LIT1108CS8 [Linear]
IC 1.5 A SWITCHING REGULATOR, 25 kHz SWITCHING FREQ-MAX, PDSO8, PLASTIC, SOIC-8, Switching Regulator or Controller;
| 型号： | LIT1108CS8 |
| 厂家： | 
Linear |
| 描述： | IC 1.5 A SWITCHING REGULATOR, 25 kHz SWITCHING FREQ-MAX, PDSO8, PLASTIC, SOIC-8, Switching Regulator or Controller 开关 光电二极管 |
| 文件： | 总12页 (文件大小：266K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT1108
Micropower
DC/DC Converter
Adjustable and Fixed 5V, 12V
U
DESCRIPTIO
EATURE
S
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
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.
Internal 1A Power Switch
TheLT1108ispin-for-pincompatiblewiththeLT1173,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.
Fixed or Adjustable Output Voltage Versions
Space Saving 8-Pin MiniDIP or S8 Package
O U
PPLICATI
S
A
■
■
■
■
■
■
■
■
Palmtop Computers
3V to 5V, 5V to 12V Converters
9V to 5V, 12V to 5V Converters
LCD Bias Generators
Switch current limit can be programmed with a single
resistor. Anauxiliarygainblockcanbeconfiguredasalow
battery detector, linear post regulator, undervoltage lock-
out circuit, or error amplifier.
Peripherals and Add-On Cards
Battery Backup Supplies
Cellular Telephones
Portable Instruments
U
O
TYPICAL APPLICATI
Palmtop Computer Logic Supply
Efficiency
84
L1*
100µH
1N5817
5V
150mA
82
V
= 3V
IN
47Ω
80
78
76
74
72
70
V
= 2.5V
= 2V
IN
V
I
V
IN
SW1
LIM
IN
AVX
+
+
2 × AA
CELLS
TPS
100µF
LT1108-5
330µF
6.3V
SENSE
SW2
GND
1
10
100
*L1 = GOWANDA GA20-103K
COILTRONICS CTX100-4
SUMIDA CD105-101K
LT1108 • TA01
LOAD CURRENT (mA)
LT1108 • TA02
1
LT1108
W W W
U
ABSOLUTE AXI U RATI GS
Supply Voltage (VIN) ............................................... 36V
SW1 Pin Voltage (VSW1) ......................................... 50V
SW2 Pin Voltage (VSW2) ............................ –0.5V to VIN
Feedback Pin Voltage (LT1108) ............................. 5.5V
Sense Pin Voltage (LT1108, -5, -12) ...................... 36V
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
W
U
/O
PACKAGE RDER I FOR ATIO
ORDER PART
ORDER PART
TOP VIEW
TOP VIEW
NUMBER
NUMBER
I
1
2
3
4
FB (SENSE*)
I
1
2
3
4
FB (SENSE*)
8
7
6
5
8
7
6
5
LIM
LIM
LT1108CN8
LT1108CN8-5
LT1108CN8-12
LT1108CS8
LT1108CS8-5
LT1108CS8-12
V
IN
SET
A0
V
IN
SET
A0
SW1
SW2
SW1
SW2
GND
GND
S8 PART MARKING
S8 PACKAGE
8-LEAD PLASTIC SOIC
N8 PACKAGE
8-LEAD PLASTIC DIP
*FIXED VERSIONS
*FIXED VERSIONS
1108
10805
10812
TJMAX = 90°C, θJA = 130°C/W
TJMAX = 90°C, θJA = 150°C/W
ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 3V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
Switch OFF
No Load
MIN
TYP
MAX
UNITS
I
Quiescent Current
●
110
150
µA
Q
Quiescent Current, Boost Mode Configuration
LT1108-5
LT1108-12
135
250
µA
µA
V
V
Input Voltage
Step-Up Mode
●
●
2
12.6
30.0
V
V
IN
Step-Down Mode
Comparator Trip Point Voltage
Output Sense Voltage
LT1108 (Note 1)
●
1.2
1.245
1.3
V
LT1108-5 (Note 2)
LT1108-12 (Note 2)
●
●
4.75
11.4
5
12
5.25
12.6
V
V
OUT
Comparator Hysteresis
Output Hysteresis
LT1108
●
5
10
mV
LT1108-5
LT1108-12
●
●
20
50
40
100
mV
mV
f
t
Oscillator Frequency
Duty Cycle
●
●
●
●
●
●
14
63
28
19
70
25
78
kHz
%
OSC
ON
Full Load, Step-Up Mode
Switch-ON Time
I
Tied to V , Step-Up Mode
36
48
µs
nA
nA
V
LIM
IN
Feedback Pin Bias Current
Set Pin Bias Current
Gain Block Output Low
Reference Line Regulation
LT1108, V = 0V
10
50
FB
V
SET
= V
20
100
0.4
REF
V
V
I
= 100µA, V = 1V
SET
0.15
OL
SINK
2V ≤ V ≤ 5V
●
●
0.20
0.02
0.400
0.075
%/V
%/V
IN
5V ≤ V ≤ 30V
IN
SW
Voltage, Step-Up Mode
V
IN
V
IN
= 3V, I = 650mA
●
0.5
0.8
0.65
1.00
V
V
SAT
SAT
SW
= 5V, I = 1A
SW
2
LT1108
TA = 25°C, VIN = 3V, unless otherwise noted.
