ● 25μA

SHDN

= 6V ● 20 35 μA

V

SHDN

= 0V ● 0.01 0.1 μA

V

SHDN

LBI Threshold Voltage 196 200 204 mV

● 194 200 206 mV

LBO Output Low I

LBO Leakage Current V

LBI Input Bias Current (Note 5) V

= 50μA ● 0.1 0.25 V

SINK

= 250mV, V

LBI

= 150mV 33 100 nA

LBI

= 5V ● 0.01 0.1 μA

LBO

Low-Battery Detector Gain 3000 V/V

Switch Leakage Current VSW = 5V ● 0.01 10 μA

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
Industrial Grade –40°C to 85°C. VIN = 1.2V, V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

Q

V

FB

I

B

g

m

A

V

Quiescent Current Not Switching, LT1308A ● 140 240 μA

Feedback Voltage ● 1.19 1.22 1.25 V

FB Pin Bias Current (Note 3) ● 27 80 nA

Reference Line Regulation 1.1V ≤ VIN ≤ 2V ● 0.05 0.4 %/V

Minimum Input Voltage 0.92 1 V

Error Amp Transconductance ΔI = 5μA60μmhos

Error Amp Voltage Gain 100 V/V

= VIN, unless otherwise noted.

SHDN

Switching, LT1308B
V

= 0V (LT1308A/LT1308B) ● 0.01 1 μA

SHDN

2V ≤ V

≤ 10V ● 0.01 0.2 %/V

IN

● 2.5 4 mA

1308abfa

3

LT1308A/LT1308B

ELECTRICAL CHARACTERISTICS

range, otherwise specifications are TA = 25°C. Industrial Grade –40°C to 85°C. VIN = 1.2V, V

The ● denotes specifications which apply over the full operating temperature

= VIN, unless otherwise noted.

SHDN

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

f

OSC

Switching Frequency ● 500 600 750 kHz

Maximum Duty Cycle ● 82 90 %

Switch Current Limit Duty Cyle = 30% (Note 4) 2 3 4.5 A

Switch V

CESAT

ISW = 2A (25°C, –40°C), VIN = 1.5V 290 350 mV

= 2A (85°C), VIN = 1.5V 330 400 mV

I

SW

Burst Mode Operation Switch Current Limit VIN = 2.5V, Circuit of Figure 1 400 mA
(LT1308A)

Shutdown Pin Current V

= 1.1V ● 2 5μA

SHDN

= 6V ● 20 35 μA

V

SHDN

= 0V 0.01 0.1 μA

V

SHDN

LBI Threshold Voltage 196 200 204 mV

● 193 200 207 mV

LBO Output Low I

LBO Leakage Current V

LBI Input Bias Current (Note 5) V

= 50μA ● 0.1 0.25 V

SINK

= 250mV, V

LBI

= 150mV 33 100 nA

LBI

= 5V ● 0.01 0.1 μA

LBO

Low-Battery Detector Gain 3000 V/V

Switch Leakage Current VSW = 5V ● 0.01 10 μA

Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.

Note 2: The LT1308ACS8, LT1308ACF, LT1308BCS8 and LT1308BCF are
designed, characterized and expected to meet the industrial temperature
limits, but are not tested at – 40°C and 85°C. I grade devices are
guaranteed over the –40°C to 85°C operating temperature range.

Note 4: Switch current limit guaranteed by design and/or correlation to
static tests. Duty cycle affects current limit due to ramp generator (see
Block Diagram).

Note 5: Bias current flows out of LBI pin.
Note 6: Connect the four GND pins (Pins 4–7) together at the device.

Similarly, connect the three SW pins (Pins 8–10) together and the two V

IN

pins (Pins 11, 12) together at the device.

Note 3: Bias current flows into FB pin.

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TYPICAL PERFORMANCE CHARACTERISTICS

LT1308B

3.3V Output Efficiency

95

90

85

80

75

70

EFFICIENCY (%)

65

60

55

50

1 100 1000

VIN = 1.8V

10

LOAD CURRENT (mA)

VIN = 2.5V

VIN = 1.2V

1308A/B G01

LT1308A

3.3V Output Efficiency

95

90

85

80

75

70

EFFICIENCY (%)

65

60

55

50

VIN = 1.8V

1 100 1000

10

LOAD CURRENT (mA)

VIN = 2.5V

VIN = 1.2V

1308A/B G02

4

LT1308A
5V Output Efficiency

95

90

VIN = 3.6V

85

80

75

70

EFFICIENCY (%)

65

60

55

50

1

LOAD CURRENT (mA)

VIN = 4.2V

VIN = 1.5V

10 100 1000

VIN = 2.5V

1308A/B G03

1308abfa

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TEMPERATURE (°C)

–50 –25

V

REF

(mV)

05025

75

100

1308 • G09

203

202

201

200

199

198

197

196

195

TYPICAL PERFORMANCE CHARACTERISTICS

LT1308A/LT1308B

LT1308B
12V Output Efficiency

90

85

80

75

70

65

EFFICIENCY (%)

