LT1108
Micropower
DC/DC Converter
Adjustable and Fixed 5V, 12V
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DESCRIPTIO
FEATURES
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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, 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.
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APPLICATI
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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
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.
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TYPICAL APPLICATI
Palmtop Computer Logic Supply
L1*
100µH
Efficiency
84
1N5817
5V
150mA
82
47Ω
2 × AA
CELLS
+
100µF
VIN
SW1
LT1108-5
SENSE
GND
+
AVX
TPS
330µF
6.3V
EFFICIENCY (%)
ILIM
VIN = 3V
80
VIN = 2.5V
VIN = 2V
78
76
74
SW2
72
70
*L1 = GOWANDA GA20-103K
COILTRONICS CTX100-4
SUMIDA CD105-101K
1
LT1108 • TA01
10
100
LOAD CURRENT (mA)
LT1108 • TA02
1
LT1108
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ABSOLUTE
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
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PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
ILIM 1
8
FB (SENSE*)
VIN 2
7
SET
SW1 3
6
A0
SW2 4
5
GND
LT1108CN8
LT1108CN8-5
LT1108CN8-12
ORDER PART
NUMBER
TOP VIEW
ILIM 1
8
FB (SENSE*)
VIN 2
7
SET
SW1 3
6
A0
SW2 4
5
GND
N8 PACKAGE
8-LEAD PLASTIC DIP
S8 PACKAGE
8-LEAD PLASTIC SOIC
*FIXED VERSIONS
*FIXED VERSIONS
TJMAX = 90°C, θJA = 130°C/W
TJMAX = 90°C, θJA = 150°C/W
ELECTRICAL CHARACTERISTICS
LT1108CS8
LT1108CS8-5
LT1108CS8-12
S8 PART MARKING
1108
10805
10812
TA = 25°C, VIN = 3V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
IQ
Quiescent Current
Switch OFF
Quiescent Current, Boost Mode Configuration
No Load
Input Voltage
Step-Up Mode
Step-Down Mode
●
●
Comparator Trip Point Voltage
LT1108 (Note 1)
●
1.2
1.245
1.3
V
Output Sense Voltage
LT1108-5 (Note 2)
LT1108-12 (Note 2)
●
●
4.75
11.4
5
12
5.25
12.6
V
V
Comparator Hysteresis
LT1108
●
5
10
mV
Output Hysteresis
LT1108-5
LT1108-12
●
●
20
50
40
100
mV
mV
kHz
VIN
VOUT
fOSC
tON
VOL
VSAT
2
MIN
●
LT1108-5
LT1108-12
Oscillator Frequency
TYP
MAX
UNITS
110
150
µA
µA
µA
135
250
2
12.6
30.0
●
14
19
25
V
V
Duty Cycle
Full Load, Step-Up Mode
●
63
70
78
%
Switch-ON Time
ILIM Tied to VIN, Step-Up Mode
●
28
36
48
µs
Feedback Pin Bias Current
LT1108, VFB = 0V
●
10
50
nA
Set Pin Bias Current
VSET = VREF
●
20
100
nA
Gain Block Output Low
ISINK = 100µA, VSET = 1V
●
0.15
0.4
V
Reference Line Regulation
2V ≤ VIN ≤ 5V
5V ≤ VIN ≤ 30V
●
●
0.20
0.02
0.400
0.075
%/V
%/V
SWSAT Voltage, Step-Up Mode
VIN = 3V, ISW = 650mA
VIN = 5V, ISW = 1A
●
0.5
0.8
0.65
1.00
V
V
LT1108
ELECTRICAL CHARACTERISTICS
TA = 25°C, VIN = 3V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
VSAT
SWSAT Voltage, Step-Down Mode
VIN = 12V, ISW = 650mA
MIN
TYP
MAX
1.1
1.5
1.7
●
AV
Gain Block Gain
RL = 100k (Note 3)
Current Limit
220Ω from ILIM to VIN
Current Limit Temperature Coefficient
VSW2
400
●
●
Switch OFF Leakage Current
Measured at SW1 Pin
Maximum Excursion Below GND
ISW1 ≤ 10µA, Switch OFF
The ● 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.
UNITS
V
V
1000
V/ V
400
mA
– 0.3
%/°C
1
10
µA
– 400
– 350
mV
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.
