LT1308A/LT1308B
1
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TYPICAL APPLICATION
DESCRIPTION
High Current, Micropower
Single Cell, 600kHz
DC/DC Converters
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 sav-
ing 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 cur-
rent 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 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.
Converter Effi ciency
FEATURES
APPLICATIONS
n 5V at 1A from a Single Li-Ion Cell
n 5V at 800mA in SEPIC Mode from Four NiCd Cells
n Fixed Frequency Operation: 600kHz
n Boost Converter Outputs up to 34V
n Starts into Heavy Loads
n
Automatic Burst Mode™ Operation at
Light Load (LT1308A)
n
Continuous Switching at Light Loads (LT1308B)
n
Low VCESAT Switch: 300mV at 2A
n
Pin-for-Pin Upgrade Compatible with LT1308
n
Lower Quiescent Current in Shutdown: 1µA (Max)
n
Improved Accuracy Low-Battery Detector
Reference: 200mV ±2%
n Available in 8-Lead SO and 14-Lead TSSOP Packages
n GSM/CDMA Phones
n
Digital Cameras
n
LCD Bias Supplies
n
Answer-Back Pagers
n
GPS Receivers
n
Battery Backup Supplies
n Handheld Computers
Figure 1. LT1308B Single Li-Ion Cell to 5V/1A DC/DC Converter
VIN SW
FB
LT1308B
L1
4.7µH D1
LBO
LBI
47k
R2
100k
R1*
309k
5V
1A
100pF
1308A/B F01a
C1
47µF
C2
220µF
Li-Ion
CELL VCGND
SHDNSHUTDOWN
C1: AVX TAJC476M010
C2: AVX TPSD227M006
D1: IR 10BQ015
+
+
L1: MURATA LQH6C4R7
*R1: 887k FOR VOUT = 12V
LOAD CURRENT (mA)
1
EFFICIENCY (%)
95
90
85
80
75
70
65
60
55
50
10 100 1000
1308A/B F01b
VIN = 4.2V
VIN = 1.5V
VIN = 2.5V
VIN = 3.6V
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.
LT1308A/LT1308B
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ABSOLUTE MAXIMUM RATINGS
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
(Note 1)
1
2
3
4
8
7
6
5
TOP VIEW
LBO
LBI
VIN
SW
VC
FB
SHDN
GND
S8 PACKAGE
8-LEAD PLASTIC SO
TJMAX = 125°C, θJA = 190°C/W
F PACKAGE
14-LEAD PLASTIC TSSOP
1
2
3
4
5
6
7
TOP VIEW
14
13
12
11
10
9
8
VC
FB
SHDN
GND
GND
GND
GND
LBO
LBI
VIN
VIN
SW
SW
SW
(NOTE 6)
TJMAX = 125°C, θJA = 80°C/W
OBSOLETE, FOR INFORMATION PURPOSES ONLY
Contact Linear Technology for Potential Replacement
PIN CONFIGURATION
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: http://www.linear.com/leadfree/
This product is only offered in trays. For more information go to: http://www.linear.com/packaging/
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
LT1308A/LT1308B
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The l denotes the specifi cations which apply over the full operating temperature
range, otherwise specifi cations are at TA = 25°C. Commercial Grade 0°C to 70°C. VIN = 1.1V, VSHDN = VIN, unless otherwise noted.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
IQQuiescent Current Not Switching, LT1308A
Switching, LT1308B