60

55

50

1 100 1000

VIN = 5V

VIN = 3.3V

10

LOAD CURRENT (mA)

SHDN Pin Bias Current vs Voltage

50

40

30

20

SHDN PIN CURRENT (μA)

10

0

2

0

SHDN PIN VOLTAGE (V)

6

4

8

1308A/B G04

–40°C

25°C

85°C

1308 G07

Switch Current Limit vs
Duty Cycle

4.0

3.5

3.0

CURRENT LIMIT (A)

2.5

2.0
20

0

DUTY CYCLE (%)

60

80

40

100

1308 • G05

FB, LBI Bias Current vs
Temperature

80

70

60

50

40

30

BIAS CURRENT (nA)

20

10

0

10

–50 –25

LBI

05025

TEMPERATURE (°C)

FB

75

100

1308 • G08

Switch Saturation Voltage
vs Current

500

400

(mV)

300

CESAT

200

SWITCH V

100

0

0

0.5
SWITCH CURRENT (A)

25°C

1.0

Low Battery Detector Reference
vs Temperature

1.5

85°C

–40°C

2.0

1308 G06

Oscillator Frequency vs
Temperature

800

750

700

650

600

550

FREQUENCY (kHz)

500

450

400

–50 –2.5

05025

TEMPERATURE (°C)

75

1308 • G10

100

LT1308A Quiescent Current vs
Temperature

180

170

160

150

140

130

120

QUIESCENT CURRENT (μA)

110

100

–50 –25

05025

TEMPERATURE (°C)

75

1308 • G11

100

Feedback Pin Voltage vs
Temperature

1.25

1.24

1.23

1.22

(V)

FB

V

1.21

1.20

1.19

1.18
–50 –25

05025

TEMPERATURE (°C)

75

1308 • G12

1308abfa

100

5

LT1308A/LT1308B

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PIN FUNCTIONS

(SO/TSSOP)

VC (Pin 1/Pin 1): Compensation Pin for Error Amplifier.

Connect a series RC from this pin to ground. Typical
values are 47kΩ and 100pF. Minimize trace area at V

FB (Pin 2/Pin 2): Feedback Pin. Reference voltage is

1.22V. Connect resistive divider tap here. Minimize trace
area at FB. Set V

V

= 1.22V(1 + R1/R2).

OUT

SHDN (Pin 3/Pin 3): Shutdown. Ground this pin to turn
off switcher. To enable, tie to 1V or more. SHDN does not
need to be at VIN to enable the device.

GND (Pin 4/Pins 4, 5, 6, 7): Ground. Connect directly to
local ground plane. Ground plane should enclose all
components associated with the LT1308. PCB copper
connected to these pins also functions as a heat sink. For
the TSSOP package, connect all pins to ground copper to
get the best heat transfer. This keeps chip heating to a
minimum.

according to:

OUT

.

C

SW (Pin 5/Pins 8, 9, 10): Switch Pins. Connect inductor/
diode here. Minimize trace area at these pins to keep EMI
down. For the TSSOP package, connect all SW pins
together at the package.

(Pin 6/Pins 11, 12): Supply Pins. Must have local

V

IN

bypass capacitor right at the pins, connected directly to
ground. For the TSSOP package, connect both V
together at the package.

LBI (Pin 7/Pin 13): Low-Battery Detector Input. 200mV
reference. Voltage on LBI must stay between –100mV
and 1V. Low-battery detector does not function with
SHDN pin grounded. Float LBI pin if not used.

LBO (Pin 8/Pin 14): Low-Battery Detector Output. Open
collector, can sink 50μA. A 220kΩ pull-up is recom-
mended. LBO is high impedance when SHDN is grounded.

IN

pins

6

1308abfa

W

BLOCK DIAGRA S

V

IN

V

OUT

R1
(EXTERNAL)

R2
(EXTERNAL)

6

FB

FB

2

R5
40k

Q1

Figure 2a. LT1308A/LT1308B Block Diagram (SO-8 Package)

Q2
×10

R6
40k

R3
30k

R4
140k

V

IN

+

g

m

–

AMPLIFIER

RAMP

GENERATOR

600kHz

OSCILLATOR

ERROR

V

IN

Q4

V

C

1

2V

BE

+

*

BIAS

–

A1

COMPARATOR

–

+

Σ

+

*HYSTERESIS IN LT1308A ONLY

+

A2

ENABLE

200mV

R

LT1308A/LT1308B

SHDN

SHUTDOWN

LBI

7

+

–

A4

FF

Q

S

DRIVER

A = 3

3

LBO

8

SW

5

Q3

+

0.03Ω

–

4

GND

1308 BD2a

V

OUT

R1
(EXTERNAL)

R2
(EXTERNAL)

