Linear LT1308ACS8, LT1308AIS8, LT1308BCS8, LT1308BIS8, LT1308ACF Schematics

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LT1308A/LT1308B

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 Mode™ 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

APPLICATIONS

GSM/CDMA Phones

Digital Cameras

LCD Bias Supplies

Answer-Back Pagers

GPS Receivers

Battery Backup Supplies

Handheld Computers

DESCRIPTION

The LT®1308A/LT1308B are micropower, ﬁ xed 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 signiﬁ cantly tighter than the LT1308. The 200mV reference is speciﬁ ed 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 V internal V

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.

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

as on the LT1308. An

TYPICAL APPLICATION

4.7µH

C1 47µF

Li-Ion CELL

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

LBI

LT1308B

SHDNSHUTDOWN

47k

100pF

L1: MURATA LQH6C4R7 *R1: 887k FOR V

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

LBO

GND

OUT

= 12V

R1* 309k

R2 100k

5V 1A

C2 220µF

1308A/B F01a

EFFICIENCY (%)

Converter Efﬁ ciency

VIN = 3.6V

VIN = 1.5V

10 100 1000

LOAD CURRENT (mA)

VIN = 4.2V

VIN = 2.5V

1308A/B F01b

1308abfb

LT1308A/LT1308B

ABSOLUTE MAXIMUM RATINGS

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

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

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

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

+ 1V

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

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

PIN CONFIGURATION

TOP VIEW

SHDN

GND

8-LEAD PLASTIC SO

= 125°C, θJA = 190°C/W

JMAX

S8 PACKAGE

LBO

LBI

(Note 1)

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

SHDN

GND

F PACKAGE

14-LEAD PLASTIC TSSOP

(NOTE 6)

= 125°C, θJA = 80°C/W

JMAX

LBO

LBI

OBSOLETE, FOR INFORMATION PURPOSES ONLY

Contact Linear Technology for Potential Replacement

ORDER INFORMATION

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 speciﬁ ed with wider operating temperature ranges. For more information on lead free part marking, go to:

This product is only offered in trays. For more information go to:

http://www.linear.com/leadfree/

http://www.linear.com/packaging/

1308abfb

LT1308A/LT1308B

ELECTRICAL CHARACTERISTICS

The l denotes the speciﬁ cations which apply over the full operating temperature range, otherwise speciﬁ cations are at T

= 25°C. Commercial Grade 0°C to 70°C. VIN = 1.1V, V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Quiescent Current Not Switching, LT1308A

Switching, LT1308B V

= 0V (LT1308A/LT1308B)

SHDN

Feedback Voltage

FB Pin Bias Current (Note 3)

Reference Line Regulation 1.1V ≤ V

2V ≤ V

≤ 10V

≤ 2V

Minimum Input Voltage 0.92 1 V

OSC

Error Amp Transconductance ∆I = 5µA 60 µmhos

Error Amp Voltage Gain 100 V/V

Switching Frequency VIN = 1.2V

Maximum Duty Cycle

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

Switch V

CESAT

Burst Mode Operation Switch Current Limit

ISW = 2A (25°C, 0°C), VIN = 1.5V I

= 2A (70°C), VIN = 1.5V

= 2.5V, Circuit of Figure 1 400 mA

(LT1308A)

Shutdown Pin Current V

SHDN

= 1.1V = 6V = 0V

LBI Threshold Voltage

LBO Output Low I

LBO Leakage Current V

LBI Input Bias Current (Note 5) V

= 50µA

SINK

= 250mV, V

LBI

= 150mV 33 100 nA

LBI

LBO

= 5V

Low-Battery Detector Gain 3000 V/V

Switch Leakage Current V

= 5V

= VIN, unless otherwise noted.

SHDN

140

2.5

0.01

1.20 1.22 1.24 V

27 80 nA

0.03

0.01

500 600 700 kHz

82 90 %

290 330

l l l

196

194

0.01

200 200

0.1 0.25 V

0.01 0.1 µA

0.01 10 µA

240

4 1

0.4

0.2

350 400

0.1

204 206

µA

%/V %/V

mV mV

µA µA µA

mV mV

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

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Quiescent Current Not Switching, LT1308A

Feedback Voltage

FB Pin Bias Current (Note 3)

Reference Line Regulation 1.1V ≤ V

Minimum Input Voltage 0.92 1 V

Error Amp Transconductance ∆I = 5µA 60 µmhos

Error Amp Voltage Gain 100 V/V

= VIN, unless otherwise noted.

