Datasheet LT1308A, LT1308B Datasheet (LINEAR TECHNOLOGY)

FEATURES
LT1308A/LT1308B
High Current, Micropower
Single Cell, 600kHz
DC/DC Converters
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DESCRIPTIO
5V at 1A from a Single Li-Ion Cell
Fixed Frequency Operation: 600kHz
Boost Converter Outputs up to 34V
Starts into Heavy Loads
Automatic Burst ModeTM Operation at Light Load (LT1308A)
Continuous Switching at Light Loads (LT1308B)
Low V
Pin-for-Pin Upgrade Compatible with LT1308
Lower Quiescent Current in Shutdown: 1μA (Max)
Improved Accuracy Low-Battery Detector
Switch: 300mV at 2A
CESAT
Reference: 200mV ± 2%
Available in 8-Lead SO and 14-Lead TSSOP Packages
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APPLICATIO S
GSM/CDMA Phones
Digital Cameras
LCD Bias Supplies
Answer-Back Pagers
GPS Receivers
Battery Backup Supplies
Handheld Computers
The LT®1308A/LT1308B are micropower, fixed frequency step-up DC/DC converters that operate over a 1V to 10V input voltage range. They are improved versions of the LT1308 and are recommended for use in new designs. The LT1308A features automatic shifting to power saving Burst Mode operation at light loads and consumes just 140μA at no load. The LT1308B features continuous switching at light loads and operates at a quiescent current of 2.5mA. Both devices consume less than 1μA in shutdown.
Low-battery detector accuracy is significantly tighter than the LT1308. The 200mV reference is specified at ± 2% at room and ± 3% over temperature. The shutdown pin enables the device when it is tied to a 1V or higher source and does not need to be tied to VIN as on the LT1308. An internal VC clamp results in improved transient response and the switch voltage rating has been increased to 36V, enabling higher output voltage applications.
The LT1308A/LT1308B are available in the 8-lead SO and the 14-lead TSSOP packages.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.
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TYPICAL APPLICATIO
L1
4.7μH
V
+
C1 47μF
Li-Ion CELL
C1: AVX TAJC476M010 C2: AVX TPSD227M006 D1: IR 10BQ015
IN
LBI
LT1308B
SHDNSHUTDOWN
V
C
47k
100pF
L1: MURATA LQH6C4R7 *R1: 887k FOR V
Figure 1. LT1308B Single Li-Ion Cell to 5V/1A DC/DC Converter
SW
LBO
GND
OUT
FB
D1
= 12V
R1* 309k
R2 100k
5V 1A
+
C2 220μF
1308A/B F01a
95
90
85
80
75
70
EFFICIENCY (%)
65
60
55
50
1
Converter Efficiency
VIN = 3.6V
VIN = 1.5V
10 100 1000
LOAD CURRENT (mA)
VIN = 4.2V
VIN = 2.5V
1308A/B F01b
1308abfa
1
LT1308A/LT1308B
A
W
O
LUTEXI TIS
S
A
WUW
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(Note 1)
ARB
G
VIN, SHDN, LBO Voltage ......................................... 10V
SW Voltage ............................................... –0.4V to 36V
FB Voltage ....................................................... VIN + 1V
VC Voltage ................................................................ 2V
LBI Voltage ................................................. –0.1V to 1V
Current into FB Pin .............................................. ±1mA
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PI CO FIGURATIO
TOP VIEW
LBO
8
LBI
7
V
6
IN
SW
5
SHDN
GND
V
C
FB
T
JMAX
1
2
3
4
S8 PACKAGE
8-LEAD PLASTIC SO
= 125°C, θJA = 190°C/W
Operating Temperature Range
Commercial ............................................ 0°C to 70°C
Extended Commerial (Note 2) ........... – 40°C to 85°C
Industrial ........................................... –40°C to 85°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
TOP VIEW
1
V
C
2
FB
3
SHDN
4
GND
5
GND
6
GND
7
GND
F PACKAGE
14-LEAD PLASTIC TSSOP
= 125°C, θJA = 80°C/W
T
JMAX
(Note 6)
LBO
14
LBI
13
V
12
IN
V
11
IN
SW
10
SW
9
SW
8
NOT RECOMMENDED FOR NEW DESIGNS
Contact Linear Technology for Potential Replacement
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ORDER I FOR ATIO
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LT1308ACS8#PBF LT1308ACS8#TRPBF 1308A 8-Lead Plastic SO 0°C to 70°C
LT1308AIS8#PBF LT1308AIS8#TRPBF 1308AI 8-Lead Plastic SO –40°C to 85°C
LT1308BCS8#PBF LT1308BCS8#TRPBF 1308B 8-Lead Plastic SO 0°C to 70°C
LT1308BIS8#PBF LT1308BIS8#TRPBF 1308BI 8-Lead Plastic SO –40°C to 85°C
LT1308ACF#PBF LT1308ACF#TRPBF LT1308ACF 14-Lead Plastic TSSOP 0°C to 70°C
LT1308BCF#PBF LT1308BCF#TRPBF LT1308BCF 14-Lead Plastic TSSOP 0°C to 70°C
LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LT1308ACS8 LT1308ACS8#TR 1308A 8-Lead Plastic SO 0°C to 70°C
LT1308AIS8 LT1308AIS8#TR 1308AI 8-Lead Plastic SO –40°C to 85°C
LT1308BCS8 LT1308BCS8#TR 1308B 8-Lead Plastic SO 0°C to 70°C
LT1308BIS8 LT1308BIS8#TR 1308BI 8-Lead Plastic SO –40°C to 85°C
LT1308ACF LT1308ACF#TR LT1308ACF 14-Lead Plastic TSSOP 0°C to 70°C
LT1308BCF LT1308BCF#TR LT1308BCF 14-Lead Plastic TSSOP 0°C to 70°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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1308abfa
LT1308A/LT1308B
ELECTRICAL CHARACTERISTICS
range, otherwise specifications are TA = 25°C. Commercial Grade 0°C to 70°C. VIN = 1.1V, V
The denotes specifications which apply over the full operating temperature
= VIN, unless otherwise noted.
