Datasheet LT1308A, LT1308B Datasheet (Linear Technology)

LT1308A/LT1308B

Final Electrical Specifications

High Current, Micropower

Single Cell, 600kHz

DC/DC Converters

FEATURES

■

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%

U

APPLICATIONS

■

GSM/CDMA Phones

■

Digital Cameras

■

LCD Bias Supplies

■

Answer-Back Pagers

■

GPS Receivers

■

Battery Backup Supplies

■

Handheld Computers

U

August 1999

DESCRIPTION

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
14-lead TSSOP packages.

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

Burst Mode is a trademark of Linear Technology Corporation.

TYPICAL APPLICATION

+

C1
47µF

Li-Ion
CELL

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

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

V

LBI

SHDNSHUTDOWN

V

L1: MURATA LQH6N4R7
*R1: 169k FOR V
887k FOR V

U

L1

4.7µH

IN

LT1308B

C

47k

100pF

D1

SW

LBO

FB

GND

OUT

OUT

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.

R1*
309k

R2
100k

= 3.3V

= 12V

5V
1A

+

C2
220µF

EFFICIENCY (%)

1308A/B F01

Converter Efficiency

95
90
85
80
75
70
65
60
55
50

1

VIN = 3.6V

VIN = 1.5V

10 100 1000

LOAD CURRENT (mA)

VIN = 4.2V

VIN = 2.5V

1308A/B F01a

1

LT1308A/LT1308B

1
2
3
4

8
7
6
5

TOP VIEW

LBO
LBI
V

IN

SW

V

C

FB

SHDN

GND

S8 PACKAGE

8-LEAD PLASTIC SO

A

W

O

LUTEXI TIS

S

A

WUW

U

ARB

G

(Note 1)

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

PACKAGE

TOP VIEW

1

V

C

2

FB

3

SHDN

4

GND

5

GND

6

GND

7

GND

F PACKAGE

14-LEAD PLASTIC TSSOP

(Note 6)

T

= 125°C, θJA = 80°C/W

JMAX

/

O

RDER I FOR ATIO

ORDER PART

LBO

14

LBI

13

V

12

IN

V

11

IN

SW

10

SW

9

SW

8

NUMBER

LT1308ACF
LT1308BCF

WU

U

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

ORDER PART

NUMBER

LT1308ACS8
LT1308AIS8
LT1308BCS8
LT1308BIS8

S8 PART MARKING

T

= 125°C, θJA = 80°C/W

JMAX

1308A
1308AI

1308B
1308BI

Consult factory for Military grade parts.

LECTRICAL C CHARA TERIST

E

ICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
Commercial Grade 0°C to 70°C. VIN = 1.1V, V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

Q

V

FB

I

B

g

m

A

V

f

OSC

Quiescent Current Not Switching, LT1308A 140 240 µA

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

Minimum Input Voltage 0.92 1 V
Error Amp Transconductance ∆I = 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

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

= VIN, unless otherwise noted.

SHDN

Switching, LT1308B 2.5 4 mA
V

= 0V (LT1308A/LT1308B) 0.01 1 µA

SHDN

≤ 10V 0.01 0.2 %/V

2V ≤ V

IN

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

= 2A (70°C), VIN = 1.5V 330 400 mV

I

SW

2

LT1308A/LT1308B

LECTRICAL C CHARA TERIST

E

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
Commercial Grade 0°C to 70°C. VIN = 1.1V, V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Shutdown Pin Current V

LBI Threshold Voltage 196 200 204 mV

LBO Output Low I
LBO Leakage Current V
LBI Input Bias Current (Note 5) V
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

V
I

g
A
f

Q

FB

B

m
V

OSC

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 ∆I = 5µA60µmhos
Error Amp Voltage Gain 100 V/V
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

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

Shutdown Pin Current V

LBI Threshold Voltage 196 200 204 mV

LBO Output Low I
LBO Leakage Current V
LBI Input Bias Current (Note 5) V
Low-Battery Detector Gain 3000 V/V
Switch Leakage Current VSW = 5V ● 0.01 10 µA

ICS

= VIN, unless otherwise noted.

