1
LT1308
Single Cell High Current
Micropower 600kHz
Boost DC/DC Converter
January 1998
Final Electrical Specifications
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
TYPICAL APPLICATIO
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DESCRIPTIO
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FEATURES
The LT
1308 is a micropower, fixed frequency boost
DC/DC converter that operates from an input voltage as
low as 1V. Capable of delivering 5V at load current of 1A
from a single Li-Ion cell, the LT1308 also features power
saving Burst Mode operation at light loads. High efficiency
is maintained over a broad 1mA to 1A load range.
The device contains a low-battery detector with a 200mV
reference and shuts down to less than 5
A quiescent
current. No-load quiescent current is 100
A and the
internal NPN power switch handles a 2A current with a
voltage drop of just 300mV.
High frequency 600kHz switching allows the use of small,
surface mount components. The LT1308's current mode
architecture provides fast response to load and line varia-
tions. The device is available in an 8-lead SO package.
s
5V at 1A from a Single Li-Ion Cell
s
3.3V at 300mA from a Single NiCd Cell
s
Low Quiescent Current: 100
A
s
Operates with V
IN
as Low as 1V
s
Fixed Frequency Operation: 600kHz
s
Current Mode PWM Delivers Low Output Ripple
s
Guaranteed Start-Up into Full Load
s
Low Shutdown Current: 3
A
s
Low-Battery Comparator
s
Automatic Burst Mode
TM
Operation at Light Load
s
Low V
CESAT
Switch: 300mV at 2A
s
GSM Terminals
s
Digital Cameras
s
Answer-Back Pagers
s
Cordless Telephones
s
DECT Phones
s
GPS Receivers
s
Battery Backup Supplies
APPLICATIO
N
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Burst Mode is a trademark of Linear Technology Corporation.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Figure 1. Single Li-Ion Cell to 5V/1A DC/DC Converter
Converter Efficiency
LOAD CURRENT (mA)
1
EFFICIENCY (%)
95
90
85
80
75
70
65
10
100
1000
1308 F01a
V
IN
= 4.2V
V
IN
= 3.6V
V
IN
= 3V
V
IN
SW
FB
LT1308
L1
4.7
H
4.2V TO 3V
D1
LBO
LBI
R
C
47k
R2
100k
5V
1A
R1
301k
C
C
22nF
1308F01
C1
10
F
C2
100
F
Li-Ion
CELL
V
C
GND
SHDN
C1: CERAMIC
C2: AVX TPS SERIES
D1: INTERNATIONAL RECTIFIER 10BQ015
L1: COILTRONICS CTX5-1
COILCRAFT DO3316-472
+
2
LT1308
A
U
G
W
A
W
U
W
A
R
BSOLUTE
XI
TI
S
V
IN
, SHDN, LBO Voltage ......................................... 10V
SW Voltage ............................................................. 30V
FB Voltage ....................................................... V
IN
+ 1V
V
C
Voltage ................................................................ 2V
LBI Voltage ............................................ 0V
V
LBI
1V
Current into FB Pin ..............................................
1mA
Junction Temperature ........................................... 125
C
Operating Temperature Range
Commercial (Note 1) ......................... 20
C to 70
C
Industrial ........................................... 40
C to 85
C
Storage Temperature Range ................ 65
C to 150
C
Lead Temperature (Soldering, 10 sec)................. 300
C
ORDER PART
NUMBER
1
2
3
4
8
7
6
5
TOP VIEW
LBO
LBI
V
IN
SW
V
C
FB
SHDN
GND
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125
C,
JA
= 80
C/W
W
U
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PACKAGE/ORDER I FOR ATIO
Consult factory for Military grade parts.
LT1308CS8
LT1308IS8
ELECTRICAL C
C
HARA TERISTICS
Commercial Grade 0
C to 70
C. V
IN
= 1.1V, V
SHDN
= V
IN
, T
A
= 25
C, unless otherwise noted.