SHDN
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
I
Q
Quiescent Current Not Switching, LT1308A 140 240 μA
Switching, LT1308B 2.5 4 mA
= 0V (LT1308A/LT1308B) 0.01 1 μA
V
SHDN
V
FB
I
B
Feedback Voltage 1.20 1.22 1.24 V
FB Pin Bias Current (Note 3) 27 80 nA
Reference Line Regulation 1.1V ≤ VIN 2V 0.03 0.4 %/V
2V ≤ V
10V 0.01 0.2 %/V
IN
Minimum Input Voltage 0.92 1 V
g
A
f
m
V
OSC
Error Amp Transconductance ΔI = 5μA60μmhos
Error Amp Voltage Gain 100 V/V
Switching Frequency VIN = 1.2V 500 600 700 kHz
Maximum Duty Cycle 82 90 %
Switch Current Limit Duty Cyle = 30% (Note 4) 2 3 4.5 A
Switch V
CESAT
ISW = 2A (25°C, 0°C), VIN = 1.5V 290 350 mV I
= 2A (70°C), VIN = 1.5V 330 400 mV
SW
Burst Mode Operation Switch Current Limit VIN = 2.5V, Circuit of Figure 1 400 mA (LT1308A)
Shutdown Pin Current V
= 1.1V 25μA
SHDN
= 6V 20 35 μA
V
SHDN
= 0V 0.01 0.1 μA
V
SHDN
LBI Threshold Voltage 196 200 204 mV
194 200 206 mV
LBO Output Low I
LBO Leakage Current V
LBI Input Bias Current (Note 5) V
= 50μA 0.1 0.25 V
SINK
= 250mV, V
LBI
= 150mV 33 100 nA
LBI
= 5V 0.01 0.1 μA
LBO
Low-Battery Detector Gain 3000 V/V
Switch Leakage Current VSW = 5V 0.01 10 μA
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. Industrial Grade –40°C to 85°C. VIN = 1.2V, V
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
I
Q
V
FB
I
B
g
m
A
V
Quiescent Current Not Switching, LT1308A 140 240 μA
Feedback Voltage 1.19 1.22 1.25 V
FB Pin Bias Current (Note 3) 27 80 nA
Reference Line Regulation 1.1V ≤ VIN 2V 0.05 0.4 %/V
Minimum Input Voltage 0.92 1 V
Error Amp Transconductance ΔI = 5μA60μmhos
Error Amp Voltage Gain 100 V/V
= VIN, unless otherwise noted.
SHDN
Switching, LT1308B V
= 0V (LT1308A/LT1308B) 0.01 1 μA
SHDN
2V ≤ V
10V 0.01 0.2 %/V
IN
2.5 4 mA
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LT1308A/LT1308B
ELECTRICAL CHARACTERISTICS
range, otherwise specifications are TA = 25°C. Industrial Grade –40°C to 85°C. VIN = 1.2V, V
The denotes specifications which apply over the full operating temperature
= VIN, unless otherwise noted.
SHDN
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
f
OSC
Switching Frequency 500 600 750 kHz
Maximum Duty Cycle 82 90 %
Switch Current Limit Duty Cyle = 30% (Note 4) 2 3 4.5 A
Switch V
CESAT
ISW = 2A (25°C, –40°C), VIN = 1.5V 290 350 mV
= 2A (85°C), VIN = 1.5V 330 400 mV
I
SW
Burst Mode Operation Switch Current Limit VIN = 2.5V, Circuit of Figure 1 400 mA (LT1308A)
Shutdown Pin Current V
= 1.1V 2 5μA
SHDN
= 6V 20 35 μA
V
SHDN
= 0V 0.01 0.1 μA
V
SHDN
LBI Threshold Voltage 196 200 204 mV
193 200 207 mV
LBO Output Low I
LBO Leakage Current V
LBI Input Bias Current (Note 5) V
= 50μA 0.1 0.25 V
SINK
= 250mV, V
LBI
= 150mV 33 100 nA
LBI
= 5V 0.01 0.1 μA
LBO
Low-Battery Detector Gain 3000 V/V
Switch Leakage Current VSW = 5V 0.01 10 μA
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.
Note 2: The LT1308ACS8, LT1308ACF, LT1308BCS8 and LT1308BCF are designed, characterized and expected to meet the industrial temperature limits, but are not tested at – 40°C and 85°C. I grade devices are guaranteed over the –40°C to 85°C operating temperature range.
Note 4: Switch current limit guaranteed by design and/or correlation to static tests. Duty cycle affects current limit due to ramp generator (see Block Diagram).
Note 5: Bias current flows out of LBI pin. Note 6: Connect the four GND pins (Pins 4–7) together at the device.
Similarly, connect the three SW pins (Pins 8–10) together and the two V
IN
pins (Pins 11, 12) together at the device.
Note 3: Bias current flows into FB pin.