V

IN

+

*

–

A1

COMPARATOR

–

+

A2

2V

BE

SHDN

3

LBO

14

SW

8SW9

SW

10

Q3

ENABLE

200mV

R

SHUTDOWN

LBI

13

+

–

A4

FF

Q

S

DRIVER

+

A = 3

0.03Ω

–

4

GND5GND6GND

GND

7

1308 BD2b

V

11

IN

V

IN

12

R5
40k

R6
40k

V

IN

+

g

m

Q4

V

C

1

–

ERROR

FB

FB

Q1

2

Q2
×10

R3
30k

R4
140k

AMPLIFIER

RAMP

GENERATOR

BIAS

+

Σ

+

600kHz

OSCILLATOR

*HYSTERESIS IN LT1308A ONLY

Figure 2b. LT1308A/LT1308B Block Diagram (TSSOP Package)

1308abfa

7

LT1308A/LT1308B

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WUU

APPLICATIONS INFORMATION

OPERATION

The LT1308A combines a current mode, fixed frequency
PWM architecture with Burst Mode micropower operation
to maintain high efficiency at light loads. Operation can be
best understood by referring to the block diagram in Figure

2. Q1 and Q2 form a bandgap reference core whose loop
is closed around the output of the converter. When VIN is
1V, the feedback voltage of 1.22V, along with an 80mV
drop across R5 and R6, forward biases Q1 and Q2’s base
collector junctions to 300mV. Because this is not enough
to saturate either transistor, FB can be at a higher voltage
than V

1.22V, causing V
decrease. When VC reaches the bias voltage on hysteretic
comparator A1, A1’s output goes low, turning off all
circuitry except the input stage, error amplifier and low­battery detector. Total current consumption in this state is
140μA. As output loading causes the FB voltage to
decrease, A1’s output goes high, enabling the rest of the
IC. Switch current is limited to approximately 400mA
initially after A1’s output goes high. If the load is light, the
output voltage (and FB voltage) will increase until A1’s
output goes low, turning off the rest of the LT1308A. Low
frequency ripple voltage appears at the output. The ripple
frequency is dependent on load current and output capaci­tance. This Burst Mode operation keeps the output regu­lated and reduces average current into the IC, resulting in
high efficiency even at load currents of 1mA or less.

If the output load increases sufficiently, A1’s output
remains high, resulting in continuous operation. When the
LT1308A is running continuously, peak switch current is
controlled by VC to regulate the output voltage. The switch
is turned on at the beginning of each switch cycle. When
the summation of a signal representing switch current and
a ramp generator (introduced to avoid subharmonic oscil­lations at duty factors greater than 50%) exceeds the V
signal, comparator A2 changes state, resetting the flip-flop
and turning off the switch. Output voltage increases as
switch current is increased. The output, attenuated by a
resistor divider, appears at the FB pin, closing the overall
loop. Frequency compensation is provided by an external
series RC network connected between the VC pin and
ground.

. When there is no load, FB rises slightly above

IN

(the error amplifier’s output) to

C

C

Low-battery detector A4’s open-collector output (LBO)
pulls low when the LBI pin voltage drops below 200mV.
There is no hysteresis in A4, allowing it to be used as an
amplifier in some applications. The entire device is dis­abled when the SHDN pin is brought low. To enable the
converter, SHDN must be at 1V or greater. It need not be
tied to VIN as on the LT1308.

The LT1308B differs from the LT1308A in that there is no
hysteresis in comparator A1. Also, the bias point on A1 is
set lower than on the LT1308B so that switching can occur
at inductor current less than 100mA. Because A1 has no
hysteresis, there is no Burst Mode operation at light loads
and the device continues switching at constant frequency.
This results in the absence of low frequency output voltage
ripple at the expense of efficiency.

The difference between the two devices is clearly illus­trated in Figure 3. The top two traces in Figure 3 shows an
LT1308A/LT1308B circuit, using the components indi­cated in Figure 1, set to a 5V output. Input voltage is 3V.
Load current is stepped from 50mA to 800mA for both
circuits. Low frequency Burst Mode operation voltage
ripple is observed on Trace A, while none is observed on
Trace B.

At light loads, the LT1308B will begin to skip alternate
cycles. The load point at which this occurs can be de­creased by increasing the inductor value. However, output
ripple will continue to be significantly less than the LT1308A
output ripple. Further, the LT1308B can be forced into
micropower mode, where I

falls from 3mA to 200μA by

Q

sinking 40μA or more out of the VC pin. This stops
switching by causing A1’s output to go low.