1308
1308I
S8 PART MARKING
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
Q
Quiescent Current
Not Switching
q
80
160
A
V
SHDN
= 0V
q
1
3
A
V
FB
Feedback Voltage
q
1.20
1.22
1.24
V
I
B
FB Pin Bias Current (Note 2)
V
FB
= V
REF
q
27
80
nA
Reference Line Regulation
1.1V
V
IN
2V (25
C, 0
C)
0.6
1.1
%/V
1.1V
V
IN
2V (70
C)
1.5
%/V
2V
V
IN
6V
q
0.3
0.8
%/V
Minimum Input Voltage
0.92
1
V
Input Voltage Range
q
1
6
V
g
m
Error Amp Transconductance
I = 5
A
40
mhos
A
V
Error Amp Voltage Gain
25
C, 0
C
100
V/V
70
C
80
V/V
f
OSC
Switching Frequency
q
500
600
700
kHz
Maximum Duty Cycle
q
80
88
95
%
Switch Current Limit (Note 3)
DC = 40%
q
2.0
2.5
A
DC = 80%
1.6
2
A
Switch V
CESAT
I
SW
= 2A (25
C, 0
C)
300
350
mV
I
SW
= 2A (70
C)
330
400
mV
Burst Mode Operation Switch Current Limit
L = 3.3
H, V
OUT
= 3.3V, V
IN
= 1.2V
200
mA
Shutdown Pin Current
V
SHDN
= 1.1V
q
2.5
4.0
A
V
SHDN
= 6V
13
26
A
V
SHDN
= 0V
q
1.5
2.5
A
LBI Threshold Voltage
q
180
200
220
mV
LBO Output Low
I
SINK
= 10
A
q
0.1
0.25
V
LBO Leakage Current
V
LBI
= 250mV, V
LBO
= 5V
q
0.01
0.1
A
LBI Input Bias Current (Note 4)
V
LBI
= 150mV
q
5
30
nA
3
LT1308
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Low-Battery Detector Gain
1M
Load (25
C, 0
C)
1000
3000
V/V
1M
Load (70
C)
500
V/V
Switch Leakage Current
V
SW
= 5V
q
0.01
10
A
Reverse Battery Current
(Note 5)
750
mA
Commercial Grade 0
C to 70
C. V
IN
= 1.1V, V
SHDN
= V
IN
, T
A
= 25
C unless otherwise noted.
ELECTRICAL C
C
HARA TERISTICS
Commercial Grade T
A
= 20
C, V
IN
= 1.1V, V
SHDN
= V
IN
, unless otherwise noted (Note 1).
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
Q
Quiescent Current
V
FB
= 1.3V, Not Switching
80
160
A
V
SHDN
= 0V
1
3
A
V
FB
Feedback Voltage
1.195
1.22
1.245
V
g
m
Error Amp Transconductance
I = 5
A
35
mhos
A
V
Error Amp Voltage Gain
100
V/V
f
OSC
Switching Frequency
500
600
750
kHz
Maximum Duty Cycle
88
%
Switch V
CESAT
I
SW
= 2A, V
IN
= 1.2V
300
350
mV
Shutdown Pin Current
V
SHDN
= V
IN
2.5
4.0
A
V
SHDN
= 0V
1.5
2.5
A
LBI Threshold Voltage
180
200
220
mV
Industrial Grade 40
C to 85
C. V
IN
= 1.2V, V
SHDN
= V
IN
, T
A
= 25
C, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
Q
Quiescent Current
Not Switching
q
80
160
A
V
SHDN
= 0V
q
1
3
A
V
FB
Feedback Voltage
q
1.195
1.22
1.245
V
I
B
FB Pin Bias Current (Note 2)
V
FB
= V
REF
q
27
80
nA
Reference Line Regulation
1.1V
V
IN
2V ( 40
C)
0.6
1.1
%/V
1.1V
V
IN
2V (85
C)
1.5
%/V
2V
V
IN
6V
q
0.3
0.8
%/V
Minimum Input Voltage ( 40
C)
1.2
V
Input Voltage Range
q
1.2
6
V
g
m
Error Amp Transconductance
I = 5
A
40
mhos
A
V
Error Amp Voltage Gain
40
C
100
V/V
85
C
80
V/V
f
OSC
Switching Frequency
V
IN
= 1.3V ( 40
C)
500
600
750
kHz
V
IN
= 1.3V (85
C)
500
600
750
kHz
Maximum Duty Cycle
40
C
80
88
95
%
85
C
75
%
Switch Current Limit (Note 3)
DC = 40%
q
2.0
2.5
A
DC = 80%
1.6
2
A
Switch V
CESAT
I
SW
= 2A ( 40
C)
300
350
mV
I
SW
= 2A (85
C)
330
400
mV
Burst Mode Operation Switch Current Limit
L = 3.3
H, V
OUT
= 3.3V
200
mA
4
LT1308
ELECTRICAL C
C
HARA TERISTICS
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Shutdown Pin Current
V
SHDN
= 1.2V
q
2.5
4.0
A
V
SHDN
= 6V
q
13
26
A
V
SHDN
= 0V
q
1.5
2.5
A
LBI Threshold Voltage
q
180
200
220
mV
LBO Output Low
I
SINK
= 10
A
q
0.1
0.25
V
LBO Leakage Current
V
LBI
= 250mV, V
LBO
= 5V
q
0.01
0.1
A
LBI Input Bias Current (Note 4)
V
LBI
= 150mV
q
5
30
nA
Low-Battery Detector Gain
1M
Load (40
C)
1000
3000
V/V
1M
Load (85
C)
300
V/V
Switch Leakage Current
V
SW
= 5V
q
0.01
10
A
Industrial Grade 40
C to 85
C. V
IN
= 1.2V, V
SHDN
= V
IN
, T
A
= 25
C, unless otherwise noted.