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TYPICAL PERFORMANCE CHARACTERISTICS
LT1308B
3.3V Output Efficiency
95
90
85
80
75
70
EFFICIENCY (%)
65
60
55
50
1 100 1000
VIN = 1.8V
10
LOAD CURRENT (mA)
VIN = 2.5V
VIN = 1.2V
1308A/B G01
LT1308A
3.3V Output Efficiency
95
90
85
80
75
70
EFFICIENCY (%)
65
60
55
50
VIN = 1.8V
1 100 1000
10
LOAD CURRENT (mA)
VIN = 2.5V
VIN = 1.2V
1308A/B G02
4
LT1308A 5V Output Efficiency
95
90
VIN = 3.6V
85
80
75
70
EFFICIENCY (%)
65
60
55
50
1
LOAD CURRENT (mA)
VIN = 4.2V
VIN = 1.5V
10 100 1000
VIN = 2.5V
1308A/B G03
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TEMPERATURE (°C)
–50 –25
V
REF
(mV)
05025
75
100
1308 • G09
203
202
201
200
199
198
197
196
195
TYPICAL PERFORMANCE CHARACTERISTICS
LT1308A/LT1308B
LT1308B 12V Output Efficiency
90
85
80
75
70
65
EFFICIENCY (%)
60
55
50
1 100 1000
VIN = 5V
VIN = 3.3V
10
LOAD CURRENT (mA)
SHDN Pin Bias Current vs Voltage
50
40
30
20
SHDN PIN CURRENT (μA)
10
0
2
0
SHDN PIN VOLTAGE (V)
6
4
8
1308A/B G04
–40°C
25°C
85°C
1308 G07
Switch Current Limit vs Duty Cycle
4.0
3.5
3.0
CURRENT LIMIT (A)
2.5
2.0 20
0
DUTY CYCLE (%)
60
80
40
100
1308 • G05
FB, LBI Bias Current vs Temperature
80
70
60
50
40
30
BIAS CURRENT (nA)
20
10
0
10
–50 –25
LBI
05025
TEMPERATURE (°C)
FB
75
100
1308 • G08
Switch Saturation Voltage vs Current
500
400
(mV)
300
CESAT
200
SWITCH V
100
0
0
0.5 SWITCH CURRENT (A)
25°C
1.0
Low Battery Detector Reference vs Temperature
1.5
85°C
–40°C
2.0
1308 G06
Oscillator Frequency vs Temperature
800
750
700
650
600
550
FREQUENCY (kHz)
500
450
400
–50 –2.5
05025
TEMPERATURE (°C)
75
1308 • G10
100
LT1308A Quiescent Current vs Temperature
180
170
160
150
140
130
120
QUIESCENT CURRENT (μA)
110
100
–50 –25
05025
TEMPERATURE (°C)
75
1308 • G11
100
Feedback Pin Voltage vs Temperature
1.25
1.24
1.23
1.22
(V)
FB
V
1.21
1.20
1.19
1.18 –50 –25
05025
TEMPERATURE (°C)
75
1308 • G12
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100
5
LT1308A/LT1308B
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PIN FUNCTIONS
(SO/TSSOP)
VC (Pin 1/Pin 1): Compensation Pin for Error Amplifier.
Connect a series RC from this pin to ground. Typical values are 47kΩ and 100pF. Minimize trace area at V
FB (Pin 2/Pin 2): Feedback Pin. Reference voltage is
1.22V. Connect resistive divider tap here. Minimize trace area at FB. Set V
V
= 1.22V(1 + R1/R2).
OUT
SHDN (Pin 3/Pin 3): Shutdown. Ground this pin to turn off switcher. To enable, tie to 1V or more. SHDN does not need to be at VIN to enable the device.
GND (Pin 4/Pins 4, 5, 6, 7): Ground. Connect directly to local ground plane. Ground plane should enclose all components associated with the LT1308. PCB copper connected to these pins also functions as a heat sink. For the TSSOP package, connect all pins to ground copper to get the best heat transfer. This keeps chip heating to a minimum.
according to:
OUT
.
C
SW (Pin 5/Pins 8, 9, 10): Switch Pins. Connect inductor/ diode here. Minimize trace area at these pins to keep EMI down. For the TSSOP package, connect all SW pins together at the package.
(Pin 6/Pins 11, 12): Supply Pins. Must have local
V
IN
bypass capacitor right at the pins, connected directly to ground. For the TSSOP package, connect both V together at the package.
LBI (Pin 7/Pin 13): Low-Battery Detector Input. 200mV reference. Voltage on LBI must stay between –100mV and 1V. Low-battery detector does not function with SHDN pin grounded. Float LBI pin if not used.
LBO (Pin 8/Pin 14): Low-Battery Detector Output. Open collector, can sink 50μA. A 220kΩ pull-up is recom- mended. LBO is high impedance when SHDN is grounded.
IN
pins
6
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BLOCK DIAGRA S
V
IN
V
OUT
R1 (EXTERNAL)
R2 (EXTERNAL)
6
FB
FB
2
R5 40k
Q1
Figure 2a. LT1308A/LT1308B Block Diagram (SO-8 Package)
Q2 ×10
R6 40k
R3 30k
R4 140k
V
IN
+
g
m
AMPLIFIER
RAMP
GENERATOR
600kHz
OSCILLATOR
ERROR
V
IN
Q4
V
C
1
2V
BE
+
*
BIAS
A1
COMPARATOR
+
Σ
+
*HYSTERESIS IN LT1308A ONLY
+
A2
ENABLE
200mV
R
LT1308A/LT1308B
SHDN
SHUTDOWN
LBI
7
+
A4
FF
Q
S
DRIVER
A = 3
3
LBO
8
SW
5
Q3
+
0.03Ω
4
GND
1308 BD2a
V
OUT
R1 (EXTERNAL)
R2 (EXTERNAL)
V
IN
+
*
A1
COMPARATOR
+
A2
2V
BE
SHDN
3
LBO
14
SW
8SW9
SW
10
Q3
ENABLE
200mV
R
SHUTDOWN
LBI
13
+
A4
FF
Q
S
DRIVER
+
A = 3
0.03Ω
4
GND5GND6GND
GND
7
1308 BD2b
V
11
IN
V
IN
12
R5 40k
R6 40k
V
IN
+
g
m
Q4
V
C
1
ERROR
FB
FB
Q1
2
Q2 ×10
R3 30k
R4 140k
AMPLIFIER
RAMP
GENERATOR
BIAS
+
Σ
+
600kHz
OSCILLATOR
*HYSTERESIS IN LT1308A ONLY
Figure 2b. LT1308A/LT1308B Block Diagram (TSSOP Package)
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7
LT1308A/LT1308B
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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 V
1.22V, causing V decrease. When VC reaches the bias voltage on hysteretic comparator A1, A1’s output goes low, turning off all circuitry except the input stage, error amplifier and low­battery detector. Total current consumption in this state is 140μA. As output loading causes the FB voltage to decrease, A1’s output goes high, enabling the rest of the IC. Switch current is limited to approximately 400mA initially after A1’s output goes high. If the load is light, the output voltage (and FB voltage) will increase until A1’s output goes low, turning off the rest of the LT1308A. Low frequency ripple voltage appears at the output. The ripple frequency is dependent on load current and output capaci­tance. This Burst Mode operation keeps the output regu­lated and reduces average current into the IC, resulting in high efficiency even at load currents of 1mA or less.
