Datasheet LT1108 Datasheet (Linear Technology)

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

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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

OPER

I

LIM

A2

A1

V

IN

GND

SET

A0

GAIN BLOCK/
ERROR AMP

COMPARATOR

DRIVER

SW1

SW2

1.245V

REFERENCE

OSCILLATOR

LT1108-5 • BD

SENSE

R1

R2

753k

LT1108-5: R1 = 250k
LT1108-12: R1 = 87.4k

AT

LT1108

U

O

I

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 suffi­cient 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.

negative input of A2 is the 1.245V reference. A resistor
divider from VIN 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 resis­tor pull-up to 5V).

A resistor connected between the I

pin and VIN sets

LIM

maximum switch current. When the switch current ex­ceeds the set value, the switch cycle is prematurely
terminated. If current limit is not used, I

should be tied

LIM

directly to VIN. Propagation delay through the current­limit circuitry is approximately 2µs.

In step-up mode the switch emitter (SW2) is connected to
ground and the switch collector (SW1) drives the induc­tor; 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.

Gain block A2 can serve as a low battery detector. The

W

BLOCK

V

1.245V

REFERENCE

GND

IDAGRA

SET

IN

FB

S

LT1108 LT1108-5/LT1108-12

A2

GAIN BLOCK/
ERROR AMP

A1

COMPARATOR

A0

OSCILLATOR

DRIVER

LT1108 • BD

I

LIM

SW1

SW2

5

LT1108

P

f

L OSC

/()02

It

V

R

e

L

IN

Rt

L

()

'

–()

–'

=











103

It

V

L

t

L

IN

()

= ()04

ELI

L

PEAK

=

1
2

052()

P V V V mA mW

L

=+

()()

=12 0 5 2 30 315 06.– ()

U

O

PPLICATI

A

INDUCTOR SELECTION

General

A DC/DC converter operates by storing energy as mag­netic flux in an inductor core, and then switching this
energy into the load. Since it is flux, not charge, that is
stored, the output voltage can be higher, lower, or oppo­site in polarity to the input voltage by choosing an appro­priate switching topology.

To operate as an efficient energy transfer element, the
inductor must fulfill three requirements. First, the induc­tance must be low enough for the inductor to store adequate
energy under the worst case condition of minimum input
voltage and switch-ON time. The inductance must also be
high enough so maximum current ratings of the LT1108
and inductor are not exceeded at the other worst case
condition of maximum input voltage and ON-time.

S

I FOR ATIO

WU

U

where VD is the diode drop (0.5V for a 1N5818 Schottky).
Energy required by the inductor per cycle must be equal or
greater than

in order for the converter to regulate the output.
When the switch is closed, current in the inductor builds

according to

where R' is the sum of the switch equivalent resistance
(0.8Ω typical at 25°C) and the inductor DC resistance.
When the drop across the switch is small compared to VIN,
the simple lossless equation

Additionally, the inductor core must be able to store the
required flux; i.e., it must not
generally encountered with LT1108 based designs, small
surface mount ferrite core units with saturation current
ratings in the 300mA to 1A range and DCR less than 0.4Ω
(depending on application) are adequate.

Lastly, the inductor must have sufficiently low DC resis­tance so excessive power is not lost as heat in the windings.
An additional consideration is Electro-Magnetic Interfer­ence (EMI). Toroid and pot core type inductors are recom­mended in applications where EMI must be kept to a
minimum; for example, where there are sensitive analog
circuitry or transducers nearby. Rod core types are a less
expensive choice where EMI is not a problem. Minimum
and maximum input voltage, output voltage and output
current must be established before an inductor can be
selected.

Step-Up Converter

In a step-up, or boost converter (Figure 1), power generated
by the inductor makes up the difference between input and
output. Power required from the inductor is determined by

saturate

. At power levels

can be used. These equations assume that at t = 0,
inductor current is zero. This situation is called “discontinu­ous mode operation” in switching regulator parlance.
Setting “t” to the switch-ON time from the LT1108 specifi­cation table (typically 36µs) will yield I
and VIN. Once I
end of the switch-ON time can be calculated as

EL must be greater than PL/f
the required power. For best efficiency I
to 1A or less. Higher switch currents will cause excessive
drop across the switch resulting in reduced efficiency. In
general, switch current should be held to as low a value as
possible in order to keep switch, diode and inductor losses
at a minimum.