SHDN

Switching, LT1308B V

= 0V (LT1308A/LT1308B)

SHDN

≤ 2V

2V ≤ V

≤ 10V

l l l

1.19 1.22 1.25 V

l l

140

240

2.5

0.01

27 80 nA

0.05

0.01

0.4

0.2

4 1

µA

%/V %/V

1308abfb

µA

LT1308A/LT1308B

The l denotes the speciﬁ cations which apply over the full operating temperature

ELECTRICAL CHARACTERISTICS

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

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

OSC

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 3: Bias current ﬂ ows into FB pin.

Switching Frequency

Maximum Duty Cycle

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

Switch V

CESAT

Burst Mode Operation Switch Current Limit

ISW = 2A (25°C, –40°C), VIN = 1.5V I

= 2A (85°C), VIN = 1.5V

= 2.5V, Circuit of Figure 1 400 mA

(LT1308A)

Shutdown Pin Current V

SHDN

= 1.1V = 6V = 0V

LBI Threshold Voltage

LBO Output Low I

LBO Leakage Current V

LBI Input Bias Current (Note 5) V

= 50µA

SINK

= 250mV, V

LBI

= 150mV 33 100 nA

LBI

LBO

= 5V

Low-Battery Detector Gain 3000 V/V

Switch Leakage Current V

= 5V

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 ﬂ ows 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.

= VIN, unless otherwise noted.

SHDN

500 600 750 kHz

82 90 %

290 330

l l

0.01

196

193

200 200

0.1 0.25 V

0.01 0.1 µA

0.01 10 µA

350 400

0.1

204 207

mV mV

µA µA µA

mV mV

TYPICAL PERFORMANCE CHARACTERISTICS

LT1308B

3.3V Output Efﬁ ciency

EFFICIENCY (%)

1 100 1000

VIN = 1.8V

LOAD CURRENT (mA)

VIN = 2.5V

VIN = 1.2V

1308A/B G01

LT1308A

3.3V Output Efﬁ ciency

EFFICIENCY (%)

VIN = 1.8V

1 100 1000

LOAD CURRENT (mA)

VIN = 2.5V

VIN = 1.2V

1308A/B G02

LT1308A 5V Output Efﬁ ciency

VIN = 3.6V

EFFICIENCY (%)

VIN = 4.2V

VIN = 1.5V

10 100 1000

LOAD CURRENT (mA)

VIN = 2.5V

1308A/B G03

1308abfb

TYPICAL PERFORMANCE CHARACTERISTICS

LT1308A/LT1308B

LT1308B 12V Output Efﬁ ciency

EFFICIENCY (%)

1 100 1000

VIN = 5V

VIN = 3.3V

LOAD CURRENT (mA)

SHDN Pin Bias Current vs Voltage

SHDN PIN CURRENT (µA)

SHDN PIN VOLTAGE (V)

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

DUTY CYCLE (%)

FB, LBI Bias Current vs Temperature

BIAS CURRENT (nA)

–50 –25

LBI

05025

TEMPERATURE (°C)

Switch Saturation Voltage vs Current

500

400

1.5

85°C

–40°C

2.0

1308 G06

(mV)

300

CESAT

200

SWITCH V

100

1308 • G05

0.5 SWITCH CURRENT (A)

25°C

1.0

Low Battery Detector Reference vs Temperature

203

202

201

200

(mV)

199

REF

100

1308 • G08

198

197

196

195

–50 –25

05025

TEMPERATURE (°C)

100

1308 • G09

Oscillator Frequency vs Temperature

800

750

700

650

600

550

FREQUENCY (kHz)

500

450

400

–50 –2.5

05025

TEMPERATURE (°C)

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)

1308 • G11

100

Feedback Pin Voltage vs Temperature

1.25

1.24

1.23

1.22

(V)

1.21

1.20

1.19

1.18 –50 –25

05025

TEMPERATURE (°C)

1308abfb

100

1308 • G12

LT1308A/LT1308B

PIN FUNCTIONS

VC (Pin 1/Pin 1): Compensation Pin for Error Ampliﬁ er. Connect a series RC from this pin to ground. Typical values are 47k 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 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 V

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.

= 1.22V(1 + R1/R2).

OUT

according to:

OUT

(SO/TSSOP)

to enable the device.