TRACE A: LT1308A

, 100mV/DIV

V

OUT

AC COUPLED

TRACE B: LT1308B

V

, 100mV/DIV

OUT

AC COUPLED

800mA

I

LOAD

50mA

V

= 3V 200μs/DIV 1308 F03

IN

(CIRCUIT OF FIGURE 1)

Figure 3. LT1308A Exhibits Burst Mode Operation Output
Voltage Ripple at 50mA Load, LT1308B Does Not

8

1308abfa

LT1308A/LT1308B

U

WUU

APPLICATIONS INFORMATION

Waveforms for a LT1308B 5V to 12V boost converter
using a 10μF ceramic output capacitor are pictured in
Figures 4 and 5. In Figure 4, the converter is operating in
continuous mode, delivering a load current of approxi­mately 500mA. The top trace is the output. The voltage
increases as inductor current is dumped into the output
capacitor during the switch off time, and the voltage
decreases when the switch is on. Ripple voltage is in this
case due to capacitance, as the ceramic capacitor has little
ESR. The middle trace is the switch voltage. This voltage
alternates between a V
The lower trace is the switch current. At the beginning of
the switch cycle, the current is 1.2A. At the end of the
switch on time, the current has increased to 2A, at which
point the switch turns off and the inductor current flows
into the output capacitor through the diode. Figure 5
depicts converter waveforms at a light load. Here the
converter operates in discontinuous mode. The inductor
current reaches zero during the switch off time, resulting
in some ringing at the switch node. The ring frequency is
set by switch capacitance, diode capacitance and induc­tance. This ringing has little energy, and its sinusoidal
shape suggests it is free from harmonics. Minimizing the
copper area at the switch node will prevent this from
causing interference problems.

V

OUT

100mV/DIV

V

SW

10V/DIV

I

SW

1A/DIV

Figure 4. 5V to 12V Boost Converter Waveforms in
Continuous Mode. 10μF Ceramic Capacitor Used at Output

V

OUT

20mV/DIV

V

SW

10V/DIV

CESAT

and V

500ns/DIV

plus the diode drop.

OUT

LAYOUT HINTS

The LT1308A/LT1308B switch current at high speed,
mandating careful attention to layout for proper perfor­mance.

careless layout

You will not get advertised performance with

. Figure 6 shows recommended component
placement for an SO-8 package boost (step-up) 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 negative terminal of output capacitor C2 should tie
close to the ground pin(s) of the LT1308A/LT1308B. 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.

LBI

GROUND PLANE

R1

SHUTDOWN

MULTIPLE

VIAs

R2

GND

1

2

LT1308A
LT1308B

3

4

C2

Figure 6. Recommended Component Placement for SO-8
Package Boost Converter. Note Direct High Current Paths
Using Wide PC Traces. Minimize Trace Area at Pin 1 (VC) and
Pin 2 (FB). Use Multiple Vias to Tie Pin 4 Copper to Ground
Plane. Use Vias at One Location Only to Avoid Introducing
Switching Currents into the Ground Plane

LBO

C1

+

8

7

6

5

V

IN

L1

+

D1

V

OUT

1308 F04

I

SW

500mA/DIV

500ns/DIV

Figure 5. Converter Waveforms in Discontinuous Mode

Figure 7 shows recommended component placement for
a boost converter using the TSSOP package. Placement is
similar to the SO-8 package layout.

1308abfa

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LT1308A/LT1308B

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APPLICATIONS INFORMATION

LBI

GROUND PLANE

R1

SHUTDOWN

MULTIPLE

VIAs

R2

GND

1

2

3

4

5

6

7

LT1308A
LT1308B

C2

Figure 7. Recommended Component
Placement for TSSOP Boost Converter.
Placement is Similar to Figure 4.

LBO

C1

+

14

13

12

11

10

9

8

+

V

OUT

V

IN

L1

D1

1308 F07

A SEPIC (Single-Ended Primary Inductance Converter)
schematic is shown in Figure 8. This converter topology
produces a regulated output over an input voltage range
that spans (i.e., can be higher or lower than) the output.
Recommended component placement for an SO-8 pack­age SEPIC is shown in Figure 9.

C2

4.7μF

CERAMIC

SW

L1B

R1

FB

GND

D1: IR 10BQ015
L1: COILTRONICS CTX10-2

309k

R2
100k

D1

V

OUT

5V
500mA

+

C3
220μF

6.3V

1308A/B F08

SHDNSHUTDOWN

V

V

CTX10-2

IN

LT1308B

C

47k

680pF

V

IN

3V TO

10V

+

C1
47μF

C1: AVX TAJC476M016
C2: TAIYO YUDEN EMK325BJ475(X5R)
C3: AVX TPSD227M006

Figure 8. SEPIC (Single-Ended Primary
Inductance Converter) Converts 3V to 10V
Input to a 5V/500mA Regulated Output

L1A

GROUND PLANE

R1

R2

SHUTDOWN

MULTIPLE

VIAs

Figure 9. Recommended Component Placement for SEPIC

GND

1

2

3

4

C1

LT1308A
LT1308B

C3

+

LBI

LBO

+

8

7

6

5

V

OUT

V

IN

L1A L1B

C2

D1

1308 F09

1308abfa

10

LT1308A/LT1308B

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APPLICATIONS INFORMATION

SHDN PIN

The LT1308A/LT1308B SHDN pin is improved over the
LT1308. The pin does not require tying to V
device, but needs only a logic level signal. The voltage on
the SHDN pin can vary from 1V to 10V independent of V
Further, floating this pin has the same effect as grounding,
which is to shut the device down, reducing current drain
to 1μA or less.