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
CONDITIONS
= 12V, I = 650mA
MIN
TYP
MAX
UNITS
V
SAT
SW Voltage, Step-Down Mode
V
1.1
1.5
1.7
V
V
SAT
IN
SW
●
●
A
V
Gain Block Gain
R = 100k (Note 3)
L
400
1000
400
–0.3
1
V/V
mA
Current Limit
220Ω from I to V
LIM IN
Current Limit Temperature Coefficient
Switch OFF Leakage Current
Maximum Excursion Below GND
●
%/°C
µA
Measured at SW1 Pin
≤ 10µA, Switch OFF
10
V
SW2
I
–400
–350
mV
SW1
The
●
denotes specifications which apply over the full operating
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.
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 3: 100k resistor connected between a 5V source and the A0 pin.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Switch ON Voltage
Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)
Maximum Switch Current
vs RLIM
Step-Down Mode
(SW1 Pin Connected to VIN)
1200
1100
1000
900
800
700
600
500
400
300
200
100
1.2
1.0
0.8
0.6
0.4
0.2
0
1.4
1.3
2V ≤ V ≤ 5V
IN
V
= 3V
IN
1.2
V
IN
= 2V
1.1
1.0
0.9
0.8
V
IN
= 5V
0.7
10
100
(Ω)
1000
0
0.4
0.6
0.8
1.0
1.2
0.2
0.1 0.2 0.3 0.4
SWITCH CURRENT (A)
0.7 0.8
0
0.5 0.6
R
SWITCH CURRENT (A)
LIM
LT1108 • TPC03
LT1108 • TPC01
LT1108 • TPC02
Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)
Supply Current vs Switch Current
Quiescent Current
1000
900
800
700
600
500
400
300
200
100
0
50
40
30
20
10
0
120
115
110
105
V
= 5V
OUT
V
= 24V
IN
L = 500µH
100
95
V
= 5V
IN
V
= 12V
IN
L = 250µH
90
85
80
V
= 2V
IN
100
1000
0
200
400
600
800
1000
–50
–25
0
50
75
100
25
R
LIM
(Ω)
SWITCH CURRENT (mA)
TEMPERATURE (°C)
LT1108 • TPC04
LTC1108 • TPC05
LT1108 • TPC06
3
LT1108
TYPICAL PERFOR A CE CHARACTERISTICS
U W
Oscillator Frequency
Duty Cycle
Switch-ON Time
80
75
70
65
60
55
50
22
21
20
19
18
17
16
15
14
44
42
40
38
36
34
32
30
13
–25
0
50
–50
75
100
25
–25
0
50
–50 –25
0
25
50
75
100
–50
75
100
25
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (˚C)
LT1108 • TPC08
LT1108 • TPC07
LT1108 • TPC09
Minimum/Maximum Frequency
vs ON-Time
Switch Saturation Voltage
Step-Up Mode
Switch Saturation Voltage
Step-Down Mode
28
1.8
1.7
0.8
0.7
0.6
0.5
I
= 650mA
SW
I
= 650mA
SW
26
24
22
20
18
16
14
12
10
0
1.6
1.5
1.4
0.4
0.3
1.3
1.2
1.1
1.0
0.9
0.8
0.2
0.1
0
25
30
35
40
45
50
–25
0
50
–50 –25
25
50
75
100
–50
75
100
25
0
ON-TIME (µs)
TEMPERATURE (°C)
TEMPERATURE (°C)
LT1108 • TPC10
LT1108 • TPC11
LT1108 • TPC12
U
O
U
U
PI
FU CTI
S
ILIM (Pin 1): Connect this pin to VIN for normal use. Where
lower current limit is desired, connect a resistor between
ILIM and VIN. A 220Ω resistor will limit the switch current
to approximately 400mA.