SHDN

= 1.1V ● 25µA

SHDN

= 6V ● 20 35 µA

V

SHDN

V

= 0V ● 0.01 0.1 µA

SHDN

● 194 200 206 mV

= 50µA ● 0.1 0.25 V

SINK

= 250mV, V

LBI

= 150mV 33 100 nA

LBI

= 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

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

= 2A (85°C), VIN = 1.5V 330 400 mV

I

SW

= 1.1V ● 2 5µA

SHDN

= 6V ● 20 35 µA

V

SHDN

= 0V 0.01 0.1 µA

V

SHDN

= 50µA ● 0.1 0.25 V

SINK

= 250mV, V

LBI

= 150mV 33 100 nA

LBI

= 5V ● 0.01 0.1 µA

LBO

● 2.5 4 mA

● 193 200 207 mV

= 5V ● 0.01 0.1 µA

LBO

3

LT1308A/LT1308B

LECTRICAL C CHARA TERIST

E

ICS

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

Note 2: The LT1308ACS8 and LT1308BCS8 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.

Note 3: Bias current flows into FB pin.

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

UW

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

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

IN

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)

1308A/B G04

V

OUT

100mV/DIV

AC COUPLED

1A

I

LOAD

0A

LT1308A Transient Response
Circuit of Figure 1

VIN = 3.6V 100µs/DIV
V

= 5V

OUT

= 220µF 1308 G05

C

OUT

Switch Saturation Voltage
vs Current

500

400

(mV)

300

CESAT

200

SWITCH V

100

0

0.5

0

SWITCH CURRENT (A)

1.0

25°C

1.5

85°C

–40°C

2.0

1308 G06

4

PIN FUNCTIONS

LT1308A/LT1308B

UUU

VC (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 VC.

FB (Pin 2): Feedback Pin. Reference voltage is 1.22V.
Connect resistive divider tap here. Minimize trace area at
FB. Set V

according to: V

OUT

= 1.22V(1 + R1/R2).

OUT

SHDN (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): Ground. Connect directly to local ground
plane. Ground plane should enclose all components
associated with the LT1308. PCB copper connected to
Pin 4 also functions as a heat sink. Maximize this area to
keep chip heating to a minimum.

W

BLOCK DIAGRAM

V

IN

V

V

OUT

R1
(EXTERNAL)

R2
(EXTERNAL)

IN

6

R5
40k

R6
40k

V

IN

+

g

m

–

ERROR

FB

FB

Q1

2

Q2
×10

R3
30k

R4
140k

AMPLIFIER

RAMP

GENERATOR

600kHz

OSCILLATOR

Figure 2. LT1308A/LT1308B Block Diagram

Q4

+

Σ

+

*HYSTERESIS IN LT1308A ONLY

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

VIN (Pin 6): Supply Pin. Must have local bypass capacitor
right at the pin, connected directly to ground.

LBI (Pin 7): 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. If not used, float LBI pin.

LBO (Pin 8): Low-Battery Detector Output. Open collec­tor, can sink 50µA. A 1MΩ pull-up is recommended. LBO
is high impedance when SHDN is grounded.

2V

BE

SHDN

3

LBO

8

SW

5

Q3

+

0.03Ω

–

4

1308 BD

GND

BIAS

V

C

1

+

*

–

A1

COMPARATOR

–

+

A2

ENABLE

200mV

R

SHUTDOWN

LBI

7

+

–

A4

FF

Q

S

DRIVER

A = 3

5

LT1308A/LT1308B

U

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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 VIN. When there is no load, FB rises slightly above

1.22V, causing VC (the error amplifier’s output) to
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
120µA. 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.