If the output load increases sufficiently, A1’s output remains high, resulting in continuous operation. When the LT1308A is running continuously, peak switch current is controlled by VC to regulate the output voltage. The switch is turned on at the beginning of each switch cycle. When the summation of a signal representing switch current and a ramp generator (introduced to avoid subharmonic oscil­lations at duty factors greater than 50%) exceeds the V signal, comparator A2 changes state, resetting the flip-flop and turning off the switch. Output voltage increases as switch current is increased. The output, attenuated by a resistor divider, appears at the FB pin, closing the overall loop. Frequency compensation is provided by an external series RC network connected between the VC pin and ground.
. When there is no load, FB rises slightly above
IN
(the error amplifier’s output) to
C
C
Low-battery detector A4’s open-collector output (LBO) pulls low when the LBI pin voltage drops below 200mV. There is no hysteresis in A4, allowing it to be used as an amplifier in some applications. The entire device is dis­abled when the SHDN pin is brought low. To enable the converter, SHDN must be at 1V or greater. It need not be tied to VIN as on the LT1308.
The LT1308B differs from the LT1308A in that there is no hysteresis in comparator A1. Also, the bias point on A1 is set lower than on the LT1308B so that switching can occur at inductor current less than 100mA. Because A1 has no hysteresis, there is no Burst Mode operation at light loads and the device continues switching at constant frequency. This results in the absence of low frequency output voltage ripple at the expense of efficiency.
The difference between the two devices is clearly illus­trated in Figure 3. The top two traces in Figure 3 shows an LT1308A/LT1308B circuit, using the components indi­cated in Figure 1, set to a 5V output. Input voltage is 3V. Load current is stepped from 50mA to 800mA for both circuits. Low frequency Burst Mode operation voltage ripple is observed on Trace A, while none is observed on Trace B.
At light loads, the LT1308B will begin to skip alternate cycles. The load point at which this occurs can be de­creased by increasing the inductor value. However, output ripple will continue to be significantly less than the LT1308A output ripple. Further, the LT1308B can be forced into micropower mode, where I
falls from 3mA to 200μA by
Q
sinking 40μA or more out of the VC pin. This stops switching by causing A1’s output to go low.
TRACE A: LT1308A
, 100mV/DIV
V
OUT
AC COUPLED
TRACE B: LT1308B
V
, 100mV/DIV
OUT
AC COUPLED
800mA
I
LOAD
50mA
V
= 3V 200μs/DIV 1308 F03
IN
(CIRCUIT OF FIGURE 1)
Figure 3. LT1308A Exhibits Burst Mode Operation Output Voltage Ripple at 50mA Load, LT1308B Does Not
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1308abfa
LT1308A/LT1308B
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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 inductor current is dumped into the output capacitor during the switch off time, and the voltage decreases when the switch is on. Ripple voltage is in this case due to capacitance, as the ceramic capacitor has little ESR. The middle trace is the switch voltage. This voltage alternates between a V The lower trace is the switch current. At the beginning of the switch cycle, the current is 1.2A. At the end of the switch on time, the current has increased to 2A, at which point the switch turns off and the inductor current flows into the output capacitor through the diode. Figure 5 depicts converter waveforms at a light load. Here the converter operates in discontinuous mode. The inductor current reaches zero during the switch off time, resulting in some ringing at the switch node. The ring frequency is set by switch capacitance, diode capacitance and induc­tance. This ringing has little energy, and its sinusoidal shape suggests it is free from harmonics. Minimizing the copper area at the switch node will prevent this from causing interference problems.
V
OUT
100mV/DIV
V
SW
10V/DIV
I
SW
1A/DIV
Figure 4. 5V to 12V Boost Converter Waveforms in Continuous Mode. 10μF Ceramic Capacitor Used at Output
V
OUT
20mV/DIV
V
SW
10V/DIV
CESAT
and V
500ns/DIV
plus the diode drop.
OUT
LAYOUT HINTS
The LT1308A/LT1308B switch current at high speed, mandating careful attention to layout for proper perfor­mance.
careless layout
You will not get advertised performance with
. Figure 6 shows recommended component placement for an SO-8 package boost (step-up) converter. Follow this closely in your PC layout. Note the direct path of the switching loops. Input capacitor C1
must
be placed close (< 5mm) to the IC package. As little as 10mm of wire or PC trace from CIN to VIN will cause problems such as inability to regulate or oscillation.
The negative terminal of output capacitor C2 should tie close to the ground pin(s) of the LT1308A/LT1308B. Doing this reduces dI/dt in the ground copper which keeps high frequency spikes to a minimum. The DC/DC converter ground should tie to the PC board ground plane at one place only, to avoid introducing dI/dt in the ground plane.
LBI
GROUND PLANE
R1
SHUTDOWN
MULTIPLE
VIAs
R2
GND
1
2
LT1308A LT1308B
3
4
C2
Figure 6. Recommended Component Placement for SO-8 Package Boost Converter. Note Direct High Current Paths Using Wide PC Traces. Minimize Trace Area at Pin 1 (VC) and Pin 2 (FB). Use Multiple Vias to Tie Pin 4 Copper to Ground Plane. Use Vias at One Location Only to Avoid Introducing Switching Currents into the Ground Plane
LBO
C1
+
8
7
6
5
V
IN
L1
+
D1
V
OUT
1308 F04
I
SW
500mA/DIV
500ns/DIV
Figure 5. Converter Waveforms in Discontinuous Mode
Figure 7 shows recommended component placement for a boost converter using the TSSOP package. Placement is similar to the SO-8 package layout.