VSHDN = 0V (LT1308A/LT1308B)
140
2.5
0.01
240
4
1
µA
mA
µA
VFB Feedback Voltage l1.20 1.22 1.24 V
IBFB Pin Bias Current (Note 3) l27 80 nA
Reference Line Regulation 1.1V ≤ VIN ≤ 2V
2V ≤ VIN ≤ 10V
l0.03
0.01
0.4
0.2
%/V
%/V
Minimum Input Voltage 0.92 1 V
gmError Amp Transconductance ∆I = 5µA 60 µmhos
AVError Amp Voltage Gain 100 V/V
fOSC Switching Frequency VIN = 1.2V l500 600 700 kHz
Maximum Duty Cycle l82 90 %
Switch Current Limit Duty Cycle = 30% (Note 4) 2 3 4.5 A
Switch VCESAT ISW = 2A (25°C, 0°C), VIN = 1.5V
ISW = 2A (70°C), VIN = 1.5V
290
330
350
400
mV
mV
Burst Mode Operation Switch Current Limit
(LT1308A)
VIN = 2.5V, Circuit of Figure 1 400 mA
Shutdown Pin Current VSHDN = 1.1V
VSHDN = 6V
VSHDN = 0V
l
l
l
2
20
0.01
5
35
0.1
µA
µA
µA
LBI Threshold Voltage
l
196
194
200
200
204
206
mV
mV
LBO Output Low ISINK = 50µA l0.1 0.25 V
LBO Leakage Current VLBI = 250mV, VLBO = 5V l0.01 0.1 µA
LBI Input Bias Current (Note 5) VLBI = 150mV 33 100 nA
Low-Battery Detector Gain 3000 V/V
Switch Leakage Current VSW = 5V l0.01 10 µA
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, VSHDN = VIN, unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
IQQuiescent Current Not Switching, LT1308A
Switching, LT1308B
VSHDN = 0V (LT1308A/LT1308B)
l
l
l
140
2.5
0.01
240
4
1
µA
mA
µA
VFB Feedback Voltage l1.19 1.22 1.25 V
IBFB Pin Bias Current (Note 3) l27 80 nA
Reference Line Regulation 1.1V ≤ VIN ≤ 2V
2V ≤ VIN ≤ 10V
l
l
0.05
0.01
0.4
0.2
%/V
%/V
Minimum Input Voltage 0.92 1 V
gmError Amp Transconductance ∆I = 5µA 60 µmhos
AVError Amp Voltage Gain 100 V/V
LT1308A/LT1308B
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ELECTRICAL CHARACTERISTICS
LT1308B
3.3V Output Effi ciency
LT1308A
3.3V Output Effi ciency
LT1308A
5V Output Effi ciency
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, VSHDN = VIN, unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
fOSC Switching Frequency l500 600 750 kHz
Maximum Duty Cycle l82 90 %
Switch Current Limit Duty Cycle = 30% (Note 4) 2 3 4.5 A
Switch VCESAT ISW = 2A (25°C, – 40°C), VIN = 1.5V
ISW = 2A (85°C), VIN = 1.5V
290
330
350
400
mV
mV
Burst Mode Operation Switch Current Limit
(LT1308A)
VIN = 2.5V, Circuit of Figure 1 400 mA
Shutdown Pin Current VSHDN = 1.1V
VSHDN = 6V
VSHDN = 0V
l
l
2
20
0.01
5
35
0.1
µA
µA
µA
LBI Threshold Voltage
l
196
193
200
200
204
207
mV
mV
LBO Output Low ISINK = 50µA l0.1 0.25 V
LBO Leakage Current VLBI = 250mV, VLBO = 5V l0.01 0.1 µA
LBI Input Bias Current (Note 5) VLBI = 150mV 33 100 nA
Low-Battery Detector Gain 3000 V/V
Switch Leakage Current VSW = 5V l0.01 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 3: Bias current fl ows 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 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 VIN
pins (Pins 11, 12) together at the device.
