LT1108
Micropower
DC/DC Converter
Adjustable and Fixed 5V, 12V
FEATURES
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DESCRIPTIO
Operates at Supply Voltages from 2V to 30V
Consumes Only 110µA Supply Current
Works in Step-Up or Step-Down Mode
Only Four External Components Required
Low Battery Detector Comparator On-Chip
User Adjustable Current Limit
Internal 1A Power Switch
Fixed or Adjustable Output Voltage Versions
Space Saving 8-Pin MiniDIP or S8 Package
The LT1108 is a versatile micropower DC/DC converter.
The device requires only four external components to
deliver a fixed output of 5V or 12V. Supply voltage ranges
from 2V to 12V in step-up mode and to 30V in step-down
mode. The LT1108 functions equally well in step-up, step-
down, or inverting applications.
The LT1108 is pin-for-pin compatible with the LT1173, but
has a duty cycle of 70%, resulting in increased output
current in many applications. The LT1108 can deliver
150mA at 5V from a 2 AA cell input and 5V at 300mA from
9V in step-down mode. Quiescent current is just 110µA,
making the LT1108 ideal for power conscious battery-
operated systems.
Switch current limit can be programmed with a single
resistor. An auxiliary gain block can be configured as a low
battery detector, linear post regulator, undervoltage lock-
out circuit, or error amplifier.
APPLICATI
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s
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S
Palmtop Computers
3V to 5V, 5V to 12V Converters
9V to 5V, 12V to 5V Converters
LCD Bias Generators
Peripherals and Add-On Cards
Battery Backup Supplies
Cellular Telephones
Portable Instruments
TYPICAL APPLICATI
Palmtop Computer Logic Supply
L1*
100µH
47Ω
I
LIM
2
×
AA
CELLS
V
IN
SW1
EFFICIENCY (%)
84
1N5817
5V
150mA
82
80
78
76
74
72
70
V
IN
= 3V
V
IN
= 2.5V
V
IN
= 2V
+
100µF
+
LT1108-5
SENSE
GND
SW2
AVX
TPS
330µF
6.3V
*L1 = GOWANDA GA20-103K
COILTRONICS CTX100-4
SUMIDA CD105-101K
LT1108 • TA01
U
Efficiency
1
10
100
LOAD CURRENT (mA)
LT1108 • TA02
UO
UO
1
LT1108
ABSOLUTE
AXI U
RATI GS
Maximum Power Dissipation ............................ 500mW
Maximum Switch Current ...................................... 1.5A
Operating Temperature Range .................... 0°C to 70°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
Supply Voltage (V
IN
) ............................................... 36V
SW1 Pin Voltage (V
SW1
) ......................................... 50V
SW2 Pin Voltage (V
SW2
) ............................ – 0.5V to V
IN
Feedback Pin Voltage (LT1108) ............................. 5.5V
Sense Pin Voltage (LT1108, -5, -12) ...................... 36V
PACKAGE/ORDER I FOR ATIO
TOP VIEW
I
LIM
1
V
IN
2
SW1 3
SW2 4
8
7
6
5
FB (SENSE*)
SET
A0
GND
ORDER PART
NUMBER
LT1108CN8
LT1108CN8-5
LT1108CN8-12
I
LIM
1
V
IN
2
SW1 3
SW2 4
N8 PACKAGE
8-LEAD PLASTIC DIP
*FIXED VERSIONS
T
JMAX
= 90°C,
θ
JA
= 130°C/W
ELECTRICAL CHARACTERISTICS
SYMBOL
I
Q
PARAMETER
Quiescent Current
Quiescent Current, Boost Mode Configuration
V
IN
Input Voltage
Comparator Trip Point Voltage
V
OUT
Output Sense Voltage
Comparator Hysteresis
Output Hysteresis
f
OSC
t
ON
Oscillator Frequency
Duty Cycle
Switch-ON Time
Feedback Pin Bias Current
Set Pin Bias Current
V
OL
Gain Block Output Low
Reference Line Regulation
V
SAT
SW
SAT
Voltage, Step-Up Mode
T
A
= 25°C, V
IN
= 3V, unless otherwise noted.