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

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 220k pull-up is recommended. LBO is high impedance when SHDN is grounded.

pins

1308abfb

BLOCK DIAGRAMS

R5 40k

OUT

R1 (EXTERNAL)

R2 (EXTERNAL)

R6 40k

Q2 ×10

R3 30k

R4 140k

–

AMPLIFIER

RAMP

GENERATOR

600kHz

OSCILLATOR

ERROR

BIAS

–

COMPARATOR

–

*HYSTERESIS IN LT1308A ONLY

LT1308A/LT1308B

SHUTDOWN

LBI

ENABLE

–

200mV

DRIVER

A = 3

–

SHDN

LBO

GND

0.03

1308 BD2a

OUT

R1 (EXTERNAL)

R2 (EXTERNAL)

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

–

COMPARATOR

–

SHDN

LBO

8SW9

ENABLE

200mV

SHUTDOWN

LBI

–

DRIVER

A = 3

0.03Ω

–

GND5GND6GND

GND

1308 BD2b

R5 40k

R6 40k

–

ERROR

Q2 ×10

R3 30k

R4 140k

AMPLIFIER

RAMP

GENERATOR

BIAS

600kHz

OSCILLATOR

*HYSTERESIS IN LT1308A ONLY

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

1308abfb

LT1308A/LT1308B

APPLICATIONS INFORMATION

OPERATION

The LT1308A combines a current mode, ﬁ xed frequency PWM architecture with Burst Mode micropower operation to maintain high efﬁ ciency 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 V

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 above 1.22V, causing V decrease. When V

. When there is no load, FB rises slightly

(the error ampliﬁ er’s output) to

reaches the bias voltage on hyster-

etic comparator A1, A1’s output goes low, turning off all circuitry except the input stage, error ampliﬁ er and low-battery 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 efﬁ ciency even at load currents of 1mA or less.

If the output load increases sufﬁ ciently, A1’s output remains high, resulting in continuous operation. When the LT1308A is running continuously, peak switch current is controlled by V

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

signal, comparator A2 changes state, resetting the ﬂ ip-

ﬂ op 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 V

and ground.

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 ampliﬁ er 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 V

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 efﬁ ciency.

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 signiﬁ cantly less than the LT1308A output ripple. Further, the LT1308B can be forced into micropower mode, where I or more out of the V

falls from 3mA to 200µA by sinking 40µA

pin. This stops switching by causing

A1’s output to go low.

TRACE A: LT1308A

, 100mV/DIV

OUT

AC COUPLED

TRACE B: LT1308B

, 100mV/DIV

OUT

AC COUPLED

800mA

LOAD

50mA

VIN = 3V (CIRCUIT OF FIGURE 1)

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

200µs/DIV

1308 F03

1308abfb

APPLICATIONS INFORMATION

LT1308A/LT1308B

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

CESAT

and V

plus the diode drop. The lower trace is

OUT

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 ﬂ ows 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.

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

. Figure 6 shows recommended component place-

layout

ment for an SO-8 package boost (step-up) converter. Follow this closely in your PC layout. Note the direct path of the

must

switching loops. Input capacitor C1

be placed close (< 5mm) to the IC package. As little as 10mm of wire or PC trace from C

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

LBO

OUT

100mV/DIV

10V/DIV

500mA/DIV

500ns/DIV

1308 F04

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

OUT

20mV/DIV

10V/DIV

500mA/DIV

500ns/DIV

1308 F05

Figure 5. Converter Waveforms in Discontinuous Mode

SHUTDOWN

MULTIPLE

VIAs

GND

LT1308A LT1308B

OUT

1308 F04

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

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

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LT1308A/LT1308B

APPLICATIONS INFORMATION

GROUND PLANE

SHUTDOWN

MULTIPLE

VIAs

GND

LBI

LBO

LT1308A

LT1308B

OUT

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

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.

3V TO

10V

C1 47µF

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

1308 F07

C3: AVX TPSD227M006

L1A

CTX10-2

LT1308B

SHDNSHUTDOWN

47k

680pF

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

4.7µF

CERAMIC

309k

GND

R2 100k

D1: IR 10BQ015 L1: COILTRONICS CTX10-2

L1B

OUT

5V 500mA

C3 220µF

6.3V

1308A/B F08

GROUND PLANE

SHUTDOWN

MULTIPLE

VIAs

LBI

LBO

LT1308A LT1308B

L1A L1B

GND

OUT

Figure 9. Recommended Component Placement for SEPIC

1308 F09

1308abfb

APPLICATIONS INFORMATION

LT1308A/LT1308B

SHDN PIN

The LT1308A/LT1308B SHDN pin is improved over the LT1308. The pin does not require tying to V

to enable

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

. Further, ﬂ oating this pin has the same effect as

of V

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.