Note 3: 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 4: Bias current flows out of LBI pin.
Note 5: The LT1308 will withstand continuous application of 1.6V applied
to GND pin while V
IN
and SW are grounded.
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1: C grade device specifications are guaranteed over the 0
C to 70
C
temperature range. In addition, C grade device specifications are assured
over the 40
C to 85
C temperature range by design or correlation, but
are not production tested.
Note 2: Bias current flows into FB pin.
TYPICAL PERFOR
M
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CE CHARACTERISTICS
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SWITCH CURRENT (A)
0
SWITCH V
CESAT
(mV)
2.0
85
C
1308 G03
0.5
1.0
1.5
500
400
300
200
100
0
25
C
40
C
LOAD CURRENT (mA)
90
85
80
75
70
65
60
55
50
1
100
1000
1308 G01
10
EFFICIENCY (%)
V
IN
= 1.2V
V
OUT
= 3.3V
R1
= 169k
V
OUT
200mV/DIV
AC COUPLED
100mA
5mA
I
LOAD
V
IN
=
1.2V
V
OUT
= 5V
C2
= 22
F
R
C
, C
C
= 47k, 6.8nF
L = 4.7
H
500
s/DIV
1308 G02
Transient Response
Switch Saturation Voltage vs
Current
Efficiency
V
C
(Pin 1): Compensation Pin for Error Amplifier. Con-
nect a series RC from this pin to ground. Typical values
are 47k
and 22nF. Minimize trace area at V
C
.
FB (Pin 2): Feedback Pin. Reference voltage is 1.22V.
Connect resistive divider tap here. Minimize trace area at
FB. Set V
OUT
according to: V
OUT
= 1.22V(1 + R1/R2).
SHDN (Pin 3): Shutdown. Ground this pin to turn off
switcher. Must be tied to V
IN
(or higher voltage) to enable
switcher. Do not float the SHDN pin.
GND (Pin 4): Ground. Connect directly to local ground
plane. Ground plane should enclose all components
associated with the LT1308.
PI
N
FU
N
CTIO
N
S
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5
LT1308
PI
N
FU
N
CTIO
N
S
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SW (Pin 5): Switch Pin. Connect inductor/diode here.
Minimize trace area at this pin to keep EMI down.
V
IN
(Pin 6): Supply Pin. Must have local bypass capacitor
right at the pin, connected directly to ground.
LBI (Pin 7): Low-Battery Detector Input. 200mV refer-
ence. Voltage on LBI must stay between ground and
700mV. Low-battery detector does not function with
SHDN pin grounded. If not used, float LBI pin.
LBO (Pin 8): Low-Battery Detector Output. Open collec-
tor, can sink 10
A. A 1M
pullup is recommended. LBO
is high impedance when SHDN is grounded.
BLOCK DIAGRA
M
W
+
+
+
+
+
+
+
COMPARATOR
RAMP
GENERATOR
R
BIAS
V
C
g
m
Q2
10
Q1
FB
FB
ENABLE
200mV
A = 3
FF
A2
A1
ERROR
AMPLIFIER
A4
0.03
DRIVER
SW
GND
1308 BD
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
V
IN
V
IN
V
OUT
6
8
R2
(EXTERNAL)
LAYOUT HINTS
The LT1308 switches current at high speed, mandating
careful attention to layout for proper performance.
You will
not get advertised performance with careless layouts.
Figure 2 shows recommended component placement.
Follow this closely in your PC layout. Note the direct path
of the switching loops. Input capacitor C
IN
must be placed
close (< 5mm) to the IC package. As little as 10mm of wire
or PC trace from C
IN
to V
IN
will cause problems such as
inability to regulate or oscillation. A 10
F ceramic bypass
capacitor is the only input capacitance required
provided
the battery has a low inductance path to the circuit. The
battery itself provides the bulk capacitance the device
requires for proper operation. If the battery is located some
APPLICATIO
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FOR
M
ATIO
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Figure 2. Recommended Component Placement. Traces
Carrying High Current Are Direct. Trace Area at FB Pin and V
C
Pin is Kept Low. Lead Length to Battery Should Be Kept Short.
Ground Plane Should Be Placed Under All Components
1
2
3
4
8
7
6
5
L
C
IN
C
OUT
D
LT1308
V
OUT
V
IN
GND
MULTIPLE
VIAs
GROUND PLANE
1308 F02
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