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 IQ falls from 3mA to 200µA by
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

6

LT1308A/LT1308B

U

WUU

APPLICATIONS INFORMATION

LAYOUT HINTS

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

You will not get advertised performance with careless
layouts

ment for a boost (step-up) converter. Follow this closely in
your PC layout. Note the direct path of the switching loops.
Input capacitor C1
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 Pin 4 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.

A SEPIC (Single-Ended Primary Inductance Converter)
schematic is shown in Figure 5. 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 a SEPIC is
shown in Figure 6.

COMPONENT SELECTION
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
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.

. Figure 4 shows recommended component place-

must

be placed close (<5mm) to the IC

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

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 4.
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

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

Diodes

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

exceeds 30V, use 40V diodes such as MBRS140 or

OUT

10BQ040.

P-

7

LT1308A/LT1308B

U

WUU

APPLICATIONS INFORMATION

LBI

GROUND PLANE

R1

SHUTDOWN

MULTIPLE

VIAs

R2

GND

1
2

LT1308A
LT1308B

3
4

C2

Figure 4. Recommended Component Placement for 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

V

IN

3V TO

10V

+

C1
47µF

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

V

SHDNSHUTDOWN

V

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

IN

C

47k

L1A

CTX10-2

SW

LT1308B

FB

GND

680pF

D1: IR 10BQ015
L1: COILTRONICS CTX10-2

C2

4.7µF

CERAMIC

R2
100k

R1

309k

L1B

D1

V

OUT

5V
500mA

+

C3
220µF

6.3V

1308A/B F05

GROUND PLANE

R1

R2

SHUTDOWN

MULTIPLE

VIAs

LBI

LBO

C1

1
2

3
4

LT1308A
LT1308B

C3

+

8
7
6
5

V

IN

L1A L1B

C2

+

GND

D1

V

OUT

Figure 6. Recommended Component Placement for SEPIC

1308 F06

8

LT1308A/LT1308B

U

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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 VIN to enable the
device, but needs only a logic level signal. The voltage on
the SHDN pin can vary from 1V to 10V independent of VIN.
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 7
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 8.

A cross plot of the low-battery detector is shown in
Figure 9. The LBI pin is swept with an input which varies
from 195mV to 205mV, and LBO with a 100k pull-up
resistor, is displayed.

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 10 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 11, the load is 5Ω and input voltage is 3V. Output
voltage reaches 5V 500µs after the device is enabled.
Figure 12 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.

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 13 (same as Figure 1,
printed again for convenience) provides a 5V, 1A output
from a Li-Ion cell. Figure 14 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 15 shows transient response of the
LT1308B under the same conditions. Note the lack of
Burst Mode ripple at 10mA load.

R1

LBI

R2
100k

V

BAT

Figure 7. Setting Low-Battery Detector Trip Point

+

–

200mV
INTERNAL
REFERENCE

GND

V

LT1308A

IN

LT1308B

LBO

1308 F07

5V

100k

TO PROCESSOR

V

LB

R1 =

– 200mV

2µA

200k

V

BAT

2N3906

V

REF

200mV

Figure 8. Accessing 200mV Reference

+

10k

LBO

LBI

10µF

V

IN

LT1308A
LT1308B

GND

1308 F08

9

LT1308A/LT1308B

U

WUU

APPLICATIONS INFORMATION

V

LBO

1V/DIV

195 200 205

Figure 9. Low-Battery Detector
Input/Output Characteristic

V

OUT

2V/DIV

I

L1

1A/DIV

V

SHDN

5V/DIV

V

(mV) 1308 F09

LBI

1ms/DIV

1308 F10

V

OUT

1V/DIV

I

L1

2A/DIV

V

SHDN

5V/DIV

500µs/DIV

Figure 11. 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

500µs/DIV

1308 F11

1308 F12

Figure 10. 5V Boost Converter of Figure 1.
Start-Up from 1.5V Input into 20Ω Load