LOW-BATTERY DETECTOR

The low-battery detector on the LT1308A/LT1308B fea­tures improved accuracy and drive capability compared to
the LT1308. The 200mV reference has an accuracy of ±2%
and the open-collector output can sink 50μA. The LT1308A/
LT1308B low-battery detector is a simple PNP input gain
stage with an open-collector NPN output. The negative
input of the gain stage is tied internally to a 200mV
reference. The positive input is the LBI pin. Arrangement
as a low-battery detector is straightforward. Figure 10
details hookup. R1 and R2 need only be low enough in
value so that the bias current of the LBI pin doesn’t cause
large errors. For R2, 100k is adequate. The 200mV refer­ence can also be accessed as shown in Figure 11.

R1

LBI

R2
100k

V

LT1308A

IN

+

LT1308B

LBO

–

to enable the

IN

5V

100k

TO PROCESSOR

IN

.

A cross plot of the low-battery detector is shown in
Figure 12. The LBI pin is swept with an input which varies
from 195mV to 205mV, and LBO (with a 100k pull-up
resistor) is displayed.

V

LBO

1V/DIV

195 200 205

(mV) 1308 F12

V

LBI

Figure 12. Low-Battery Detector
Input/Output Characteristic

START-UP

The LT1308A/LT1308B can start up into heavy loads,
unlike many CMOS DC/DC converters that derive operat­ing voltage from the output (a technique known as
“bootstrapping”). Figure 13 details start-up waveforms of
Figure 1’s circuit with a 20Ω load and VIN of 1.5V. Inductor
current rises to 3.5A as the output capacitor is charged.
After the output reaches 5V, inductor current is about 1A.
In Figure 14, the load is 5Ω and input voltage is 3V. Output
voltage reaches 5V in 500μs after the device is enabled.
Figure 15 shows start-up behavior of Figure 5’s SEPIC
circuit, driven from a 9V input with a 10Ω load. The output
reaches 5V in about 1ms after the device is enabled.

200mV

V

BAT

INTERNAL
REFERENCE

GND

1308 F10

Figure 10. Setting Low-Battery Detector Trip Point

200k

V

BAT

2N3906

V

REF

200mV

+

10k

LBO

LBI

10μF

Figure 11. Accessing 200mV Reference

R1 =

V

IN

LT1308A
LT1308B

GND

1308 F11

V

LB

– 200mV

2μA

V

OUT

2V/DIV

I

L1

1A/DIV

V

SHDN

5V/DIV

1ms/DIV

Figure 13. 5V Boost Converter of Figure 1.
Start-Up from 1.5V Input into 20Ω Load

1308 F13

1308abfa

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LT1308A/LT1308B

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APPLICATIONS INFORMATION

V

OUT

1V/DIV

I

L1

2A/DIV

V

SHDN

5V/DIV

Figure 14. 5V Boost Converter of Figure 1.
Start-Up from 3V Input into 5Ω Load

V

OUT

2V/DIV

I

SW

2A/DIV

V

SHDN

5V/DIV

Figure 15. 5V SEPIC Start-Up from 9V Input into 10Ω Load

Soft-Start

In some cases it may be undesirable for the LT1308A/
LT1308B to operate at current limit during start-up, e.g.,

500μs/DIV

1308 F14

500μs/DIV

1308 F15

when operating from a battery composed of alkaline
cells. The inrush current may cause sufficiency internal
voltage drop to trigger a low-battery indicator. A pro­grammable soft-start can be implemented with 4 discrete
components. A 5V to 12V boost converter using the
LT1308B is detailed in Figure 16. C4 differentiates V
causing a current to flow into R3 as V

increases.

OUT

OUT

,

When this current exceeds 0.7V/33k, or 21μA, current
flows into the base of Q1. Q1’s collector then pulls
current out the VC pin, creating a feedback loop where the
slope of V

ΔΔV

OUT

With C4 = 33nF, V

is limited as follows:

OUT

V

07

=

t

kC

33 4.•

OUT

/t is limited to 640mV/ms. Start-up

waveforms for Figure 16’s circuit are pictured in Figure

17. Without the soft-start circuit implemented, the inrush
current reaches 3A. The circuit reaches final output
voltage in approximately 250μs. Adding the soft-start
components reduces inductor current to less than 1A, as
detailed in Figure 18, while the time required to reach final
output voltage increases to about 15ms. C4 can be
adjusted to achieve any output slew rate desired.

12

V

IN

5V

+

C1
47μF

C4
33nF

R3
33k

C1: AVX TAJ476M010
C2: TAIYO YUDEN TMK432BJ106MM
D1: IR 10BQ015
L1: MURATA LQH6C4R7
Q1: 2N3904

SHUTDOWN

R4

33k

Q1

V

IN

SHDN

LT1308B

V

C

R

C

47k

SOFT-START
COMPONENTS

L1

4.7μH

C

C

100pF

SW

GND

D1

330pF

100k

10k

FB

11.3k

1308 F16

C2
10μF

V

OUT

12V
500mA

Figure 16. 5V to 12V Boost Converter with Soft-Start Components Q1, C4, R3 and R4.