TYPICAL PERFORMANCE CHARACTERISTICS
LOAD CURRENT (mA)
95
90
85
80
75
70
65
60
55
50
1 100 1000
1308A/B G01
10
EFFICIENCY (%)
VIN = 1.8V VIN = 2.5V
VIN = 1.2V
LOAD CURRENT (mA)
95
90
85
80
75
70
65
60
55
50
1 100 1000
1308A/B G02
10
EFFICIENCY (%)
VIN = 1.8V VIN = 2.5V
VIN = 1.2V
LOAD CURRENT (mA)
1
EFFICIENCY (%)
95
90
85
80
75
70
65
60
55
50
10 100 1000
1308A/B G03
VIN = 4.2V
VIN = 2.5V
VIN = 3.6V
VIN = 1.5V
LT1308A/LT1308B
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TYPICAL PERFORMANCE CHARACTERISTICS
SHDN Pin Bias Current vs Voltage
FB, LBI Bias Current vs
Temperature
Low Battery Detector Reference
vs Temperature
Oscillator Frequency vs
Temperature
LT1308A Quiescent Current vs
Temperature
Feedback Pin Voltage vs
Temperature
LT1308B
12V Output Effi ciency
Switch Current Limit vs
Duty Cycle
Switch Saturation Voltage
vs Current
LOAD CURRENT (mA)
90
85
80
75
70
65
60
55
50
1 100 1000
1308A/B G04
10
EFFICIENCY (%)
VIN = 5V
VIN = 3.3V
DUTY CYCLE (%)
0
CURRENT LIMIT (A)
3.0
3.5
80
1308 • G05
2.5
2.0 20 40 60 100
4.0
SWITCH CURRENT (A)
0
SWITCH VCESAT (mV)
2.0
85°C
1308 G06
0.5 1.0 1.5
500
400
300
200
100
0
25°C
–40°C
SHDN PIN VOLTAGE (V)
0
SHDN PIN CURRENT (µA)
50
40
30
20
10
08
1308 G07
24610
–40°C
25°C
85°C
TEMPERATURE (°C)
–50 –25
BIAS CURRENT (nA)
05025 75 100
1308 • G08
80
70
60
50
40
30
20
10
0
LBI
FB
TEMPERATURE (°C)
–50 –25
VREF (mV)
05025 75 100
1308 • G09
203
202
201
200
199
198
197
196
195
TEMPERATURE (°C)
–50 –2.5
FREQUENCY (kHz)
05025 75 100
1308 • G10
800
750
700
650
600
550
500
450
400
TEMPERATURE (°C)
–50 –25
QUIESCENT CURRENT (µA)
05025 75 100
1308 • G11
180
170
160
150
140
130
120
110
100
TEMPERATURE (°C)
–50 –25
VFB (V)
05025 75 100
1308 • G12
1.25
1.24
1.23
1.22
1.21
1.20
1.19
1.18
LT1308A/LT1308B
6
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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 VC.
FB (Pin 2/Pin 2): Feedback Pin. Reference voltage is
1.22V. Connect resistive divider tap here. Minimize trace
area at FB. Set VOUT according to:
V
OUT = 1.22V(1 + R1/R2).
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.
SW (Pin 5/Pins 8, 9, 10): Switch Pins. Connect induc-
tor/diode here. Minimize trace area at these pins to keep
EMI down. For the TSSOP package, connect all SW pins
together at the package.
VIN (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 VIN pins
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 recommend-
ed. LBO is high impedance when SHDN is grounded.
(SO/TSSOP)
LT1308A/LT1308B
7
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BLOCK DIAGRAMS
Figure 2a. LT1308A/LT1308B Block Diagram (SO-8 Package)
Figure 2b. LT1308A/LT1308B Block Diagram (TSSOP Package)
–
+
–
+
–
+
–
+
–
+
+
+
Σ
COMPARATOR
RAMP
GENERATOR R
BIAS
VC
2VBE
gm
Q2
×10
Q1
FB
FB ENABLE
*HYSTERESIS IN LT1308A ONLY
200mV
A = 3
FF
A2
A1
Q4
*
ERROR
AMPLIFIER
A4
0.03
DRIVER
SW
GND 1308 BD2a
Q3
Q
S
600kHz
OSCILLATOR
5
LBO
LBI
SHDN
SHUTDOWN 3
7
1
4
R6
40k
R5
40k
R1
(EXTERNAL)
R3
30k
R4
140k
2
VIN
VIN
VIN
VOUT
6
8
R2
(EXTERNAL)
–
+
–
+
–
+
–
+
–
+
+
+
Σ
COMPARATOR
RAMP
GENERATOR R
BIAS
VC
2VBE
gm
Q2
×10
Q1
FB
FB ENABLE
*HYSTERESIS IN LT1308A ONLY
200mV
A = 3
FF
A2
A1
Q4
*
ERROR
AMPLIFIER
A4
0.03Ω
DRIVER
SW
GND 1308 BD2b
Q3
Q
S
600kHz
OSCILLATOR
8
SW
9
SW
LBO
LBI
SHDN
SHUTDOWN 3
13
1
4
GND
5
GND
6
GND
7
R6
40k
R5
40k
R1
(EXTERNAL)
R3
30k
R4
140k
2
VIN
VIN
VIN
VIN
VOUT
11
12
14
R2
(EXTERNAL) 10
LT1308A/LT1308B
8
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OPERATION
The LT1308A combines a current mode, fi xed frequency
PWM architecture with Burst Mode micropower opera-
tion 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 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 amplifi er’s output) to
decrease. When VC 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 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
VC 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 VC pin
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 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 effi ciency.
