Linear Technology LT1108 Datasheet

LT1108

Micropower

DC/DC Converter

Adjustable and Fixed 5V, 12V

EATU

F

■

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

PPLICATI

A

■

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

RE

S

O

U
S

DUESCRIPTIO

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, step­down, 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 battery­operated 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 lock­out circuit, or error amplifier.

A

2 × AA
CELLS

*L1 =

+

GOWANDA GA20-103K
COILTRONICS CTX100-4
SUMIDA CD105-101K

U

O

PPLICATITYPICAL

EfficiencyPalmtop Computer Logic Supply

84

82

80

78

76

EFFICIENCY (%)

74

72

70

1

VIN = 3V

VIN = 2.5V

V

= 2V

IN

10 100

LOAD CURRENT (mA)

LT1108 • TA02

100µF

47Ω

I

LIM

GND SW2

V

LT1108-5

SENSE

IN

SW1

L1*

100µH

1N5817

+

5V
150mA

AVX
TPS
330µF

6.3V

LT1108 • TA01

1

LT1108

A

W

O

LUTEXI T

S

A

WUW

ARB

U
G

I

S

Supply Voltage (VIN)............................................... 36V

SW1 Pin Voltage (V
SW2 Pin Voltage (V

) ......................................... 50V

SW1

) ............................ –0.5V to V

SW2

Feedback Pin Voltage (LT1108) ............................. 5.5V

Sense Pin Voltage (LT1108, -5, -12) ...................... 36V

WU

/

PACKAGE



I

LIM



V

IN

SW1
SW2

T

JMAX

O

RDER I FOR ATIO

TOP VIEW

1
2
3
4

N8 PACKAGE

8-LEAD PLASTIC DIP

*FIXED VERSIONS

= 90°C, θJA = 130°C/W

FB (SENSE*)

8

SET

7

A0

6

GND

5



ORDER PART

NUMBER

LT1108CN8
LT1108CN8-5
LT1108CN8-12

Maximum Power Dissipation ............................ 500mW

Maximum Switch Current ...................................... 1.5A

Operating Temperature Range .................... 0°C to 70°C

IN

Storage Temperature Range ................ –65°C to 150°C

Lead Temperature (Soldering, 10 sec)................. 300°C

U

ORDER PART



I

1

LIM



V

2

IN

SW1

3

SW2

4

S8 PACKAGE

8-LEAD PLASTIC SOIC

*FIXED VERSIONS

T

= 90°C, θJA = 150°C/W

JMAX

TOP VIEW

FB (SENSE*)

8

SET

7

A0

6

GND

5



NUMBER

LT1108CS8
LT1108CS8-5
LT1108CS8-12

S8 PART MARKING

1108
10805
10812

LECTRICAL C CHARA TERIST

E

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

V

V

f

t

V

V

Q

IN

OUT

OSC

ON

OL

SAT

Quiescent Current Switch OFF ● 110 150 µA
Quiescent Current, Boost Mode Configuration No Load LT1108-5 135 µA

Input Voltage Step-Up Mode ● 2 12.6 V

Comparator Trip Point Voltage LT1108 (Note 1) ● 1.2 1.245 1.3 V
Output Sense Voltage LT1108-5 (Note 2) ● 4.75 5 5.25 V

Comparator Hysteresis LT1108 ● 510 mV
Output Hysteresis LT1108-5 ● 20 40 mV

Oscillator Frequency ● 14 19 25 kHz
Duty Cycle Full Load, Step-Up Mode ● 63 70 78 %
Switch-ON Time I
Feedback Pin Bias Current LT1108, VFB = 0V ● 10 50 nA
Set Pin Bias Current V
Gain Block Output Low I
Reference Line Regulation 2V ≤ VIN ≤ 5V ● 0.20 0.400 %/V

SW

Voltage, Step-Up Mode VIN = 3V, ISW = 650mA ● 0.5 0.65 V

SAT

ICS

TA = 25°C, VIN = 3V, unless otherwise noted.