GND (Pin 5): Ground.
AO(Pin6):Auxiliarygainblock(GB)output.Opencollector,
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,thispingoestotheinternalapplicationresistor
that sets output voltage.
VIN (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 VIN.
SW2 (Pin 4): Emitter of power transistor. For step-up
mode connect to ground. For step-down mode connect to
inductor/diode. Thispinmustneverbeallowedtogomore
than a Schottky diode drop below ground.
4
LT1108
U
O
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.
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 resis-
tor pull-up to 5V).
A resistor connected between the ILIM pin and VIN 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, ILIM should be tied
directly to VIN. 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 VIN
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.
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
W
BLOCK DIAGRA S
LT1108
LT1108-5/LT1108-12
SET
SET
A2
A2
A0
A0
V
IN
V
IN
GAIN BLOCK/
ERROR AMP
GAIN BLOCK/
ERROR AMP
I
SW1
SW2
I
SW1
SW2
LIM
LIM
1.245V
REFERENCE
1.245V
REFERENCE
A1
A1
OSCILLATOR
OSCILLATOR
DRIVER
DRIVER
COMPARATOR
COMPARATOR
GND
LT1108 • BD
LT1108-5 • BD
FB
R2
753k
R1
LT1108-5: R1 = 250k
LT1108-12: R1 = 87.4k
GND
SENSE
5
LT1108
PPLICATI
O U
W
U
A
S I FOR ATIO
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
INDUCTOR SELECTION
General
A DC/DC converter operates by storing energy as mag-
netic 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 oppo-
site in polarity to the input voltage by choosing an appro-
priate switching topology.
P /
(02)
f
L
OSC
in order for the converter to regulate the output.
When the switch is closed, current in the inductor builds
according to
–R't
L
V
R'
To operate as an efficient energy transfer element, the
inductor must fulfill three requirements. First, the induc-
tancemustbelowenoughfortheinductortostoreadequate
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.
IN
I (t) =
1–e
(03)
L
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
V
L
IN
Additionally, the inductor core must be able to store the
required flux; i.e., it must not saturate. At power levels
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.
I
t =
( )
t
(04)
L
can be used. These equations assume that at t = 0,
inductorcurrentiszero.Thissituationiscalled“discontinu-
ous mode operation” in switching regulator parlance.
Setting “t” to the switch-ON time from the LT1108 specifi-
cationtable(typically36µs)willyieldIPEAK foraspecific“L”
and VIN. Once IPEAK is known, energy in the inductor at the
end of the switch-ON time can be calculated as
Lastly, the inductor must have sufficiently low DC resis-
tancesoexcessivepowerisnotlostasheatinthewindings.
An additional consideration is Electro-Magnetic Interfer-
ence (EMI). Toroid and pot core type inductors are recom-
mended 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.
1
2
2
E = LI
(05)
L
PEAK
EL must be greater than PL/fOSC for the converter to deliver
therequiredpower. ForbestefficiencyIPEAK shouldbekept
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.
Step-Up Converter
Inastep-up,orboostconverter(Figure1),powergenerated
by the inductor makes up the difference between input and
output. Power required from the inductor is determined by
As an example, suppose 12V at 30mA is to be generated
from a 2V to 3V input. Recalling equation (01),
P = 12V + 0.5V – 2V 30mA = 315mW
(06)
L
P = V
+ V – V
I
) (
OUT
(01)
(
)
MIN
L
OUT
D
IN
6
LT1108
O U
W
U
PPLICATI
A
S I FOR ATIO
where DC = duty cycle (0.60)
VSW = switch drop in step-down mode
VD = diode drop (0.5V for a 1N5818)
IOUT = output current
Energy required from the inductor is
P
315mW
19kHz
L
=
= 16.6µJ
(07)
f
OSC
VOUT = output voltage
VIN = minimum input voltage
Picking an inductor value of 100µH with 0.2Ω DCR results
in a peak switch current of
VSW is actually a function of switch current which is in turn
a function of VIN, L, time, and VOUT. To simplify, 1.5V can
be used for VSW as a very conservative value.