As an example, suppose 12V at 30mA is to be generated
from a 2V to 3V input. Recalling equation (01),

is known, energy in the inductor at the

PEAK

for the converter to deliver

OSC

for a specific “L”

PEAK

should be kept

PEAK

PV VV I

=+

()()

L OUT D IN

6

–()01

MIN

OUT

LT1108

L

VVV

I

t

IN MIN SW OUT

PEAK

ON

=

−−
× ()11

I

mA

mA

PEAK

=

()

+

+











=

2 300

060

505

12 15 05

500 12

.

.

–. .

()

L

mA

sH==

12 1 5 5

500

36 396 13

–.–

()µµ

U

O

PPLICATI

A

Energy required from the inductor is

315

P

L

f

OSC

Picking an inductor value of 100µH with 0.2Ω DCR results
in a peak switch current of

I

=

PEAK

Substituting I

EHAJ

=

L

Since 18.3µJ > 16.6µJ, the 100µH inductor will work. This
trial-and-error approach can be used to select the optimum
inductor. Keep in mind the switch current maximum rating
of 1.5A. If the calculated peak current exceeds this, an
external power transistor can be used.

A resistor can be added in series with the I
switch current limit. The resistor should be picked so the
calculated I
Switch Current (from Typical Performance Characteristic
curves). Then, as VIN increases, switch current is held
constant, resulting in increasing efficiency.

mW

==

19

kHz

V

2
.

10

PEAK

1

100 6 605 18 3 09

()( )

2

PEAK

S

I FOR ATIO

16 6 07.()µ

J

×

10 36

–.ΩΩµ



emA

–()

1 605 08




into Equation 04 results in

µµ.. ()

at minimum VIN is equal to the Maximum

100

H

µ

2

=

WU

s



=




pin to invoke

LIM

U

where DC = duty cycle (0.60)

VSW = switch drop in step-down mode
VD = diode drop (0.5V for a 1N5818)
I

= output current

OUT

V

= output voltage

OUT

VIN = minimum input voltage

VSW is actually a function of switch current which is in turn
a function of VIN, L, time, and V
be used for VSW as a very conservative value.

Once I

where tON = switch-ON time (36µs).
Next, the current limit resistor R

from the R
resistor keeps maximum switch current constant as the
input voltage is increased.

As an example, suppose 5V at 300mA is to be generated
from a 12V to 24V input. Recalling Equation (10),

is known, inductor value can be derived from

PEAK

Step-Down Mode curve. The addition of this

LIM

. To simplify, 1.5V can

OUT

is selected to give I

LIM

PEAK

Step-Down Converter

The step-down case (Figure 2) differs from the step-up in
that the inductor current flows through the load during both
the charge and discharge periods of the inductor. Current
through the switch should be limited to ~650mA in this
mode. Higher current can be obtained by using an external
switch (see Figure 3). The I
operation over varying inputs.

After establishing output voltage, output current and input
voltage range, peak switch current can be calculated by the
formula:

I

PEAK

2

=

pin is the key to successful

LIM



I

OUT OUT D

DC

VV

+



VV V

IN SW D



+






10–()

Next, inductor value is calculated using Equation (11)

Use the next lowest standard value (330µH).
Then pick R

R

= 220Ω.

LIM

Positive-to-Negative Converter

Figure 4 shows hookup for positive-to-negative conver­sion. All of the output power must come from the inductor.
In this case,

P

= (V

L

from the curve. For I

LIM

+ V

)(I

OUT

D

) (14)

OUT

= 500mA,

PEAK

7

LT1108

I

V

L

t

PEA K

IN

ON

= *()20

V

R

R

V

OUT

=+











()

1

2

1

1 245 21.()

L1

LT1108 • F01

GND SW2

SW1

LIM

I

IN

V

D1

R3


LT1108

+

V

OUT

R2

R1

C1

V

IN

FB

PPLICATI

A

U

O

S

I FOR ATIO

WU

U

In this mode the switch is arranged in common collector or
step-down mode. The switch drop can be modeled as a

0.75V source in series with a 0.65Ω resistor. When the
switch closes, current in the inductor builds according to

–'



V

=

L

'

R




It

()

L

where: R' = 0.65Ω + DCR

Rt



–()

e

115

L




L

VL = VIN – 0.75V

As an example, suppose –5V at 100mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),

P

= (–5V+ 0.5V)(100mA) = 550mW. (16)

L

Energy required from the inductor is

P

f

OSC

550

L

mW

==

19

kHz

28 9 17.()µ

J

The usual step-up configuration for the LT1108 is shown in
Figure 1. The LT1108 first pulls SW1 low causing VIN –
V

to appear across L1. A current then builds up in L1.