1308abfa

LT1308A/LT1308B

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APPLICATIONS INFORMATION

12V

V

OUT

5V/DIV

5V

I

L1

1A/DIV

V

SHDN

10V/DIV

50μs/DIV

Figure 17. Start-Up Waveforms of Figure 16’s Circuit
without Soft-Start Components

12V

V

OUT

5V

I

L1

1A/DIV

V

SHDN

10V/DIV

5ms/DIV

Figure 18. Start-Up Waveforms of Figure 16’s Circuit
with Soft-Start Components Added

COMPONENT SELECTION

Diodes

We have found ON Semiconductor MBRS130 and Interna­tional Rectifier 10BQ015 to perform well. For applications
where V

exceeds 30V, use 40V diodes such as

OUT

MBRS140 or 10BQ040.

Height limited applications may benefit from the use of the
MBRM120. This component is only 1mm tall and offers
performance similar to the MBRS130.

Inductors

Suitable inductors for use with the LT1308A/LT1308B
must fulfill two requirements. First, the inductor must be
able to handle current of 2A steady-state, as well as
support transient and start-up current over 3A without
inductance decreasing by more than 50% to 60%. Second,
the DCR of the inductor should have low DCR, under 0.05Ω

1308 F17

1308 F18

so that copper loss is minimized. Acceptable inductance
values range between 2μH and 20μH, with 4.7μH best for
most applications. Lower value inductors are physically
smaller than higher value inductors for the same current
capability.

Table 1 lists some inductors we have found to perform well
in LT1308A/LT1308B application circuits. This is not an
exclusive list.

Table 1

VENDOR PART NO. VALUE PHONE NO.

Murata LQH6C4R7 4.7μH 770-436-1300

Sumida CDRH734R7 4.7μH 847-956-0666

Coiltronics CTX5-1 5μH 561-241-7876

Coilcraft LPO2506IB-472 4.7μH 847-639-6400

Capacitors

Equivalent Series Resistance (ESR) is the main issue
regarding selection of capacitors, especially the output
capacitors.

The output capacitors specified for use with the LT1308A/
LT1308B circuits have low ESR and are specifically
designed for power supply applications. Output voltage
ripple of a boost converter is equal to ESR multiplied by
switch current. The performance of the AVX TPSD227M006
220μF tantalum can be evaluated by referring to Figure 3.
When the load is 800mA, the peak switch current is
approximately 2A. Output voltage ripple is about 60mV

, so the ESR of the output capacitor is 60mV/2A or 0.03Ω.

P

P-

Ripple can be further reduced by paralleling ceramic units.

Table 2 lists some capacitors we have found to perform
well in the LT1308A/LT1308B application circuits. This is
not an exclusive list.

Table 2

VENDOR SERIES PART NO. VALUE PHONE NO.

AVX TPS TPSD227M006 220μF, 6V 803-448-9411

AVX TPS TPSD107M010 100μF, 10V 803-448-9411

Taiyo Yuden X5R LMK432BJ226 22μF, 10V 408-573-4150

Taiyo Yuden X5R TMK432BJ106 10μF, 25V 408-573-4150

1308abfa

13

LT1308A/LT1308B

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APPLICATIONS INFORMATION

Ceramic Capacitors

Multilayer ceramic capacitors have become popular, due
to their small size, low cost, and near-zero ESR. Ceramic
capacitors can be used successfully in LT1308A/LT1308B
designs provided loop stability is considered. A tantalum
capacitor has some ESR and this causes an "ESR zero" in
the regulator loop. This zero is beneficial to loop stability.
Ceramics do not have appreciable ESR, so the zero is lost
when they are used. However, the LT1308A/LT1308B
have external compensation pin (VC) so component val­ues can be adjusted to achieve stability. A phase lead
capacitor can also be used to tune up load step response
to optimum levels, as detailed in the following paragraphs.

Figure 19 details a 5V to 12V boost converter using either
a tantalum or ceramic capacitor for C2. The input capaci­tor has little effect on loop stability, as long as minimum
capacitance requirements are met. The phase lead capaci­tor CPL parallels feedback resistor R1. Figure 20 shows
load step response of a 50mA to 500mA load step using a
47μF tantalum capacitor at the output. Without the phase
lead capacitor, there is some ringing, suggesting the
phase margin is low. CPL is then added, and response to
the same load step is pictured in Figure 21. Some phase
margin is restored, improving the response. Next, C2 is
replaced by a 10μF, X5R dielectric, ceramic capacitor.

Without CPL, load step response is pictured in Figure 22.
Although the output settles faster than the tantalum case,
there is appreciable ringing, again suggesting phase mar­gin is low. Figure 23 depicts load step response using the
10μF ceramic output capacitor and CPL. Response is clean
and no ringing is evident. Ceramic capacitors have the
added benefit of lowering ripple at the switching frequency
due to their very low ESR. By applying CPL in tandem with
the series RC at the V

pin, loop response can be tailored

C

to optimize response using ceramic output capacitors.