LBI

R2 100k

BAT

–

200mV INTERNAL REFERENCE

GND

LT1308A

LT1308B

LBO

1308 F10

100k

R1 =

TO PROCESSOR

– 200mV

2µA

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.

LBO

1V/DIV

195 205

200

(mV)

LBI

1308 F12

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 20 load and V

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 5 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 10 load. The output reaches 5V in about 1ms after the device is enabled.

OUT

2V/DIV

Figure 10. Setting Low-Battery Detector Trip Point

200k

BAT

2N3906

REF

200mV

10k

10µF

LBO

LBI

LT1308A LT1308B

GND

1308 F11

Figure 11. Accessing 200mV Reference

1A/DIV

SHDN

5V/DIV

1ms/DIV

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

1308 F13

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LT1308A/LT1308B

APPLICATIONS INFORMATION

OUT

1V/DIV

2A/DIV

SHDN

5V/DIV

500µs/DIV

1308 F14

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

OUT

2V/DIV

2A/DIV

SHDN

5V/DIV

500µs/DIV

1308 F15

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

Soft-Start

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

when operating from a battery composed of alkaline cells. The inrush current may cause sufﬁ ciency 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 a current to ﬂ ow into R3 as V

increases. When this

OUT

, causing

OUT

current exceeds 0.7V/33k, or 21µA, current ﬂ ows into the base of Q1. Q1’s collector then pulls current out the

pin, creating a feedback loop where the slope of V

is limited as follows:

ΔV

OUT

Δt

With C4 = 33nF, V

0.7V

33k •C4

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

OUT

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 ﬁ nal 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 ﬁ nal output voltage increases to about 15ms. C4 can be adjusted to achieve any output slew rate desired.

OUT

C1 47µF

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

SHUTDOWN

C4 33nF

R3 33k

4.7µH

SHDN

33k

SOFT-START COMPONENTS

LT1308B

47k

100pF

GND

100k

10k

330pF

11.3k

1308 F16

C2 10µF

OUT

12V 500mA

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

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APPLICATIONS INFORMATION

LT1308A/LT1308B

12V

OUT

5V/DIV

1A/DIV

SHDN

10V/DIV

50µs/DIV

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

12V

OUT

1A/DIV

SHDN

10V/DIV

5ms/DIV

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

1308 F17

1308 F18

COMPONENT SELECTION

Diodes

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

exceeds 30V, use 40V diodes such as

OUT

MBRS140 or 10BQ040.

Height limited applications may beneﬁ t 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 fulﬁ ll 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 0.05 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 speciﬁ ed for use with the LT1308A/ LT1308B circuits have low ESR and are speciﬁ cally 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

P-P

ESR of the output capacitor is 60mV/2A or 0.03. 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

1308abfb

LT1308A/LT1308B

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 beneﬁ cial 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 (V

) 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 C

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. C

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.

SHDN

4.7µH

LT1308B

47k

100pF

C1 47µF

GND

100k

10k

11.3k

330pF

V 12V 500mA

OUT

Without C

, 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 C

. Response is clean and

no ringing is evident. Ceramic capacitors have the added beneﬁ t of lowering ripple at the switching frequency due to their very low ESR. By applying C series RC at the V

pin, loop response can be tailored to

in tandem with the

optimize response using ceramic output capacitors.

OUT

500mV/DIV

1A/DIV

500mA

LOAD

CURRENT

50mA

200µs/DIV

1308 F20

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

OUT

500mV/DIV

1A/DIV

500mA

LOAD

50mA

200µs/DIV

1308 F21

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

OUT

500mV/DIV

1A/DIV

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

1308 F19

LOAD

CURRENT

500mA

50mA

200µs/DIV

1308 F22

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

1308abfb

APPLICATIONS INFORMATION

OUT

500mV/DIV

1A/DIV

500mA

LOAD

CURRENT

50mA

200µs/DIV

1308 F23

OUT

VIN = 4.2V

OUT

VIN = 3.6V

OUT

VIN = 3V

LOAD

1mA

V 200mV/DIV

TRACES =

OUT

LT1308A/LT1308B

200µs/DIV

1308 F25

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 ampliﬁ ers 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 V

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.