LT1108-12 250 µA

Step-Down Mode ● 30.0 V

LT1108-12 (Note 2) ● 11.4 12 12.6 V

LT1108-12

Tied to VIN, Step-Up Mode ● 28 36 48 µs

LIM

= V

SET

REF

= 100µA, V

SINK

5V ≤ V

≤ 30V ● 0.02 0.075 %/V

IN

VIN = 5V, ISW = 1A 0.8 1.00 V

= 1V ● 0.15 0.4 V

SET

● 50 100 mV

● 20 100 nA

2

LT1108

R

LIM

(Ω)

10

SWITCH CURRENT (mA)

100 1000

LT1108 • TPC03

1000

1200
1100

900
800
700
600
500
400
300
200
100

2V ≤ VIN ≤ 5V

LECTRICAL C CHARA TERIST

E

ICS

TA = 25°C, VIN = 3V, unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

SAT

A

V

SW

Voltage, Step-Down Mode VIN = 12V, ISW = 650mA 1.1 1.5 V

SAT

● 1.7 V

Gain Block Gain RL = 100k (Note 3) ● 400 1000 V/V
Current Limit 220Ω from I

LIM

to V

IN

400 mA

Current Limit Temperature Coefficient ● –0.3 %/°C
Switch OFF Leakage Current Measured at SW1 Pin 1 10 µA

V

SW2

● denotes specifications which apply over the full operating

The
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.

Maximum Excursion Below GND I

≤ 10µA, Switch OFF –400 –350 mV

SW1

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.

UW

Y

PICA

1.2

LPER

F

O

R

AT

Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)

CCHARA TERIST

E

C

Switch ON Voltage
Step-Down Mode
(SW1 Pin Connected to VIN)

1.4

ICS

Maximum Switch Current
vs R

LIM

1.0

0.8

(V)

0.6

CESAT

V

0.4

0.2

0

0

Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)

1000

900
800
700
600

500
400

300

SWITCH CURRENT (mA)

200
100

0

100

VIN = 3V

VIN = 2V

0.4 0.6 0.8

0.2
SWITCH CURRENT (A)

VIN = 24V
L = 500µH

VIN = 12V

L = 250µH

R

(Ω)

LIM

VIN = 5V

1.0 1.2

LT1108 • TPC01

V

= 5V

OUT

LT1108 • TPC04

1000

1.3

1.2

1.1

1.0

0.9

SWITCH ON VOLTAGE (V)

0.8

0.7

0.1 0.3 0.7

0.2 0.4 0.8

0

SWITCH CURRENT (A)

0.5

0.6

LT1108 • TPC02

Supply Current vs Switch Current Quiescent Current

50

40

30

20

SUPPLY CURRENT (mA)

10

0

VIN = 5V

200

0

SWITCH CURRENT (mA)

VIN = 2V

400

600

800

LTC1108 • TPC05

1000



120

115

110

105

100

95

90

QUIESCENT CURRENT (µA)

85

80

–25 0 50

–50

25

TEMPERATURE (°C)

75

LT1108 • TPC06

3

100

LT1108

TEMPERATURE (˚C)

–50



38

40

42

25 75

LT1108 • TPC09

36

34

–25 0

50 100

32

30

SWITCH-ON TIME (µs)

44

Y

PICA

LPER

F

O

R

AT

UW

CCHARA TERIST

E

C

ICS

Oscillator Frequency Duty Cycle Switch-ON Time

22

21

20

19

18

17

16

FREQUENCY (kHz)

15

14

13

–25 0 50

–50

25

TEMPERATURE (°C)

75

LT1108 • TPC07

100



80

75

70

65

DUTY CYCLE (%)

60

55

50

–25 0 50

–50

25

TEMPERATURE (°C)

75

LT1108 • TPC08

100

Minimum/Maximum Frequency Switch Saturation Voltage Switch Saturation Voltage
vs ON-Time Step-Up Mode Step-Down Mode

28
26
24
22
20
18
16

FREQUENCY (kHz)

14
12
10

0

30

25

40

35

ON-TIME (µs)

45

LT1108 • TPC10

0.8
ISW = 650mA

0.7

0.6

0.5

(V)

0.4

CESAT

V

0.3

0.2

0.1

50

0

–25 0 50

–50

25

TEMPERATURE (°C)

75

LT1108 • TPC11

100

(V)

SAT

V

1.8

1.6

1.5

1.4

1.2

0.8

1.7

1.3

1.1

1.0

0.9

–50

ISW = 650mA

–25

0

TEMPERATURE (°C)

25 50

75

LT1108 • TPC12

100

U

PI

I

lower current limit is desired, connect a resistor between
I
to approximately 400mA.

V
SW1 (Pin 3): Collector of power transistor. For step-up

mode connect to inductor/diode. For step-down mode
connect to VIN.

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

FUUC

(Pin 1): Connect this pin to VIN for normal use. Where

LIM

and VIN. A 220Ω resistor will limit the switch current

LIM

(Pin 2): Input supply voltage.

IN

TI

O

U
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.

than a Schottky diode drop below ground.

4