V

OUT

500mV/DIV

I

L1

1A/DIV

500mA

LOAD

CURRENT

50mA

Figure 20. Load Step Response of LT1308B 5V to 12V
Boost Converter with 47μF Tantalum Output Capacitor

V

OUT

500mV/DIV

200μs/DIV

1308 F20

V

IN

5V

V

IN

SHDN

V

C

+

C1
47μF

C1: AVX TAJC476M010
C2: AVX TPSD476M016 (47μF) OR

TAIYO YUDEN TMK432BJ106MM (10μF)
D1: IR 10BQ015
L1: MURATA LQH6C4R7

Figure 19. 5V to 12V Boost Converter

L1

4.7μH

LT1308B

47k

100pF

SW

GND

I

D1

R1

R3

100k

10k

FB

R2

11.3k

C

PL

330pF

1308 F19

C2

V

OUT

12V
500mA

CURRENT

CURRENT

L1

1A/DIV

500mA

LOAD

50mA

200μs/DIV

1308 F21

Figure 21. Load Step Response with 47μF Tantalum
Output Capacitor and Phase Lead Capacitor C

V

OUT

1V/DIV

I

L1

1A/DIV

500mA

LOAD

50mA

200μs/DIV

1308 F22

PL

Figure 22. Load Step Response with 10μF X5R
Ceramic Output Capacitor

1308abfa

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LT1308A/LT1308B

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APPLICATIONS INFORMATION

V

OUT

500mV/DIV

I

L1

1A/DIV

500mA

LOAD

CURRENT

50mA

Figure 23. Load Step Response with 10μF X5R
Ceramic Output Capacitor and C

GSM AND CDMA PHONES

The LT1308A/LT1308B are suitable for converting a single
Li-Ion cell to 5V for powering RF power stages in GSM or
CDMA phones. Improvements in the LT1308A/LT1308B
error amplifiers allow external compensation values to be
reduced, resulting in faster transient response compared
to the LT1308. The circuit of Figure 24 (same as Figure 1,
printed again for convenience) provides a 5V, 1A output
from a Li-Ion cell. Figure 25 details transient response at
the LT1308A operating at a VIN of 4.2V, 3.6V and 3V.
Ripple voltage in Burst Mode operation can be seen at
10mA load. Figure 26 shows transient response of the
LT1308B under the same conditions. Note the lack of
Burst Mode ripple at 10mA load.

+

C1
47μF

Li-Ion
CELL

200μs/DIV

V

IN

SHDNSHUTDOWN

V

C

L1

4.7μH

LT1308B

47k

100pF

SW

GND

1308 F23

PL

D1

R1
309k

FB

R2
100k

5V
1A

+

C2
220μF

V

OUT

VIN = 4.2V

V

OUT

VIN = 3.6V

V

OUT

VIN = 3V

I

LOAD

1A

10mA

V

TRACES = 200μs/DIV

OUT

200mV/DIV

1308 F25

Figure 25. LT1308A Li-Ion to 5V Boost Converter
Transient Response to 1A Load Step

V

OUT

VIN = 4.2V

V

OUT

VIN = 3.6V

V

OUT

VIN = 3V

I

LOAD

1A

10mA

V

TRACES = 100μs/DIV

OUT

200mV/DIV

1308 F26

Figure 26. LT1308B Li-Ion to 5V Boost
Converter Transient Response to 1A Load Step

C1: AVX TAJC476M010
C2: AVX TPSD227M006

D1: IR 10BQ015
L1: MURATA LQH6N4R7

Figure 24. Li-Ion to 5V Boost Converter Delivers 1A

1308A/B F24

1308abfa

15

LT1308A/LT1308B

TYPICAL APPLICATIO S

U

Triple Output TFTLCD Bias Supply

V

IN

5V

V

3

SHDN

C1

4.7μF

C1:TAIYO-YUDEN JMK212BJ475MG
C2, C3:TAIYO-YUDEN LMK325BJ106MN
C4, C5, C6:TAIYO-YUDEN EMK212BJ105MG
D1: MBRM120
D2,D3,D4: BAT54S
L1: TOKO 817FY-4R7M