CONDITIONS
Switch OFF
No Load
Step-Up Mode
Step-Down Mode
LT1108 (Note 1)
LT1108-5 (Note 2)
LT1108-12 (Note 2)
LT1108
LT1108-5
LT1108-12
Full Load, Step-Up Mode
I
LIM
Tied to V
IN
, Step-Up Mode
LT1108, V
FB
= 0V
V
SET
= V
REF
I
SINK
= 100µA, V
SET
= 1V
2V
≤
V
IN
≤
5V
5V
≤
V
IN
≤
30V
V
IN
= 3V, I
SW
= 650mA
V
IN
= 5V, I
SW
= 1A
LT1108-5
LT1108-12
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
2
U
U
W
W W
U
W
TOP VIEW
8
7
6
5
FB (SENSE*)
SET
A0
GND
ORDER PART
NUMBER
LT1108CS8
LT1108CS8-5
LT1108CS8-12
S8 PART MARKING
1108
10805
10812
S8 PACKAGE
8-LEAD PLASTIC SOIC
*FIXED VERSIONS
T
JMAX
= 90°C,
θ
JA
= 150°C/W
MIN
TYP
110
135
250
MAX
150
UNITS
µA
µA
µA
2
1.2
4.75
11.4
1.245
5
12
5
20
50
14
63
28
19
70
36
10
20
0.15
0.20
0.02
0.5
0.8
12.6
30.0
1.3
5.25
12.6
10
40
100
25
78
48
50
100
0.4
0.400
0.075
0.65
1.00
V
V
V
V
V
mV
mV
mV
kHz
%
µs
nA
nA
V
%/V
%/V
V
V
LT1108
ELECTRICAL CHARACTERISTICS
SYMBOL
V
SAT
A
V
PARAMETER
SW
SAT
Voltage, Step-Down Mode
Gain Block Gain
Current Limit
Current Limit Temperature Coefficient
Switch OFF Leakage Current
V
SW2
Maximum Excursion Below GND
T
A
= 25°C, V
IN
= 3V, unless otherwise noted.
CONDITIONS
V
IN
= 12V, I
SW
= 650mA
q
MIN
TYP
1.1
MAX
1.5
1.7
UNITS
V
V
V/ V
mA
%/°C
R
L
= 100k (Note 3)
220Ω from I
LIM
to V
IN
q
400
1000
400
– 0.3
1
– 400
10
– 350
q
Measured at SW1 Pin
I
SW1
≤
10µA, Switch OFF
µA
mV
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1:
This specification guarantees that both the high and low trip points
of the comparator fall within the 1.2V to 1.3V range.
Note 2:
The output voltage waveform will exhibit a sawtooth shape due to
the comparator hysteresis. The output voltage on the fixed output versions
will always be within the specified range.
Note 3:
100k resistor connected between a 5V source and the A0 pin.
TYPICAL PERFOR A CE CHARACTERISTICS
Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)
1.2
1.0
SWITCH ON VOLTAGE (V)
SWITCH CURRENT (mA)
V
IN
= 3V
0.8
V
CESAT
(V)
V
IN
= 2V
0.6
V
IN
= 5V
0.4
0.2
0
0
0.2
0.4
0.6
0.8
SWITCH CURRENT (A)
Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)
1000
900
800
SWITCH CURRENT (mA)
V
OUT
= 5V
QUIESCENT CURRENT (µA)
700
600
500
400
300
200
100
0
100
SUPPLY CURRENT (mA)
V
IN
= 24V
L = 500µH
V
IN
= 12V
L = 250µH
R
LIM
(Ω)
LT1108 • TPC04
U W
1.0
1.2
LT1108 • TPC01
Switch ON Voltage
Step-Down Mode
(SW1 Pin Connected to V
IN
)
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0
0.1 0.2
0.5 0.6
SWITCH CURRENT (A)
0.3
0.4
0.7 0.8
LT1108 • TPC02
Maximum Switch Current
vs R
LIM
1200
1100
1000
900
800
700
600
500
400
300
200
100
10
100
R
LIM
(Ω)
1000
LT1108 • TPC03
2V
≤
V
IN
≤
5V
Supply Current vs Switch Current
50
Quiescent Current
120
115
40
110
105
100
95
90
85
30
V
IN
= 5V
20
10
V
IN
= 2V
1000
0
0
200
600
800
400
SWITCH CURRENT (mA)
1000
80
–50
–25
0
50
25
TEMPERATURE (°C)
75
100
LTC1108 • TPC05
LT1108 • TPC06
3
LT1108
TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
22
21
20
SWITCH-ON TIME (µs)
FREQUENCY (kHz)
DUTY CYCLE (%)
19
18
17
16
15
14
13
–50
–25
0
50
25
TEMPERATURE (°C)
75
100
Minimum/Maximum Frequency
vs ON-Time
28
26
24
0.8
0.7
0.6
FREQUENCY (kHz)
22
V
CESAT
(V)
20
18
16
14
12
10
0
25
30
40
35
ON-TIME (µs)
45
50
V
SAT
(V)
PI FU CTI
S
GND (Pin 5):
Ground.
AO (Pin 6):
Auxiliary gain block (GB) output. Open collector,
can sink 100µA.