CESAT

At the end of the switch-ON time the current in L1 is

Figure 1. Step-Up Mode Hookup

Picking an inductor value of 220µH with 0.3Ω DCR results
in a peak switch current of

×

095 36

–.

45 075

.–.

VV

I

PEAK

Substituting I

()

=

0 ΩΩ

()

568

=

1

EHAJ

=

L

2

+

65 0 3

..

mA

into Equation (04) results in

PEAK

220 0 568 35 5 19

µµ.. ()

()()



1

–




2

Ωµ

e

220

=

µ

H

s

(18)






Since 35.5µJ > 28.9µJ, the 220µH inductor will work.
Finally, R

Current vs R

STEP-UP (BOOST MODE) OPERATION

A step-up DC/DC converter delivers an output voltage
higher than the input voltage. Step-up converters are not
short-circuit protected since there is a DC path from input
to output.

should be selected by looking at the Switch

LIM

curve. In this example, R

LIM

= 150Ω.

LIM

Immediately after switch turn-off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches V
inductor current flows through D1 into C1, increasing V
This action is repeated as needed by the LT1108 to keep V

+ VD, the

OUT

OUT

.

FB

at the internal reference voltage of 1.245V. R1 and R2 set
the output voltage according to the formula

STEP-DOWN (BUCK MODE) OPERATION

A step-down DC/DC converter converts a higher voltage to
a lower voltage. The usual hookup for an LT1108 based
step-down converter is shown in Figure 2.

When the switch turns on, SW2 pulls up to V
puts a voltage across L1 equal to VIN – VSW – V

– VSW. This

IN

, causing

OUT

a current to build up in L1. At the end of the switch- ON time,
the current in L1 is equal to

*Expression 20 neglects the effect of switch and coil resistance. This is taken into account in the
"Inductor Selection" section.

8

LT1108

VVV V

SW R Q SAT

=+ ≈

11

10 24.()

LT1108 • F03

D1
1N5821

+

+

V

OUT

V

IN



30V

MAX

L1

R1

0.15Ω

R2
100Ω

Q1

ZETEX ZTX749

R3

330Ω

R4

R5

C1

LT1108

GND

SW2

SW1

V

IN

I

L

FB

C2

R6

100Ω

V

OUT

= 1.245V 1 +





R4
R5

()

PPLICATI

A

I

=

PEAK

O

V

VV

−−

IN

SW OUT

L

U

S

I FOR ATIO

t

ON

WU

U

()22

When the switch turns off, the SW2 pin falls rapidly and
actually goes below ground. D1 turns on when SW2
reaches 0.4V below ground.

DIODE

. The voltage at SW2 must never be allowed to go

D1 MUST BE A SCHOTTKY

below –0.5V. A silicon diode such as the 1N4933 will allow
SW2 to go to –0.8V, causing potentially destructive power
dissipation inside the LT1108. Output voltage is deter­mined by

V

OUT



=+

1







R

2

()



R

1



V

1 245 23.()

R3 programs switch current limit. This is especially impor­tant in applications where the input varies over a wide
range. Without R3, the switch stays on for a fixed time each
cycle. Under certain conditions the current in L1 can build
up to excessive levels, exceeding the switch rating and/or
saturating the inductor. The 100Ω resistor programs the
switch to turn off when the current reaches approximately
700mA. When using the LT1108 in step-down mode,
output voltage should be limited to 6.2V or less. Higher
output voltages can be accommodated by inserting a
1N5818 diode in series with the SW2 pin (anode connected
to SW2).

HIGHER CURRENT STEP-DOWN OPERATION

Output current can be increased by using a discrete PNP
pass transistor as shown in Figure 3. R1 serves as a
current limit sense. When the voltage drop across R1
equals 0.5VBE, the switch turns off. As shown, switch
current is limited to 2A. Inductor value can be calculated
based on formulas in the Inductor Selection Step-Down
Converter section with the following conservative expres­sion for VSW:

R2 provides a current path to turn off Q1. R3 provides base
drive to Q1. R4 and R5 set output voltage. A PMOS FET can
be used in place of Q1 when VIN is between 10V and 20V.