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9
LT1308A/LT1308B
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APPLICATIONS INFORMATION
LBI
GROUND PLANE
R1
SHUTDOWN
MULTIPLE
VIAs
R2
GND
1
2
3
4
5
6
7
LT1308A LT1308B
C2
Figure 7. Recommended Component Placement for TSSOP Boost Converter. Placement is Similar to Figure 4.
LBO
C1
+
14
13
12
11
10
9
8
+
V
OUT
V
IN
L1
D1
1308 F07
A SEPIC (Single-Ended Primary Inductance Converter) schematic is shown in Figure 8. This converter topology produces a regulated output over an input voltage range that spans (i.e., can be higher or lower than) the output. Recommended component placement for an SO-8 pack­age SEPIC is shown in Figure 9.
C2
4.7μF
CERAMIC
SW
L1B
R1
FB
GND
D1: IR 10BQ015 L1: COILTRONICS CTX10-2
309k
R2 100k
D1
V
OUT
5V 500mA
+
C3 220μF
6.3V
1308A/B F08
SHDNSHUTDOWN
V
V
CTX10-2
IN
LT1308B
C
47k
680pF
V
IN
3V TO
10V
+
C1 47μF
C1: AVX TAJC476M016 C2: TAIYO YUDEN EMK325BJ475(X5R) C3: AVX TPSD227M006
Figure 8. SEPIC (Single-Ended Primary Inductance Converter) Converts 3V to 10V Input to a 5V/500mA Regulated Output
L1A
GROUND PLANE
R1
R2
SHUTDOWN
MULTIPLE
VIAs
Figure 9. Recommended Component Placement for SEPIC
GND
1
2
3
4
C1
LT1308A LT1308B
C3
+
LBI
LBO
+
8
7
6
5
V
OUT
V
IN
L1A L1B
C2
D1
1308 F09
1308abfa
10
LT1308A/LT1308B
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APPLICATIONS INFORMATION
SHDN PIN
The LT1308A/LT1308B SHDN pin is improved over the LT1308. The pin does not require tying to V device, but needs only a logic level signal. The voltage on the SHDN pin can vary from 1V to 10V independent of V Further, floating this pin has the same effect as grounding, which is to shut the device down, reducing current drain to 1μA or less.
LOW-BATTERY DETECTOR
The low-battery detector on the LT1308A/LT1308B fea­tures improved accuracy and drive capability compared to the LT1308. The 200mV reference has an accuracy of ±2% and the open-collector output can sink 50μA. The LT1308A/ LT1308B low-battery detector is a simple PNP input gain stage with an open-collector NPN output. The negative input of the gain stage is tied internally to a 200mV reference. The positive input is the LBI pin. Arrangement as a low-battery detector is straightforward. Figure 10 details hookup. R1 and R2 need only be low enough in value so that the bias current of the LBI pin doesn’t cause large errors. For R2, 100k is adequate. The 200mV refer­ence can also be accessed as shown in Figure 11.
R1
LBI
R2 100k
V
LT1308A
IN
+
LT1308B
LBO
to enable the
IN
5V
100k
TO PROCESSOR
IN
.
A cross plot of the low-battery detector is shown in Figure 12. The LBI pin is swept with an input which varies from 195mV to 205mV, and LBO (with a 100k pull-up resistor) is displayed.
V
LBO
1V/DIV
195 200 205
(mV) 1308 F12
V
LBI
Figure 12. Low-Battery Detector Input/Output Characteristic
START-UP
The LT1308A/LT1308B can start up into heavy loads, unlike many CMOS DC/DC converters that derive operat­ing voltage from the output (a technique known as “bootstrapping”). Figure 13 details start-up waveforms of Figure 1’s circuit with a 20Ω load and VIN of 1.5V. Inductor current rises to 3.5A as the output capacitor is charged. After the output reaches 5V, inductor current is about 1A. In Figure 14, the load is 5Ω and input voltage is 3V. Output voltage reaches 5V in 500μs after the device is enabled. Figure 15 shows start-up behavior of Figure 5’s SEPIC circuit, driven from a 9V input with a 10Ω load. The output reaches 5V in about 1ms after the device is enabled.
200mV
V
BAT
INTERNAL REFERENCE
GND
1308 F10
Figure 10. Setting Low-Battery Detector Trip Point
200k
V
BAT
2N3906
V
REF
200mV
+
10k
LBO
LBI
10μF
Figure 11. Accessing 200mV Reference
R1 =
V
IN
LT1308A LT1308B
GND
1308 F11
V
LB
– 200mV
2μA
V
OUT
2V/DIV
I
L1
1A/DIV
V
SHDN
5V/DIV
1ms/DIV
Figure 13. 5V Boost Converter of Figure 1. Start-Up from 1.5V Input into 20Ω Load
1308 F13
1308abfa
11
LT1308A/LT1308B
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APPLICATIONS INFORMATION
V
OUT
1V/DIV
I
L1
2A/DIV
V
SHDN
5V/DIV
Figure 14. 5V Boost Converter of Figure 1. Start-Up from 3V Input into 5Ω Load
V
OUT
2V/DIV
I
SW
2A/DIV
V
SHDN
5V/DIV
Figure 15. 5V SEPIC Start-Up from 9V Input into 10Ω Load
Soft-Start
In some cases it may be undesirable for the LT1308A/ LT1308B to operate at current limit during start-up, e.g.,
500μs/DIV
1308 F14
500μs/DIV
1308 F15
when operating from a battery composed of alkaline cells. The inrush current may cause sufficiency internal voltage drop to trigger a low-battery indicator. A pro­grammable soft-start can be implemented with 4 discrete components. A 5V to 12V boost converter using the LT1308B is detailed in Figure 16. C4 differentiates V causing a current to flow into R3 as V
increases.