The difference between the two devices is clearly illus-
trated 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 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.
APPLICATIONS INFORMATION
Figure 3. LT1308A Exhibits Burst Mode Operation Output
Voltage Ripple at 50mA Load, LT1308B Does Not
1308 F03
VIN = 3V
(CIRCUIT OF FIGURE 1)
800mA
50mA
TRACE A: LT1308A
VOUT, 100mV/DIV
AC COUPLED
TRACE B: LT1308B
VOUT, 100mV/DIV
AC COUPLED
200µs/DIV
ILOAD
LT1308A/LT1308B
9
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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 induc-
tor 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
VCESAT and VOUT plus the diode drop. 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 fl ows into the output capacitor
through the diode. Figure 5 depicts converter waveforms
at a light load. Here the converter operates in discontinu-
ous 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, man-
dating careful attention to layout for proper performance.
You will not get advertised performance with careless
layout
. Figure 6 shows recommended component place-
ment 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.
Figure 7 shows recommended component placement for
a boost converter using the TSSOP package. Placement
is similar to the SO-8 package layout.
Figure 4. 5V to 12V Boost Converter Waveforms in
Continuous Mode. 10μF Ceramic Capacitor Used at Output
Figure 5. Converter Waveforms in Discontinuous Mode
1308 F04
VOUT
100mV/DIV
VSW
10V/DIV
ISW
500mA/DIV
500ns/DIV
1308 F05
VOUT
20mV/DIV
VSW
10V/DIV
ISW
500mA/DIV
500ns/DIV
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
1
2
8
7
3
4
6
5
L1
C2
D1
LBO
LBI
LT1308A
LT1308B
VOUT
VIN
GND
SHUTDOWN
R1
R2
MULTIPLE
VIAs
GROUND PLANE
1308 F04
+
C1 +
LT1308A/LT1308B
10
1308abfb
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.
APPLICATIONS INFORMATION
Figure 7. Recommended Component
Placement for TSSOP Boost Converter.
Placement is Similar to Figure 4
Figure 8. SEPIC (Single-Ended Primary
Inductance Converter) Converts 3V to 10V
Input to a 5V/500mA Regulated Output
Figure 9. Recommended Component Placement for SEPIC
1
2
14
13
3
4
12
11
10
5
6
7
9
8
L1
C2
D1
LBO
LBI
LT1308A
LT1308B
VOUT
VIN
GND
SHUTDOWN
R1
R2
MULTIPLE
VIAs
GROUND PLANE
1308 F07
+
C1 +
VIN SW
FB
LT1308B
L1A
CTX10-2
L1B
D1
47k R2
100k
R1
309k
680pF
1308A/B F08
C1
47µF
C3
220µF
6.3V
C2
4.7µF
CERAMIC
VOUT
5V
500mA
VIN
3V TO
10V
VCGND
SHDNSHUTDOWN
C1: AVX TAJC476M016
C2: TAIYO YUDEN EMK325BJ475(X5R)
C3: AVX TPSD227M006
+
+
D1: IR 10BQ015
L1: COILTRONICS CTX10-2
1
2
8
7
3
4
6
5
C3
L1A L1B
D1
LBO
LBI
LT1308A
LT1308B
VOUT
VIN
GND
SHUTDOWN
R1
R2
GROUND PLANE
1308 F09
MULTIPLE
VIAs +C2
C1 +
LT1308A/LT1308B
11
1308abfb
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, fl oating 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 fea-
tures 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.