V

IN

R3
100Ω

+

I

V

LIM

C2

Figure 2. Step-Down Mode Hookup

SW1

IN

FB

LT1108

SW2

GND

Figure 3. Q1 Permits Higher Current Switching
The LT1108 Functions as Controller

INVERTING CONFIGURATIONS

L1

D1
1N5818

V

OUT

+

C1

R2

R1

LT1108 • F02

The LT1108 can be configured as a positive-to-negative
converter (Figure 4), or a negative-to-positive converter
(Figure 5). In Figure 4, the arrangement is very similar to a
step-down, except that the high side of the feedback is
referred to ground. This level shifts the output negative. As
in the step-down mode, D1 must be a Schottky diode,
and V

should be less than 6.2V. More negative output

OUT

voltages can be accommodated as in the prior section.
In Figure 5, the input is negative while the output is positive.

In this configuration, the magnitude of the input voltage can
be higher or lower than the output voltage. A level shift,

9

LT1108

V

V

VV DC

OUT DIODE

IN SW

+

−

<

−11

25.()

LT1108 • F06

OFF

ON

SWITCH

I

L

LT1108 • F07

ON

OFF

SWITCH

PROGRAMMED CURRENT LIMIT

I

L

PPLICATI

A

U

O

S

I FOR ATIO

WU

U

provided by the PNP transistor, supplies proper polarity
feedback information to the regulator.

V

IN

R3

–V

I

LIMVIN

+

C2

Figure 4. Positive-to-Negative Converter

I

LIM

+

C2

AO

GND SW2

IN

LT1108

GND

LT1108

V

SW1

SW2

IN

SW1

FB

L1

+

D1
1N5818

L1

C1

D1

+

FB

R2

V

= 1.245V + 0.6V

OUT

R1

R2

–V

OUT

LT1108 • F04

V

OUT

R1

C1

2N3906

R1

()

R2

LT1108 • F05

Another situation where the I

feature is useful occurs

LIM

when the device goes into continuous mode operation. This
occurs in step-up mode when

When the input and output voltages satisfy this relation­ship, inductor current does not go to zero during the switch­OFF time. When the switch turns on again, the current ramp
starts from the non-zero current level in the inductor just
prior to switch turn-on. As shown in Figure 6, the inductor
current increases to a high level before the comparator
turns off the oscillator. This high current can cause exces­sive output ripple and requires oversizing the output ca­pacitor and inductor. With the I

feature, however, the

LIM

switch current turns off at a programmed level as shown in
Figure 7, keeping output ripple to a minimum.

Figure 5. Negative-to-Positive Converter

USING THE I

LIM

The LT1108 switch can be programmed to turn off at a set
switch current, a feature not found on competing devices.
This enables the input to vary over a wide range without
exceeding the maximum switch rating or saturating the
inductor. Consider the case where analysis shows the
LT1108 must operate at an 800mA peak switch current with
a 2.0V input. If VIN rises to 4V, the peak switch current will
rise to 1.6A, exceeding the maximum switch current rating.
With the proper resistor selected (see the “Maximum
Switch Current vs R
will be limited to 800mA, even if the input voltage increases.

10

PIN

” characteristic), the switch current

LIM

Figure 6. No Current Limit Causes Large Inductor
Current Build-Up

Figure 7. Current Limit Keeps Inductor Current Under Control

LT1108

PPLICATI

A

U

O

S

I FOR ATIO

WU

U

Figure 8 details current limit circuitry. Sense transistor Q1,
whose base and emitter are paralleled with power switch
Q2, is ratioed such that approximately 0.5% of Q2’s
collector current flows in Q1’s collector. This current
passed through internal 80Ω resistor R1 and out through
the I
between I
switch current flows to develop a VBE across R1 + R

pin. The value of the external resistor connected

LIM

and VIN sets the current limit. When sufficient

LIM

LIM

, Q3
turns on and injects current into the oscillator, turning off
the switch. Delay through this circuitry is approximately
2µs. The current trip point becomes less accurate for
switch-ON times less than 5µs. Resistor values program­ming switch-ON time for 2µs or less will cause spurious
response in the switch circuitry although the device will
still maintain output regulation.