OUT
OUT
,
When this current exceeds 0.7V/33k, or 21μA, current flows into the base of Q1. Q1’s collector then pulls current out the VC pin, creating a feedback loop where the slope of V
ΔΔV
OUT
With C4 = 33nF, V
is limited as follows:
OUT
V
07
=
t
kC
33 4.•
OUT
/t is limited to 640mV/ms. Start-up
waveforms for Figure 16’s circuit are pictured in Figure
17. Without the soft-start circuit implemented, the inrush current reaches 3A. The circuit reaches final output voltage in approximately 250μs. Adding the soft-start components reduces inductor current to less than 1A, as detailed in Figure 18, while the time required to reach final output voltage increases to about 15ms. C4 can be adjusted to achieve any output slew rate desired.
12
V
IN
5V
+
C1 47μF
C4 33nF
R3 33k
C1: AVX TAJ476M010 C2: TAIYO YUDEN TMK432BJ106MM D1: IR 10BQ015 L1: MURATA LQH6C4R7 Q1: 2N3904
SHUTDOWN
R4
33k
Q1
V
IN
SHDN
LT1308B
V
C
R
C
47k
SOFT-START COMPONENTS
L1
4.7μH
C
C
100pF
SW
GND
D1
330pF
100k
10k
FB
11.3k
1308 F16
C2 10μF
V
OUT
12V 500mA
Figure 16. 5V to 12V Boost Converter with Soft-Start Components Q1, C4, R3 and R4.
1308abfa
LT1308A/LT1308B
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APPLICATIONS INFORMATION
12V
V
OUT
5V/DIV
5V
I
L1
1A/DIV
V
SHDN
10V/DIV
50μs/DIV
Figure 17. Start-Up Waveforms of Figure 16’s Circuit without Soft-Start Components
12V
V
OUT
5V
I
L1
1A/DIV
V
SHDN
10V/DIV
5ms/DIV
Figure 18. Start-Up Waveforms of Figure 16’s Circuit with Soft-Start Components Added
COMPONENT SELECTION
Diodes
We have found ON Semiconductor MBRS130 and Interna­tional Rectifier 10BQ015 to perform well. For applications where V
exceeds 30V, use 40V diodes such as
OUT
MBRS140 or 10BQ040.
Height limited applications may benefit from the use of the MBRM120. This component is only 1mm tall and offers performance similar to the MBRS130.
Inductors
Suitable inductors for use with the LT1308A/LT1308B must fulfill two requirements. First, the inductor must be able to handle current of 2A steady-state, as well as support transient and start-up current over 3A without inductance decreasing by more than 50% to 60%. Second, the DCR of the inductor should have low DCR, under 0.05Ω
1308 F17
1308 F18
so that copper loss is minimized. Acceptable inductance values range between 2μH and 20μH, with 4.7μH best for most applications. Lower value inductors are physically smaller than higher value inductors for the same current capability.
Table 1 lists some inductors we have found to perform well in LT1308A/LT1308B application circuits. This is not an exclusive list.
Table 1
VENDOR PART NO. VALUE PHONE NO.
Murata LQH6C4R7 4.7μH 770-436-1300
Sumida CDRH734R7 4.7μH 847-956-0666
Coiltronics CTX5-1 5μH 561-241-7876
Coilcraft LPO2506IB-472 4.7μH 847-639-6400
Capacitors
Equivalent Series Resistance (ESR) is the main issue regarding selection of capacitors, especially the output capacitors.
The output capacitors specified for use with the LT1308A/ LT1308B circuits have low ESR and are specifically designed for power supply applications. Output voltage ripple of a boost converter is equal to ESR multiplied by switch current. The performance of the AVX TPSD227M006 220μF tantalum can be evaluated by referring to Figure 3. When the load is 800mA, the peak switch current is approximately 2A. Output voltage ripple is about 60mV
, so the ESR of the output capacitor is 60mV/2A or 0.03Ω.
P
P-
Ripple can be further reduced by paralleling ceramic units.
Table 2 lists some capacitors we have found to perform well in the LT1308A/LT1308B application circuits. This is not an exclusive list.
Table 2
VENDOR SERIES PART NO. VALUE PHONE NO.
AVX TPS TPSD227M006 220μF, 6V 803-448-9411
AVX TPS TPSD107M010 100μF, 10V 803-448-9411
Taiyo Yuden X5R LMK432BJ226 22μF, 10V 408-573-4150
Taiyo Yuden X5R TMK432BJ106 10μF, 25V 408-573-4150
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13
LT1308A/LT1308B
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APPLICATIONS INFORMATION
Ceramic Capacitors
Multilayer ceramic capacitors have become popular, due to their small size, low cost, and near-zero ESR. Ceramic capacitors can be used successfully in LT1308A/LT1308B designs provided loop stability is considered. A tantalum capacitor has some ESR and this causes an "ESR zero" in the regulator loop. This zero is beneficial to loop stability. Ceramics do not have appreciable ESR, so the zero is lost when they are used. However, the LT1308A/LT1308B have external compensation pin (VC) so component val­ues can be adjusted to achieve stability. A phase lead capacitor can also be used to tune up load step response to optimum levels, as detailed in the following paragraphs.
Figure 19 details a 5V to 12V boost converter using either a tantalum or ceramic capacitor for C2. The input capaci­tor has little effect on loop stability, as long as minimum capacitance requirements are met. The phase lead capaci­tor CPL parallels feedback resistor R1. Figure 20 shows load step response of a 50mA to 500mA load step using a 47μF tantalum capacitor at the output. Without the phase lead capacitor, there is some ringing, suggesting the phase margin is low. CPL is then added, and response to the same load step is pictured in Figure 21. Some phase margin is restored, improving the response. Next, C2 is replaced by a 10μF, X5R dielectric, ceramic capacitor.
Without CPL, load step response is pictured in Figure 22. Although the output settles faster than the tantalum case, there is appreciable ringing, again suggesting phase mar­gin is low. Figure 23 depicts load step response using the 10μF ceramic output capacitor and CPL. Response is clean and no ringing is evident. Ceramic capacitors have the added benefit of lowering ripple at the switching frequency due to their very low ESR. By applying CPL in tandem with the series RC at the V
pin, loop response can be tailored
C
to optimize response using ceramic output capacitors.