A cross plot of the low-battery detector is shown in
Figure 12. The LBI pin is swept with an input which var-
ies 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 13 details start-up waveforms of Figure 1’s circuit
with a 20 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 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.
Figure 11. Accessing 200mV Reference
Figure 10. Setting Low-Battery Detector Trip Point
Figure 12. Low-Battery Detector
Input/Output Characteristic
Figure 13. 5V Boost Converter of Figure 1.
Start-Up from 1.5V Input into 20 Load
LBO
LBI
TO PROCESSOR
R1
100k
R2
100k
VIN
VBAT
LT1308A
LT1308B
1308 F10
5V
GND
200mV
INTERNAL
REFERENCE
–
+
R1 = VLB – 200mV
2µA
VIN
VBAT LT1308A
LT1308B
LBI
LBO
200k
10µF GND
10k
1308 F11
2N3906
VREF
200mV +
1308 F12
VLBO
1V/DIV
200
VLBI (mV)
195 205
1308 F13
VOUT
2V/DIV
IL1
1A/DIV
VSHDN
5V/DIV
1ms/DIV
LT1308A/LT1308B
12
1308abfb
APPLICATIONS INFORMATION
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 compo-
nents. A 5V to 12V boost converter using the LT1308B
is detailed in Figure 16. C4 differentiates VOUT
, causing
a current to fl ow into R3 as VOUT increases. When this
current exceeds 0.7V/33k, or 21µA, current fl ows into
the base of Q1. Q1’s collector then pulls current out the
VC pin, creating a feedback loop where the slope of VOUT
is limited as follows:
ΔVOUT
Δt=0.7V
33k • C4
With C4 = 33nF, VOUT/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 cur-
rent 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.
Figure 14. 5V Boost Converter of Figure 1.
Start-Up from 3V Input into 5 Load
Figure 15. 5V SEPIC Start-Up from 9V Input into 10 Load
1308 F14
VOUT
1V/DIV
IL1
2A/DIV
VSHDN
5V/DIV
500µs/DIV
1308 F15
VOUT
2V/DIV
ISW
2A/DIV
VSHDN
5V/DIV
500µs/DIV
Figure 16. 5V to 12V Boost Converter with Soft-Start Components Q1, C4, R3 and R4
VIN SW
LT1308B
GND
VC
FB
SHDN
+
330pF
100k
SHUTDOWN
SOFT-START
COMPONENTS
C1
47µF
C4
33nF
Q1
R4
33k
R3
33k
VIN
5V
D1
L1
4.7µH VOUT
12V
500mA
C2
10µF
CC
100pF
11.3k
RC
47k
10k
C1: AVX TAJ476M010
C2: TAIYO YUDEN TMK432BJ106MM
D1: IR 10BQ015
L1: MURATA LQH6C4R7
Q1: 2N3904
1308 F16
LT1308A/LT1308B
13
1308abfb
APPLICATIONS INFORMATION
COMPONENT SELECTION
Diodes
We have found ON Semiconductor MBRS130 and Inter-
national Rectifi er 10BQ015 to perform well. For applica-
tions where VOUT exceeds 30V, use 40V diodes such as
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 approxi-
mately 2A. Output voltage ripple is about 60mVP-P, so the
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
Figure 17. Start-Up Waveforms of Figure 16’s Circuit
without Soft-Start Components
Figure 18. Start-Up Waveforms of Figure 16’s Circuit
with Soft-Start Components Added
1308 F17
VOUT
5V/DIV
IL1
1A/DIV
12V
5V
VSHDN
10V/DIV
50µs/DIV
1308 F18
VOUT
IL1
1A/DIV
12V
5V
VSHDN
10V/DIV
5ms/DIV
LT1308A/LT1308B
14
1308abfb
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 (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 capaci-
tor has little effect on loop stability, as long as minimum
capacitance requirements are met. The phase lead capaci-
tor 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
benefi t of lowering ripple at the switching frequency due
to their very low ESR. By applying CPL in tandem with the
series RC at the VC pin, loop response can be tailored to
optimize response using ceramic output capacitors.