–1.0Ω × 36µs
2V
1.0Ω
I
=
1– e
= 605mA
(08)
100µH
PEAK
Once IPEAK is known, inductor value can be derived from
Substituting IPEAK into Equation 04 results in
V
− V
− V
OUT
IN MIN
SW
1
2
2
L =
× t
(11)
ON
E = 100µH 6.605A = 18.3µJ
) (
(09)
(
)
L
I
PEAK
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.
where tON = switch-ON time (36µs).
Next, the current limit resistor RLIM is selected to give IPEAK
from the RLIM Step-Down Mode curve. The addition of this
resistor keeps maximum switch current constant as the
input voltage is increased.
A resistor can be added in series with the ILIM pin to invoke
switch current limit. The resistor should be picked so the
calculated IPEAK at minimum VIN is equal to the Maximum
Switch Current (from Typical Performance Characteristic
curves). Then, as VIN increases, switch current is held
constant, resulting in increasing efficiency.
As an example, suppose 5V at 300mA is to be generated
from a 12V to 24V input. Recalling Equation (10),
2 300mA
(
)
5 + 0.5
I
=
= 500mA (12)
PEAK
0.60
12 – 1.5 + 0.5
Step-Down Converter
Next, inductor value is calculated using Equation (11)
The step-down case (Figure 2) differs from the step-up in
thattheinductorcurrentflowsthroughtheloadduringboth
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 ILIM pin is the key to successful
operation over varying inputs.
12 – 1.5 – 5
L =
36µs = 396µH
(13)
500mA
Use the next lowest standard value (330µH).
Then pick RLIM from the curve. For IPEAK = 500mA,
RLIM = 220Ω.
After establishing output voltage, output current and input
voltage range, peak switch current can be calculated by the
formula:
Positive-to-Negative Converter
Figure 4 shows hookup for positive-to-negative conver-
sion. All of the output power must come from the inductor.
In this case,
2I
V
+ V
OUT D
OUT
I
=
(10)
PEAK
DC
V
– V
+ V
IN
SW D
PL = ( VOUT + VD)(IOUT)
(14)
7
LT1108
PPLICATI
O U
W
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A
S I FOR ATIO
In this mode the switch is arranged in common collector or The usual step-up configuration for the LT1108 is shown in
step-down mode. The switch drop can be modeled as a Figure 1. The LT1108 first pulls SW1 low causing VIN –
0.75V source in series with a 0.65Ω resistor. When the VCESAT to appear across L1. A current then builds up in L1.
switch closes, current in the inductor builds according to
At the end of the switch-ON time the current in L1 is
V
–R't
L
IN
V
R'
L
I
=
t
*
(20)
I
t =
( )
1– e
(15)
PEAK
ON
L
L
D1
L1
where: R' = 0.65Ω + DCRL
V
V
IN
OUT
VL = VIN – 0.75V
R3
R2
R1
As an example, suppose –5V at 100mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
I
V
LIM
IN
SW1
+
C1
LT1108
FB
PL = ( –5V + 0.5V)(100mA) = 550mW.
Energy required from the inductor is
(16)
GND
SW2
LT1108 • F01
P
550mW
19kHz
L
=
= 28.9µJ
(17)
f
Figure 1. Step-Up Mode Hookup
OSC
Picking an inductor value of 220µH with 0.3Ω DCR results
Immediately after switch turn-off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches VOUT + VD, the
in a peak switch current of
–0.95Ω × 36µs
220µH
4.5V – 0.75V
(18)
(
(
)
1– e
)
inductorcurrentflowsthroughD1intoC1,increasingVOUT
.
I
=
PEAK
ThisactionisrepeatedasneededbytheLT1108tokeepVFB
at the internal reference voltage of 1.245V. R1 and R2 set
the output voltage according to the formula
0.65Ω + 0.3Ω
= 568mA
Substituting IPEAK into Equation (04) results in
R2
R1
V
= 1+
1.245V
(21)
(
)
OUT
1
2
E = 220µH 0.568A = 35.5µJ
) (
(19)
(
)
L
2
STEP-DOWN (BUCK MODE) OPERATION
Since 35.5µJ > 28.9µJ, the 220µH inductor will work.
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.
Finally, RLIM should be selected by looking at the Switch
Current vs RLIM curve. In this example, RLIM = 150Ω.