+

C1
47µF

Li-Ion
CELL

C1: AVX TAJC476M010
C2: AVX TPSD227M006

Figure 13. Li-Ion to 5V Boost Converter Delivers 1A

L1

4.7µH

V

IN

LBI

SHDNSHUTDOWN

V

C

47k

D1: IR 10BQ015
L1: MURATA LQH6N4R7

SW

LBO

LT1308B

FB

GND

100pF

Figure 12. 5V SEPIC Start-Up from
9V Input into 10Ω Load

D1

R1
309k

R2
100k

5V
1A

+

C2
220µF

1308A/B F13

10

LT1308A/LT1308B

U

WUU

APPLICATIONS INFORMATION

V

OUT

VIN = 4.2V

V

OUT

VIN = 3.6V

V

OUT

VIN = 3V

I

LOAD

1A

10mA

TRACES = 200µs/DIV

V

OUT

200mV/DIV

1308 F14

Figure 14. LT1308A Li-Ion to 5V Boost Converter
Transient Response to 1A Load Step

U

PACKAGE DESCRIPTION

Dimensions in inches (millimeters) unless otherwise noted.

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 F15

Figure 15. LT1308B Li-Ion to 5V Boost
Converter Transient Response to 1A Load Step

F Package

14-Lead Plastic TSSOP (4.4mm)

(LTC DWG # 05-08-1650)

4.30 – 4.48**
(0.169 – 0.176)

° – 8°

0

0.09 – 0.18

(0.0035 – 0.0071)

NOTE: DIMENSIONS ARE IN MILLIMETERS

*

DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.152mm (0.006") PER SIDE

**

DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.254mm (0.010") PER SIDE

0.50 – 0.70

(0.020 – 0.028)

0.65

(0.0256)

BSC

0.18 – 0.30

(0.0071 – 0.0118)

4.90 – 5.10*

(0.193 – 0.201)

14 13 12 11 10 9

13456

2

8

6.25 – 6.50

(0.246 – 0.256)

7

1.10

(0.0433)

MAX

0.05 – 0.15

(0.002 – 0.006)

F14 TSSOP 1299

11

LT1308A/LT1308B

U

TYPICAL APPLICATION

SEPIC Converts 3V to 10V Input to a 5V/500mA Regulated Output 3.3V to 12V/300mA Step-Up DC/DC Converter

C2

4.7µF

CERAMIC

SW

FB

GND

D1: IR 10BQ015
L1: COILTRONICS CTX10-2

R2
100k

R1

309k

L1B

D1

+

C1

V

OUT

5V
500mA

+

C3
220µF

6.3V

C1: AVX TAJC476M010

1308A/B F05

C2: AVX TPSD107M016
D1: IR 10BQ015

Li-Ion
CELL

47µF

L1

4.7µH

V

LBI

SHDNSHUTDOWN

V

L1: MURATA LQH6N4R7

IN

C

47k

LT1308B

100pF

SW

LBO

GND

FB

D1

R1
887k

+

C2
100µF

R2
100k

1308A/B TA01

V

IN

3V TO

10V

+

C1
47µF

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

V

SHDNSHUTDOWN

V

IN

C

L1A

CTX10-2

LT1308B

47k

680pF

U

PACKAGE DESCRIPTION

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°– 8° TYP

Dimensions in inches (millimeters) unless otherwise noted.

S8 Package

8-Lead Plastic Small Outline (Narrow 0.150)

0.053 – 0.069

(1.346 – 1.752)

0.014 – 0.019

(0.355 – 0.483)

TYP

(LTC DWG # 05-08-1610)

0.004 – 0.010

(0.101 – 0.254)

0.050

(1.270)

BSC

0.228 – 0.244

(5.791 – 6.197)

0.189 – 0.197*
(4.801 – 5.004)

8

1

7

6

2

3

5

0.150 – 0.157**
(3.810 – 3.988)

4

SO8 1298

12V
300mA

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1308abis, sn1308ab LT/TP 0899 4K • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1999

12

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear-tech.com

Q

Output Noise

RMS