220k

100pF

1

V

C

D2

1μF

0.22μF

0.22μF

L1

4.7μH

65

IN

LT1308B

SW

GND

D1

2

FB

4

C4

0.22μF

76.8k

10.7k

V

OFF

–9V
10mA

D3

D4

C5
1μF

C6
1μF

C2, C3
10μF
×2

1308 TA02

V

ON

27V
15mA

AV

DD

10V
500mA

500mV/DIV

500mV/DIV

500mV/DIV

I

LOAD

TFTLCD Bias Supply Transient Response

AV

DD

V

ON

V

OFF

800mA

200mA

100μs/DIV

1308abfa

16

TYPICAL APPLICATIO S

3.3V

REGULATED

Q1

150k

3.3k

U

100k

47k

324k

22nF

40nF EL Panel Driver

V

BAT

3V TO 6V

+

1μF

V

IN

LBI

V

C

LT1308A

100pF

47pF

2M

49.9k

C1
47μF

1

SW

SHDN

GND

LT1308A/LT1308B

T1

D2

1:12

4

3

6

D1

FBLBO

D3

4.3M

17k

SHUTDOWN

400V

C2
1μF
200V

Q2

EL PANEL
≤40nF

10k

V

IN

2.7V TO 6V

100pF

+

SHUTDOWN

47k

D1, D2, D3: BAV21 200mA, 250V
D4: MBR0540
T1: MIDCOM 31105R L

C1: AVX TAJC476M010
C2: VITRAMON VJ225Y105KXCAT
D1: BAT54
D2, D3: BAV21

High Voltage Supply 350V at 1.2mA

10nF

C1
47μF

10nF

V

SHDN

V

C

IN

LT1308A

= 1.5μH

P

GND

250V

T1

1:12

3

4

1

6

D4

SW

FB

D3

D2

D1

10M

34.8k

10nF
250V

10nF
250V

1308 TA04

Q1: MMBT3906
Q2: ZETEX FCX458
T1: MIDCOM 31105

SEPIC Converts 3V to 10V Input to a 5V/500mA Regulated Output

V

OUT

350V

1.2mA

V

IN

3V TO

10V

C1: AVX TAJC476M016
C2: TAIYO YUDEN EMK325BJ475(X5R)
C3: AVX TPSD227M006

1308 TA03

C2

L1A

CTX10-2

V

SHDNSHUTDOWN

IN

V

C

LT1308B

47k

680pF

+

C1
47μF

4.7μF

CERAMIC

SW

FB

GND

D1: IR 10BQ015
L1: COILTRONICS CTX10-2

309k

R2
100k

D1

L1B

R1

+

V

OUT

5V
500mA

C3
220μF

6.3V

1308A/B TA05

1308abfa

17

LT1308A/LT1308B

PACKAGE DESCRIPTION

8-Lead Plastic Small Outline (Narrow .150 Inch)

.050 BSC

U

S8 Package

(Reference LTC DWG # 05-08-1610)

.045 ±.005

.189 – .197

(4.801 – 5.004)

8

NOTE 3

7

5

6

.245

MIN

.030 ±.005

TYP

RECOMMENDED SOLDER PAD LAYOUT

.010 – .020

(0.254 – 0.508)

.008 – .010

(0.203 – 0.254)

NOTE:

1. DIMENSIONS IN

2. DRAWING NOT TO SCALE

3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)

× 45°

.016 – .050

(0.406 – 1.270)

INCHES

(MILLIMETERS)

.160 ±.005

.228 – .244

(5.791 – 6.197)

0°– 8° TYP

.053 – .069

(1.346 – 1.752)

.014 – .019

(0.355 – 0.483)

TYP

.150 – .157

(3.810 – 3.988)

NOTE 3

1

3

2

4

.004 – .010

(0.101 – 0.254)

.050

(1.270)

BSC

SO8 0303

18

1308abfa

PACKAGE DESCRIPTION

U

F Package

14-Lead Plastic TSSOP (4.4mm)

(Reference LTC DWG # 05-08-1650)

1.05 ±0.10

4.90 – 5.10*
(.193 – .201)

14 13 12 11 10 9

LT1308A/LT1308B

8

6.60 ±0.10

0.45 ± 0.05

RECOMMENDED SOLDER PAD LAYOUT

4.30 – 4.50**
(.169 – .177)

0.09 – 0.20

(.0035 – .0079)

NOTE:

1. CONTROLLING DIMENSION: MILLIMETERS

2. DIMENSIONS ARE IN

3. DRAWING NOT TO SCALE
*

DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED .152mm (.006") PER SIDE

**

DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED .254mm (.010") PER SIDE

0.50 – 0.75

(.020 – .030)

4.50 ±0.10

MILLIMETERS

(INCHES)

0.65 BSC

0.25
REF

0° – 8°

0.65

(.0256)

BSC

0.19 – 0.30

(.0075 – .0118)

13456

2

TYP

6.40

(.252)

BSC

7

1.10

(.0433)

MAX

0.05 – 0.15

(.002 – .006)

F14 TSSOP 0204

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.

1308abfa

19

LT1308A/LT1308B

TYPICAL APPLICATIO

U

Li-Ion to 12V/300mA Step-Up DC/DC Converter

2.7V TO 4.2V

+

C1
47μF

Li-Ion
CELL

C1: AVX TAJC476M010
C2: AVX TPSD107M016
D1: IR 10BQ015

L1

4.7μH

V

IN

LT1308B

SHDNSHUTDOWN

V

C

47k

330pF

L1: MURATA LQH6C4R7

SW

GND

D1

R1
887k

FB

R2
100k

+

12V
300mA

C2
100μF

1308A/B TA01

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Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear-tech.com

1308abfa

LT 0807 REV A • PRINTED IN USA

© LINEAR TECHNOLOGY CORPORATION 1999