SET (Pin 7):
GB input. GB is an op amp with positive input
connected to SET pin and negative input connected to
1.245V reference.
FB/SENSE (Pin 8):
On the LT1108 (adjustable) this pin
goes to the comparator input. On the LT1108-5 and
LT1108-12, this pin goes to the internal application resistor
that sets output voltage.
I
LIM
(Pin 1):
Connect this pin to V
IN
for normal use. Where
lower current limit is desired, connect a resistor between
I
LIM
and V
IN
. A 220Ω resistor will limit the switch current
to approximately 400mA.
V
IN
(Pin 2):
Input supply voltage.
SW1 (Pin 3):
Collector of power transistor. For step-up
mode connect to inductor/diode. For step-down mode
connect to V
IN
.
SW2 (Pin 4):
Emitter of power transistor. For step-up
mode connect to ground. For step-down mode connect to
inductor/diode. This pin must never be allowed to go more
than a Schottky diode drop below ground.
4
U W
LT1108 • TPC07
Duty Cycle
80
75
70
65
60
55
50
–50
44
42
40
38
36
34
32
Switch-ON Time
–25
0
50
25
TEMPERATURE (°C)
75
100
30
–50
–25
50
25
0
TEMPERATURE (˚C)
75
100
LT1108 • TPC08
LT1108 • TPC09
Switch Saturation Voltage
Step-Up Mode
1.8
I
SW
= 650mA
Switch Saturation Voltage
Step-Down Mode
1.7
1.6
1.5
I
SW
= 650mA
0.5
0.4
0.3
0.2
0.1
0
–50
–25
50
25
TEMPERATURE (°C)
0
75
100
1.4
1.3
1.2
1.1
1.0
0.9
0.8
–50
–25
25
50
0
TEMPERATURE (°C)
75
100
LT1108 • TPC10
LT1108 • TPC11
LT1108 • TPC12
UO
U
U
LT1108
1 OPERATI
The LT1108 is a gated oscillator switcher. This type
architecture has very low supply current because the
switch is cycled when the feedback pin voltage drops
below the reference voltage. Circuit operation can best be
understood by referring to the LT1108 block diagram.
Comparator A1 compares the feedback (FB) pin voltage
with the 1.245V reference signal. When FB drops below
1.245V, A1 switches on the 19kHz oscillator. The driver
amplifier boosts the signal level to drive the output NPN
power switch. The switch cycling action raises the output
voltage and FB pin voltage. When the FB voltage is suffi-
cient to trip A1, the oscillator is gated off. A small amount
of hysteresis built into A1 ensures loop stability without
external frequency compensation. When the comparator
output is low, the oscillator and all high current circuitry
is turned off, lowering device quiescent current to just
110µA.
The oscillator is set internally for 36µs ON-time and 17µs
OFF-time, allowing continuous mode operation in many
cases such as 2V to 5V converters. Continuous mode
greatly increases available output power.
Gain block A2 can serve as a low battery detector. The
BLOCK DIAGRA S
LT1108
LT1108-5/LT1108-12
SET
A2
V
IN
GAIN BLOCK/
ERROR AMP
1.245V
REFERENCE
A1
OSCILLATOR
DRIVER
GND
FB
COMPARATOR
LT1108 • BD
W
UO
negative input of A2 is the 1.245V reference. A resistor
divider from V
IN
to GND, with the mid-point connected to
the SET pin provides the trip voltage in a low battery
detector application. A0 can sink 100µA (use a 47k resis-
tor pull-up to 5V).
A resistor connected between the I
LIM
pin and V
IN
sets
maximum switch current. When the switch current ex-
ceeds the set value, the switch cycle is prematurely
terminated. If current limit is not used, I
LIM
should be tied
directly to V
IN
. Propagation delay through the current-
limit circuitry is approximately 2µs.
In step-up mode the switch emitter (SW2) is connected to
ground and the switch collector (SW1) drives the induc-
tor; in step-down mode the collector is connected to V
IN
and the emitter drives the inductor.
The LT1108-5 and LT1108-12 are functionally identical to
the LT1108. The -5 and -12 versions have on-chip voltage
setting resistors for fixed 5V or 12V outputs. Pin 8 on the
fixed versions should be connected to the output. No
external resistors are needed.
SET
A0
V
IN
I
LIM
SW1
1.245V
REFERENCE
A1
OSCILLATOR
DRIVER
COMPARATOR
SW2
R1
GND
R2
753k
SENSE
LT1108-5: R1 = 250k
LT1108-12: R1 = 87.4k
LT1108-5 • BD
A2
A0
GAIN BLOCK/
ERROR AMP
I
LIM
SW1
SW2
5
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