Figure 19. 5V to 12V Boost Converter
Figure 20. Load Step Response of LT1308B 5V to 12V
Boost Converter with 47μF Tantalum Output Capacitor
Figure 21. Load Step Response with 47μF Tantalum
Output Capacitor and Phase Lead Capacitor CPL
Figure 22. Load Step Response with 10μF X5R
Ceramic Output Capacitor
1308 F20
LOAD
CURRENT
IL1
1A/DIV
500mA
50mA
VOUT
500mV/DIV
200µs/DIV
1308 F21
LOAD
CURRENT
IL1
1A/DIV
500mA
50mA
VOUT
500mV/DIV
200µs/DIV
1308 F22
LOAD
CURRENT
IL1
1A/DIV
500mA
50mA
VOUT
500mV/DIV
200µs/DIV
VIN SW
LT1308B
GND
VC
FB
SHDN
+
CPL
330pF
R1
100k
C1
47µF
VIN
5V
D1
L1
4.7µH VOUT
12V
500mA
C2
100pF
R2
11.3k
47k
R3
10k
C1: AVX TAJC476M010
C2: AVX TPSD476M016 (47µF) OR
TAIYO YUDEN TMK432BJ106MM (10µF)
D1: IR 10BQ015
L1: MURATA LQH6C4R7
1308 F19
LT1308A/LT1308B
15
1308abfb
APPLICATIONS INFORMATION
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 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.
Figure 23. Load Step Response with 10μF X5R
Ceramic Output Capacitor and CPL
Figure 25. LT1308A Li-Ion to 5V Boost Converter
Transient Response to 1A Load Step
Figure 26. LT1308B Li-Ion to 5V Boost
Converter Transient Response to 1A Load Step
1308 F23
LOAD
CURRENT
IL1
1A/DIV
500mA
50mA
VOUT
500mV/DIV
200µs/DIV 1308 F25
ILOAD
1A
1mA
VOUT
VIN = 4.2V
VOUT
VIN = 3.6V
VOUT
VIN = 3V
200µs/DIV
VOUT TRACES =
200mV/DIV
1308 F26
ILOAD
1A
10mA
VOUT
VIN = 4.2V
VOUT
VIN = 3.6V
VOUT
VIN = 3V
100µs/DIV
VOUT TRACES =
200mV/DIV
Figure 24. Li-Ion to 5V Boost Converter Delivers 1A
VIN SW
FB
LT1308B
L1
4.7µH D1
47k
R2
100k
R1
309k
5V
1A
100pF
1308A/B F24
C1
47µF
C2
220µF
Li-Ion
CELL VCGND
SHDNSHUTDOWN
C1: AVX TAJC476M010
C2: AVX TPSD227M006
+
+
D1: IR 10BQ015
L1: MURATA LQH6N4R7
LT1308A/LT1308B
16
1308abfb
TYPICAL APPLICATIONS
Triple Output TFTLCD Bias Supply
TFTLCD Bias Supply Transient Response
D1
D4
0.22µF
L1
4.7µH
65
2
4
3
1
VIN SW
LT1308B
GND
VCFB
SHDN
0.22µF
220k
10.7k
1308 TA02
76.8k
C1
4.7µF
VIN
5V
C2, C3
10µF
×2
C6
1µF
C5
1µF
C4
1µF
0.22µF
AVDD
10V
500mA
VON
27V
15mA
VOFF
–9V
10mA
100pF
D3
D2
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
AVDD
500mV/DIV
VON
500mV/DIV
VOFF
500mV/DIV
100µs/DIV
ILOAD 800mA
200mA
LT1308A/LT1308B
17
1308abfb
TYPICAL APPLICATIONS
40nF EL Panel Driver
47pF 10k
47k
17k
C1
47µF
VBAT
3V TO 6V
100pF
D3
D2
D1
SHUTDOWN
2M
4.3M
1µF
Q1
100k
150k 324k
3.3k 22nF 49.9k
Q2
400V
1308 TA03
C2
1µF
200V
EL PANEL
≤40nF
+3
1
4
6
VIN SW
LT1308A
GND
VC
FBLBO
LBI SHDN
3.3V
REGULATED
T1