When the switch turns on, SW2 pulls up to VIN – VSW. This
putsavoltageacrossL1equaltoVIN –VSW –VOUT, causing
acurrenttobuildupinL1. Attheendoftheswitch-ONtime,
the current in L1 is equal to
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.
*Expression 20 neglects the effect of switch and coil resistance. This is taken into account in the
"Inductor Selection" section.
8
LT1108
O U
W
U
PPLICATI
A
S I FOR ATIO
HIGHER CURRENT STEP-DOWN OPERATION
V
− V − V
IN
SW
OUT
I
=
t
(22)
PEAK
ON
L
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 expres-
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. D1 MUST BE A SCHOTTKY
DIODE. The voltage at SW2 must never be allowed to go
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 deter-
mined by
sion for VSW
= V + V ≈ 1.0V
Q1SAT
:
V
(24)
SW
R1
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.
R2
R1
V
= 1+
1.245V
(23)
(
)
OUT
R3 programs switch current limit. This is especially impor-
tant 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).
R1
Q1
V
L1
0.15Ω
ZETEX ZTX749
IN
30V
V
OUT
MAX
R2
100Ω
R6
100Ω
D1
1N5821
R3
330Ω
V
I
L
SW1
IN
+
+
C2
C1
LT1108
R4
FB
SW2
GND
R5
R4
V
= 1.245V 1 +
(
)
OUT
R5
LT1108 • F03
Figure 3. Q1 Permits Higher Current Switching
The LT1108 Functions as Controller
V
IN
R3
100Ω
+
I
V
IN
SW1
FB
LIM
C2
INVERTING CONFIGURATIONS
LT1108
GND
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 VOUT should be less than 6.2V. More negative output
voltages can be accommodated as in the prior section.
L1
V
SW2
OUT
R2
R1
+
D1
1N5818
C1
LT1108 • F02
Figure 2. Step-Down Mode Hookup
InFigure5, theinputisnegativewhiletheoutputispositive.
Inthisconfiguration,themagnitudeoftheinputvoltagecan
be higher or lower than the output voltage. A level shift,
9
LT1108
PPLICATI
provided by the PNP transistor, supplies proper polarity
feedback information to the regulator.
O U
W
U
A
S I FOR ATIO
Another situation where the ILIM feature is useful occurs
whenthedevicegoesintocontinuousmodeoperation.This
occurs in step-up mode when
V
IN
R3
V
+
V
1
OUT
DIODE
<
.
(25)
I
V
IN
SW1
FB
LIM
V − V
1− DC
IN
SW
+
C2
LT1108
GND
When the input and output voltages satisfy this relation-
ship,inductorcurrentdoesnotgotozeroduringtheswitch-
OFFtime. Whentheswitchturnsonagain, thecurrentramp
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 exces-
sive output ripple and requires oversizing the output ca-
pacitor and inductor. With the ILIM feature, however, the
switch current turns off at a programmed level as shown in
Figure 7, keeping output ripple to a minimum.
L1
SW2
R1
R2
+
D1
1N5818
C1
–V
OUT
LT1108 • F04
Figure 4. Positive-to-Negative Converter
D1
L1
V
OUT
+
R1
C1
I
V
2N3906
LIM
IN
SW1
+
LT1108
C2
AO
GND
FB
SW2
I
L
R2
R1
R2
V
=
1.245V + 0.6V
OUT
(
)
LT1108 • F05
–V
IN
ON
SWITCH
Figure 5. Negative-to-Positive Converter
OFF
LT1108 • F06
USING THE ILIM PIN
Figure 6. No Current Limit Causes Large Inductor
Current Build-Up
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
LT1108mustoperateatan800mApeakswitchcurrentwith
a 2.0V input. If VIN rises to 4V, the peak switch current will
riseto1.6A, exceedingthemaximumswitchcurrentrating.
With the proper resistor selected (see the “Maximum
SwitchCurrent vs RLIM” characteristic), the switch current
willbelimitedto800mA,eveniftheinputvoltageincreases.