V
OUT
500mV/DIV
I
L1
1A/DIV
500mA
LOAD
CURRENT
50mA
Figure 20. Load Step Response of LT1308B 5V to 12V Boost Converter with 47μF Tantalum Output Capacitor
V
OUT
500mV/DIV
200μs/DIV
1308 F20
V
IN
5V
V
IN
SHDN
V
C
+
C1 47μF
C1: AVX TAJC476M010 C2: AVX TPSD476M016 (47μF) OR
TAIYO YUDEN TMK432BJ106MM (10μF) D1: IR 10BQ015 L1: MURATA LQH6C4R7
Figure 19. 5V to 12V Boost Converter
L1
4.7μH
LT1308B
47k
100pF
SW
GND
I
D1
R1
R3
100k
10k
FB
R2
11.3k
C
PL
330pF
1308 F19
C2
V
OUT
12V 500mA
CURRENT
CURRENT
L1
1A/DIV
500mA
LOAD
50mA
200μs/DIV
1308 F21
Figure 21. Load Step Response with 47μF Tantalum Output Capacitor and Phase Lead Capacitor C
V
OUT
1V/DIV
I
L1
1A/DIV
500mA
LOAD
50mA
200μs/DIV
1308 F22
PL
Figure 22. Load Step Response with 10μF X5R Ceramic Output Capacitor
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14
LT1308A/LT1308B
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APPLICATIONS INFORMATION
V
OUT
500mV/DIV
I
L1
1A/DIV
500mA
LOAD
CURRENT
50mA
Figure 23. Load Step Response with 10μF X5R Ceramic Output Capacitor and C
GSM AND CDMA PHONES
The LT1308A/LT1308B are suitable for converting a single Li-Ion cell to 5V for powering RF power stages in GSM or CDMA phones. Improvements in the LT1308A/LT1308B error amplifiers allow external compensation values to be reduced, resulting in faster transient response compared to the LT1308. The circuit of Figure 24 (same as Figure 1, printed again for convenience) provides a 5V, 1A output from a Li-Ion cell. Figure 25 details transient response at the LT1308A operating at a VIN of 4.2V, 3.6V and 3V. Ripple voltage in Burst Mode operation can be seen at 10mA load. Figure 26 shows transient response of the LT1308B under the same conditions. Note the lack of Burst Mode ripple at 10mA load.
+
C1 47μF
Li-Ion CELL
200μs/DIV
V
IN
SHDNSHUTDOWN
V
C
L1
4.7μH
LT1308B
47k
100pF
SW
GND
1308 F23
PL
D1
R1 309k
FB
R2 100k
5V 1A
+
C2 220μF
V
OUT
VIN = 4.2V
V
OUT
VIN = 3.6V
V
OUT
VIN = 3V
I
LOAD
1A
10mA
V
TRACES = 200μs/DIV
OUT
200mV/DIV
1308 F25
Figure 25. LT1308A Li-Ion to 5V Boost Converter Transient Response to 1A Load Step
V
OUT
VIN = 4.2V
V
OUT
VIN = 3.6V
V
OUT
VIN = 3V
I
LOAD
1A
10mA
V
TRACES = 100μs/DIV
OUT
200mV/DIV
1308 F26
Figure 26. LT1308B Li-Ion to 5V Boost Converter Transient Response to 1A Load Step
C1: AVX TAJC476M010 C2: AVX TPSD227M006
D1: IR 10BQ015 L1: MURATA LQH6N4R7
Figure 24. Li-Ion to 5V Boost Converter Delivers 1A
1308A/B F24
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15
LT1308A/LT1308B
TYPICAL APPLICATIO S
U
Triple Output TFTLCD Bias Supply
V
IN
5V
V
3
SHDN
C1
4.7μF
C1:TAIYO-YUDEN JMK212BJ475MG C2, C3:TAIYO-YUDEN LMK325BJ106MN C4, C5, C6:TAIYO-YUDEN EMK212BJ105MG D1: MBRM120 D2,D3,D4: BAT54S L1: TOKO 817FY-4R7M
220k
100pF
1
V
C
D2
1μF
0.22μF
0.22μF
L1
4.7μH
65
IN
LT1308B
SW
GND
D1
2
FB
4
C4
0.22μF
76.8k
10.7k
V
OFF
–9V 10mA
D3
D4
C5 1μF
C6 1μF
C2, C3 10μF ×2
1308 TA02
V
ON
27V 15mA
AV
DD
10V 500mA
500mV/DIV
500mV/DIV
500mV/DIV
I
LOAD
TFTLCD Bias Supply Transient Response
AV
DD
V
ON
V
OFF
800mA
200mA
100μs/DIV
1308abfa
16
TYPICAL APPLICATIO S
3.3V
REGULATED
Q1
150k
3.3k
U
100k
47k
324k
22nF
40nF EL Panel Driver
V
BAT
3V TO 6V
+
1μF
V
IN
LBI
V
C
LT1308A
100pF
47pF
2M
49.9k
C1 47μF
1
SW
SHDN
GND
LT1308A/LT1308B
T1
D2
1:12
4
3
6
D1
FBLBO
D3
4.3M
17k
SHUTDOWN
400V
C2 1μF 200V
Q2
EL PANEL 40nF
10k
V
IN
2.7V TO 6V
100pF
+
SHUTDOWN
47k
D1, D2, D3: BAV21 200mA, 250V D4: MBR0540 T1: MIDCOM 31105R L
C1: AVX TAJC476M010 C2: VITRAMON VJ225Y105KXCAT D1: BAT54 D2, D3: BAV21
High Voltage Supply 350V at 1.2mA
10nF
C1 47μF
10nF
V
SHDN
V
C
IN
LT1308A
= 1.5μH
P
GND
250V
T1
1:12
3
4
1
6
D4
SW
FB
D3
D2
D1
10M
34.8k
10nF 250V
10nF 250V
1308 TA04
Q1: MMBT3906 Q2: ZETEX FCX458 T1: MIDCOM 31105
SEPIC Converts 3V to 10V Input to a 5V/500mA Regulated Output
V
OUT
350V
1.2mA
V
IN
3V TO
10V
C1: AVX TAJC476M016 C2: TAIYO YUDEN EMK325BJ475(X5R) C3: AVX TPSD227M006