1:12
C1: AVX TAJC476M010
C2: VITRAMON VJ225Y105KXCAT
D1: BAT54
D2, D3: BAV21
Q1: MMBT3906
Q2: ZETEX FCX458
T1: MIDCOM 31105
High Voltage Supply 350V at 1.2mA SEPIC Converts 3V to 10V Input to a 5V/500mA Regulated Output
D4
VIN SW
LT1308A
GND
VCFB
SHDN
47k
1308 TA04
C1
47µF
10nF
100pF
D2
D1
D3
34.8k
10M
10nF
250V
10nF
250V
10nF
250V
+
VOUT
350V
1.2mA
VIN
2.7V TO 6V
T1
1:12
3
1
4
6
SHUTDOWN
D1, D2, D3: BAV21 200mA, 250V
D4: MBR0540
T1: MIDCOM 31105R LP = 1.5µH
VIN SW
FB
LT1308B
L1A
CTX10-2
L1B
D1
47k R2
100k
R1
309k
680pF
1308A/B TA05
C1
47µF
C3
220µF
6.3V
C2
4.7µF
CERAMIC
VOUT
5V
500mA
VIN
3V TO
10V
VCGND
SHDNSHUTDOWN
C1: AVX TAJC476M016
C2: TAIYO YUDEN EMK325BJ475(X5R)
C3: AVX TPSD227M006
+
+
D1: IR 10BQ015
L1: COILTRONICS CTX10-2
LT1308A/LT1308B
18
1308abfb
F Package
14-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1650)
PACKAGE DESCRIPTION
S8 Package
8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.016 – .050
(0.406 – 1.270)
.010 – .020
(0.254 – 0.508)× 45°
0°– 8° TYP
.008 – .010
(0.203 – 0.254)
SO8 0303
.053 – .069
(1.346 – 1.752)
.014 – .019
(0.355 – 0.483)
TYP
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
1234
.150 – .157
(3.810 – 3.988)
NOTE 3
8765
.189 – .197
(4.801 – 5.004)
NOTE 3
.228 – .244
(5.791 – 6.197)
.245
MIN .160 ±.005
RECOMMENDED SOLDER PAD LAYOUT
.045 ±.005
.050 BSC
.030 ±.005
TYP
INCHES
(MILLIMETERS)
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)
F14 TSSOP 0204
0.09 – 0.20
(.0035 – .0079)
0° – 8°
0.25
REF
0.50 – 0.75
(.020 – .030)
4.30 – 4.50**
(.169 – .177)
6.40
(.252)
BSC
134
567
8
4.90 – 5.10*
(.193 – .201)
14 13 12 11 10 9
1.10
(.0433)
MAX
0.05 – 0.15
(.002 – .006)
0.65
(.0256)
BSC 0.19 – 0.30
(.0075 – .0118)
TYP
2
MILLIMETERS
(INCHES)
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
*
**
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
1.05 ±0.10
0.65 BSC
0.45 ±0.05
RECOMMENDED SOLDER PAD LAYOUT
4.50 ±0.10
6.60 ±0.10
LT1308A/LT1308B
19
1308abfb
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
B 12/10 Obsoleted F Package 2
(Revision history begins at Rev B)
LT1308A/LT1308B
20
1308abfb
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1999
LT 1210 REV B • PRINTED IN USA
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LT1308B
L1
4.7µH D1
47k
R2
100k
R1
887k
12V
300mA
330pF
1308A/B TA01
C1
47µF
C2
100µF
Li-Ion
CELL VCGND
SHDNSHUTDOWN
C1: AVX TAJC476M010
C2: AVX TPSD107M016
D1: IR 10BQ015
+
+
L1: MURATA LQH6C4R7
2.7V TO 4.2V