PROGRAMMED CURRENT LIMIT
I
L
ON
SWITCH
OFF
LT1108 • F07
Figure 7. Current Limit Keeps Inductor Current Under Control
10
LT1108
O U
W
U
PPLICATI
A
S I FOR ATIO
5V
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 ILIM pin. The value of the external resistor connected
betweenILIM andVIN setsthecurrentlimit. Whensufficient
switchcurrentflowstodevelopaVBE across R1 +RLIM, 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 program-
ming 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
LT1108
47k
R1
R2
1.245V
REF
–
+
AO
V
TO
BAT
PROCESSOR
SET
GND
R3
V
– 1.25V
35.1µA
LB
R1 =
V
= BATTERY TRIP POINT
LB
R2 = 33k
R3 = 1.6M
LT1108 • F09
Figure 9. Setting Low Battery Detector Trip Point
R
I
LIM
LIM
(EXTERNAL)
Table 1. Inductor Manufacturers
MANUFACTURER
V
IN
R1
80Ω
(INTERNAL)
PART NUMBERS
OCTA-PACTM
Series
Coiltronics International
984 S.W. 13th Court
Pompano Beach, FL 33069
305-781-8900
Q3
SW1
Q2
DRIVER
Q1
OSCILLATOR
Sumida Electric Co. USA
708-956-0666
CD54
CDR74
CDR105
SW2
LT1108 • F08
Figure 8. LT1108 Current Limit Circuitry
Table 2. Capacitor Manufacturers
MANUFACTURER
PART NUMBERS
USING THE GAIN BLOCK
Sanyo Video Components
1201 Sanyo Avenue
San Diego, CA 92073
619-661-6322
OS-CON Series
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
blockistiedinternallytothe1.245Vreference.Thepositive
input comes out on the SET pin.
Nichicon America Corporation
927 East State Parkway
Schaumberg, IL 60173
708-843-7500
PL Series
AVX Corporation
Myrtle Beach, SC
803-946-0690
TPS Series
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
isadequate. R3canbeaddedtointroduceasmallamount
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.
Table 3. Transistor Manufacturers
MANUFACTURER
PART NUMBERS
Zetex Inc.
ZTX 749 (NPN)
ZTX 849 (NPN)
ZTX 949 (PNP)
87 Modular Avenue
Commack, NY 11725
516-543-7100
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-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
11
LT1108
U
O
TYPICAL APPLICATI S
5V to –5V Converter
6.5V-20V to 5V Step-Down Converter
V
IN
L1*
ZETEX
5V INPUT
V
IN
100µH
5V
0.22Ω
OUT
200mA AT 6.5V
500mA AT 8V
IN
ZTX-949
6.5V
TO
220Ω
IN
20V
100Ω
+
I
V
LIM
IN
100Ω
220Ω
47µF
1N5818
+
SW1
330µF
+
V
I
LIM
33pF
IN
LT1108-5
SW1
SENSE
SW2
LT1108-5
GND
L1*
300µH
LT1108 • TA04
SENSE
SW2
GND
+
MBRS130T3
330µF
* L1= COILTRONICS CTX100-4
–5V OUTPUT
150mA
* L1= COILTRONICS CTX300-4
LT1108 • TA03
U
PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted.
N8 Package
8-Lead Plastic DIP
0.400
(10.160)
MAX
0.130 ± 0.005
0.300 – 0.320
0.045 – 0.065
(3.302 ± 0.127)
(1.143 – 1.651)
(7.620 – 8.128)
8
1
7
6
5
4
0.065
(1.651)
TYP
0.250 ± 0.010
(6.350 ± 0.254)
0.009 – 0.015
(0.229 – 0.381)
0.125
0.020
(0.508)
MIN
(3.175)
MIN
+0.025
–0.015
2
3
0.045 ± 0.015
(1.143 ± 0.381)
0.325
N8 0393
+0.635
–0.381
8.255
(
)
0.100 ± 0.010
(2.540 ± 0.254)
0.018 ± 0.003
(0.457 ± 0.076)
S8 Package
8-Lead Plastic SOIC
0.189 – 0.197
(4.801 – 5.004)
0.010 – 0.020
(0.254 – 0.508)
7
5
8
6
× 45°
0.053 – 0.069
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
0.008 – 0.010
(0.203 – 0.254)
0°– 8° TYP
0.228 – 0.244
0.150 – 0.157
(5.791 – 6.197)
(3.810 – 3.988)
0.016 – 0.050
0.406 – 1.270
0.050
(1.270)
BSC
0.014 – 0.019
(0.355 – 0.483)
SO8 0393
1
3
4
2
LT/GP 0493 10K REV 0
12 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
●
●
(408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977
LINEAR TECHNOLOGY CORPORATION 1993
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