V

IN

Q3

OSCILLATOR

R

LIM

(EXTERNAL)

DRIVER

I

LIM

R1
80Ω
(INTERNAL)

Q1

SW1

Q2

SW2

LT1108 • F08

5V

V

IN

LT1108

R1

1.245V

V

BAT

SET

R2

REF

–

+

GND

R3

– 1.25V

V

LB

R1 =

35.1µA

= BATTERY TRIP POINT

V

LB

R2 = 33k
R3 = 1.6M

47k

AO



TO 
PROCESSOR

LT1108 • F09

Figure 9. Setting Low Battery Detector Trip Point

Table 1. Inductor Manufacturers

MANUFACTURER PART NUMBERS

Coiltronics International OCTA-PAC
984 S.W. 13th Court Series
Pompano Beach, FL 33069
305-781-8900

Sumida Electric Co. USA CD54
708-956-0666 CDR74

CDR105

TM

Figure 8. LT1108 Current Limit Circuitry

USING THE GAIN BLOCK

The gain block (GB) on the LT1108 can be used as an error
amplifier, low battery detector or linear post regulator. The
gain block itself is a very simple PNP input op amp with an
open collector NPN output. The negative input of the gain
block is tied internally to the 1.245V reference. The positive
input comes out on the SET pin.

Arrangement of the gain block as a low battery detector
is straightforward. Figure 9 shows hookup. R1 and R2
need only be low enough in value so that the bias current
of the SET input does not cause large errors. 33k for R2
is adequate. R3 can be added to introduce a small amount
of hysteresis. This will cause the gain block to “snap”
when the trip point is reached. Values in the 1M to 10M
range are optimal. The addition however, of R3 will
change the trip point.

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.

Table 2. Capacitor Manufacturers

MANUFACTURER PART NUMBERS

Sanyo Video Components OS-CON Series
1201 Sanyo Avenue
San Diego, CA 92073
619-661-6322

Nichicon America Corporation PL Series
927 East State Parkway
Schaumberg, IL 60173
708-843-7500

AVX Corporation TPS Series
Myrtle Beach, SC
803-946-0690

Table 3. Transistor Manufacturers

MANUFACTURER PART NUMBERS

Zetex Inc. ZTX 749 (NPN)
87 Modular Avenue ZTX 849 (NPN)
Commack, NY 11725 ZTX 949 (PNP)
516-543-7100

11

LT1108

U

O

PPLICATITYPICAL

SA

5V to –5V Converter

VIN

5V INPUT

220Ω

I

LIM

+

33pF

GND

* L1 = COILTRONICS CTX300-4

LT1108-5

V

IN

SW1

SENSE
SW2

MBRS130T3

PACKAGEDESCRIPTI

0.300 – 0.320

(7.620 – 8.128)

L1*

300µH

O

6.5V-20V to 5V Step-Down Converter

ZETEX

ZTX-949

100Ω

220Ω

* L1 = COILTRONICS CTX100-4

+

330µF

–5V OUTPUT
150mA

LT1108 • TA03

V

6.5V
TO

20V



IN

+

47µF

V

IN

LT1108-5

GND

0.22Ω

I

LIM

SW1

SENSE
SW2

100Ω

U

Dimensions in inches (millimeters) unless otherwise noted.

N8 Package

8-Lead Plastic DIP

0.045 – 0.065

(1.143 – 1.651)

0.130 ± 0.005

(3.302 ± 0.127)

876



(10.160)

0.400

MAX

L1*

100µH

1N5818

LT1108 • TA04

5

5V



OUT

200mA AT 6.5V
500mA AT 8V

+

330µF

IN

IN

0.065

(1.651)

0.009 – 0.015

(0.229 – 0.381)

+0.025

0.325

–0.015

+0.635

8.255

()

–0.381

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

× 45°

0.016 – 0.050

0.406 – 1.270

TYP

0.045 ± 0.015

(1.143 ± 0.381)

0.100 ± 0.010

(2.540 ± 0.254)

0.053 – 0.069

(1.346 – 1.752)

0°– 8° TYP

0.014 – 0.019

(0.355 – 0.483)

Linear Technology Corporation

12

1630 McCarthy Blvd., Milpitas, CA 95035-7487

[phone]

●

FAX

: [phone]

●

TELEX

: 499-3977

0.125

(3.175)

MIN



0.018 ± 0.003

(0.457 ± 0.076)

S8 Package

8-Lead Plastic SOIC

(0.101 – 0.254)

0.050

(1.270)

BSC

0.020

(0.508)

MIN

0.004 – 0.010

0.228 – 0.244

(5.791 – 6.197)

0.250 ± 0.010

(6.350 ± 0.254)



12

0.189 – 0.197

(4.801 – 5.004)

7

8

1

2

 LINEAR TECHNOLOGY CORPORATION 1993

3

6

4

3

N8 0393

5

0.150 – 0.157

(3.810 – 3.988)

SO8 0393

4

LT/GP 0493 10K REV 0