Li-Ion CELL

4.7µH

C1 47µF

LT1308B

SHDNSHUTDOWN

47k

100pF

GND

R1 309k

R2 100k

5V 1A

C2 220µF

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

OUT

VIN = 4.2V

OUT

VIN = 3.6V

OUT

VIN = 3V

LOAD

10mA

TRACES =

OUT

200mV/DIV

100µs/DIV

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

1308 F26

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

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LT1308A/LT1308B

TYPICAL APPLICATIONS

Triple Output TFTLCD Bias Supply

SHDN

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

1µF

0.22µF

4.7µH

LT1308B

GND

0.22µF

76.8k

10.7k

OFF

–9V 10mA

C5 1µF

C6 1µF

C2, C3 10µF ×2

1308 TA02

27V 15mA

10V 500mA

500mV/DIV

LOAD

OFF

800mA

200mA

TFTLCD Bias Supply Transient Response

100µs/DIV

1308abfb

TYPICAL APPLICATIONS

3.3V

REGULATED

150k

3.3k

100k

47k

324k

40nF EL Panel Driver

BAT

3V TO 6V

1µF

LBI

22nF

49.9k

100pF

C1 47µF

LT1308A

47pF

146

FBLBO

SHDN

GND

1:12

4.3M

17k

SHUTDOWN

LT1308A/LT1308B

C2 1µF

10k

200V

EL PANEL ≤40nF

400V

2.7V TO 6V

100pF

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

Q1: MMBT3906 Q2: ZETEX FCX458 T1: MIDCOM 31105

1308 TA03

High Voltage Supply 350V at 1.2mA SEPIC Converts 3V to 10V Input to a 5V/500mA Regulated Output

10nF

250V

SHUTDOWN

47k

C1 47µF

10nF

SHDN

LT1308A

GND

1:12

146

10M

34.8k

10nF 250V

OUT

350V

1.2mA V

3V TO

10V

C1 47µF

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

SHDNSHUTDOWN

47k

L1A

CTX10-2

LT1308B

GND

680pF

D1: IR 10BQ015 L1: COILTRONICS CTX10-2

4.7µF

CERAMIC

R2 100k

309k

L1B

OUT

5V 500mA

C3 220µF

6.3V

1308A/B TA05

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

= 1.5µH

1308 TA04

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LT1308A/LT1308B

PACKAGE DESCRIPTION

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

.050 BSC

S8 Package

(Reference LTC DWG # 05-08-1610)

.045 ±.005

(4.801 – 5.004)

.189 – .197

NOTE 3

.245

MIN

.030 ±.005

TYP

RECOMMENDED SOLDER PAD LAYOUT

.010 – .020

(0.254 – 0.508)

.008 – .010

(0.203 – 0.254)

.016 – .050

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)

(0.406 – 1.270)

INCHES

(MILLIMETERS)

× 45°

.160 ±.005

0°– 8° TYP

.228 – .244

(5.791 – 6.197)

F Package

14-Lead Plastic TSSOP (4.4mm)

(Reference LTC DWG # 05-08-1650)

1.05 ±0.10

.053 – .069

(1.346 – 1.752)

.014 – .019

(0.355 – 0.483)

TYP

4.90 – 5.10* (.193 – .201)

14 13 12 11 10 9

.150 – .157

(3.810 – 3.988)

NOTE 3

.004 – .010

(0.101 – 0.254)

.050

(1.270)

BSC

SO8 0303

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

TYP

6.40

(.252)

BSC

1.10

(.0433)

MAX

0.05 – 0.15

(.002 – .006)

F14 TSSOP 0204

1308abfb

LT1308A/LT1308B

REVISION HISTORY

REV DATE DESCRIPTION PAGE NUMBER

B 12/10 Obsoleted F Package 2

(Revision history begins at Rev B)

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.

1308abfb

LT1308A/LT1308B

TYPICAL APPLICATION

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

4.7µH

LT1308B

SHDNSHUTDOWN

47k

330pF

L1: MURATA LQH6C4R7

GND

R1 887k

R2 100k

12V 300mA

C2 100µF

1308A/B TA01

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

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LT1617 Micropower Inverting DC/DC Converter in SOT-23 V

, 36V, 350mA Switch

= 1V to 15V; V

OUT

to –34V

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LT1949 600kHz, 1A Switch PWM DC/DC Converter 1.1A, 0.5, 30V Internal Switch, V

as Low as 1.5V

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Output Noise

RMS

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507

●

www.linear.com

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LT 1210 REV B • PRINTED IN USA

Linear LT1308ACS8, LT1308AIS8, LT1308BCS8, LT1308BIS8, LT1308ACF Schematics

Specifications and Main Features

Frequently Asked Questions

User Manual