1308 TA03
C2
L1A
CTX10-2
V
SHDNSHUTDOWN
IN
V
C
LT1308B
47k
680pF
+
C1 47μF
4.7μF
CERAMIC
SW
FB
GND
D1: IR 10BQ015 L1: COILTRONICS CTX10-2
309k
R2 100k
D1
L1B
R1
+
V
OUT
5V 500mA
C3 220μF
6.3V
1308A/B TA05
1308abfa
17
LT1308A/LT1308B
PACKAGE DESCRIPTION
8-Lead Plastic Small Outline (Narrow .150 Inch)
.050 BSC
U
S8 Package
(Reference LTC DWG # 05-08-1610)
.045 ±.005
.189 – .197
(4.801 – 5.004)
8
NOTE 3
7
5
6
.245
MIN
.030 ±.005
TYP
RECOMMENDED SOLDER PAD LAYOUT
.010 – .020
(0.254 – 0.508)
.008 – .010
(0.203 – 0.254)
NOTE:
1. DIMENSIONS IN
2. DRAWING NOT TO SCALE
3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
× 45°
.016 – .050
(0.406 – 1.270)
INCHES
(MILLIMETERS)
.160 ±.005
.228 – .244
(5.791 – 6.197)
0°– 8° TYP
.053 – .069
(1.346 – 1.752)
.014 – .019
(0.355 – 0.483)
TYP
.150 – .157
(3.810 – 3.988)
NOTE 3
1
3
2
4
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
SO8 0303
18
1308abfa
PACKAGE DESCRIPTION
U
F Package
14-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1650)
1.05 ±0.10
4.90 – 5.10* (.193 – .201)
14 13 12 11 10 9
LT1308A/LT1308B
8
6.60 ±0.10
0.45 ± 0.05
RECOMMENDED SOLDER PAD LAYOUT
4.30 – 4.50** (.169 – .177)
0.09 – 0.20
(.0035 – .0079)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE *
DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED .152mm (.006") PER SIDE
**
DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED .254mm (.010") PER SIDE
0.50 – 0.75
(.020 – .030)
4.50 ±0.10
MILLIMETERS
(INCHES)
0.65 BSC
0.25 REF
0° – 8°
0.65
(.0256)
BSC
0.19 – 0.30
(.0075 – .0118)
13456
2
TYP
6.40
(.252)
BSC
7
1.10
(.0433)
MAX
0.05 – 0.15
(.002 – .006)
F14 TSSOP 0204
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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19
LT1308A/LT1308B
TYPICAL APPLICATIO
U
Li-Ion to 12V/300mA Step-Up DC/DC Converter
2.7V TO 4.2V
+
C1 47μF
Li-Ion CELL
C1: AVX TAJC476M010 C2: AVX TPSD107M016 D1: IR 10BQ015
L1
4.7μH
V
IN
LT1308B
SHDNSHUTDOWN
V
C
47k
330pF
L1: MURATA LQH6C4R7
SW
GND
D1
R1 887k
FB
R2 100k
+
12V 300mA
C2 100μF
1308A/B TA01
RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS
LT1302 High Output Current Micropower DC/DC Converter 5V/600mA from 2V, 2A Internal Switch, 200μA I
LT1304 2-Cell Micropower DC/DC Converter 5V/200mA, Low-Battery Detector Active in Shutdown
LT1307/LT1307B Single Cell, Micropower, 600kHz PWM DC/DC Converters 3.3V at 75mA from One Cell, MSOP Package
LT1316 Burst Mode Operation DC/DC with Programmable Current Limit 1.5V Minimum, Precise Control of Peak Current Limit
LT1317/LT1317B Micropower, 600kHz PWM DC/DC Converters 100μA IQ, Operate with VIN as Low as 1.5V
LTC®1474 Micropower Step-Down DC/DC Converter 94% Efficiency, 10μA IQ, 9V to 5V at 250mA
LTC1516 2-Cell to 5V Regulated Charge Pump 12μA IQ, No Inudctors, 5V at 50mA from 3V Input
LTC1522 Micropower, 5V Charge Pump DC/DC Converter Regulated 5V ± 4% Output, 20mA from 3V Input
LT1610 Single-Cell Micropower DC/DC Converter 3V at 30mA from 1V, 1.7MHz Fixed Frequency
LT1611 Inverting 1.4MHz Switching Regulator in 5-Lead SOT-23 – 5V at 150mA from 5V Input, Tiny SOT-23 package
LT1613 1.4MHz Switching Regulator in 5-Lead SOT-23 5V at 200mA from 4.4V Input, Tiny SOT-23 package
LT1615 Micropower Step-Up DC/DC in 5-Lead SOT-23 20μA IQ, 36V, 350mA Switch
LT1617 Micropower Inverting DC/DC Converter in SOT-23 VIN = 1V to 15V; V
OUT
to –34V
LTC1682 Doubler Charge Pump with Low Noise LDO Adjustable or Fixed 3.3V, 5V Outputs, 60μV
LT1949 600kHz, 1A Switch PWM DC/DC Converter 1.1A, 0.5Ω, 30V Internal Switch, VIN as Low as 1.5V
LT1949-1 1.1MHz, 1A Switch DC/DC Converter 1.1MHz Version of LT1949
Q
Output Noise
RMS
20
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
www.linear-tech.com
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LT 0807 REV A • PRINTED IN USA
© LINEAR TECHNOLOGY CORPORATION 1999
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