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TYPICAL PERFOR A CE CHARACTERISTICS
Switch ON Voltage
Step-Down Mode
(SW1 Pin Connected to VIN)
Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)
1.2
1.4
1.0
1.3
Maximum Switch Current
vs RLIM
1200
2V ≤ VIN ≤ 5V
1100
VIN = 2V
0.6
VIN = 5V
0.4
0.2
0
1.2
1.1
1.0
0.9
0
0.2
0.4
0.6
0.8
SWITCH CURRENT (A)
1.0
0.1 0.2
0.3
400
0.5 0.6
SWITCH CURRENT (A)
100
RLIM (Ω)
10
0.7 0.8
0.4
VOUT = 5V
1000
LT1108 • TPC03
Quiescent Current
50
120
115
800
40
VIN = 24V
L = 500µH
SUPPLY CURRENT (mA)
SWITCH CURRENT (mA)
500
Supply Current vs Switch Current
1000
600
500
400
600
LT1108 • TPC02
Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)
700
700
100
0
LT1108 • TPC01
900
800
200
0.7
1.2
900
300
0.8
VIN = 12V
L = 250µH
300
200
QUIESCENT CURRENT (µA)
VCESAT (V)
0.8
1000
SWITCH CURRENT (mA)
SWITCH ON VOLTAGE (V)
VIN = 3V
30
VIN = 5V
20
VIN = 2V
10
110
105
100
95
90
85
100
0
100
1000
R LIM (Ω)
LT1108 • TPC04
0
0
200
600
800
400
SWITCH CURRENT (mA)
1000
LTC1108 • TPC05
80
–50
–25
0
50
25
TEMPERATURE (°C)
75
100
LT1108 • TPC06
3
LT1108
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TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
Duty Cycle
22
21
Switch-ON Time
80
44
75
42
19
18
17
16
SWITCH-ON TIME (µs)
DUTY CYCLE (%)
FREQUENCY (kHz)
20
70
65
60
40
38
36
34
15
55
32
14
13
–50
–25
0
50
25
TEMPERATURE (°C)
75
50
–50
100
–25
0
50
25
TEMPERATURE (°C)
75
30
–50
100
Minimum/Maximum Frequency
vs ON-Time
1.8
ISW = 650mA
1.7
0.7
ISW = 650mA
1.6
0.6
1.5
18
16
0.5
VSAT (V)
20
VCESAT (V)
FREQUENCY (kHz)
24
22
100
Switch Saturation Voltage
Step-Down Mode
0.8
26
75
LT1108 • TPC09
Switch Saturation Voltage
Step-Up Mode
28
0.4
0.3
14
1.4
1.3
1.2
1.1
0.2
12
1.0
0.1
10
25
30
40
35
ON-TIME (µs)
45
50
0
–50
0.9
–25
0
50
25
TEMPERATURE (°C)
LT1108 • TPC10
75
100
LT1108 • TPC11
0.8
–50
–25
25
50
0
TEMPERATURE (°C)
75
100
LT1108 • TPC12
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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.
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. This pin must never be allowed to go more
than a Schottky diode drop below ground.
4
50
25
0
TEMPERATURE (˚C)
LT1108 • TPC08
LT1108 • TPC07
0
–25
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.
LT1108
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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 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 ILIM pin and VIN sets
maximum switch current. When the switch current exceeds 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 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 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.
Gain block A2 can serve as a low battery detector. The
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BLOCK DIAGRA S
LT1108
LT1108-5/LT1108-12
SET
SET
A2
A2
A0
VIN
A0
VIN
GAIN BLOCK/
ERROR AMP
ILIM
GAIN BLOCK/
ERROR AMP
SW1
ILIM
SW1
1.245V
REFERENCE
1.245V
REFERENCE
A1
A1
OSCILLATOR
OSCILLATOR
DRIVER
DRIVER
COMPARATOR
COMPARATOR
GND
LT1108 • BD
FB
SW2
LT1108-5 • BD
R1
GND
R2
753k
SENSE
SW2
LT1108-5: R1 = 250k
LT1108-12: R1 = 87.4k
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LT1108
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APPLICATI
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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 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.
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.
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
(
)(
PL = VOUT + V D – VIN MIN IOUT
6
)
(01)
PL / f OSC
(02)
in order for the converter to regulate the output.
When the switch is closed, current in the inductor builds
according to
–R't
V
IL ( t) = IN 1– e L
R'
(03)
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
IL t = IN t
L
(04)
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 IPEAK for a specific “L”
and VIN. Once IPEAK is known, energy in the inductor at the
end of the switch-ON time can be calculated as
EL =
1 2
LI
2 PEAK
(05)
EL must be greater than PL/fOSC for the converter to deliver
the required power. For best efficiency IPEAK should be kept
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),
(
)(
)
PL = 12V + 0.5V – 2V 30mA = 315mW
(06)
LT1108
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Energy required from the inductor is
PL
f OSC
=
315mW
= 16.6µJ
19kHz
(07)
Picking an inductor value of 100µH with 0.2Ω DCR results
in a peak switch current of
2V
I PEAK =
1.0Ω
–1.0Ω × 36µs
e
–
1
100µH
= 605mA
(08)
(
)(
)
1
100µH 6.605 A 2 = 18.3µJ
2
(09)
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 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.
Step-Down Converter
After establishing output voltage, output current and input
voltage range, peak switch current can be calculated by the
formula:
2 I OUT
DC
V OUT + V D
V – V
SW + V D
IN
L=
VIN MIN − V SW − V OUT
I PEAK
× t ON
(11)
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.
As an example, suppose 5V at 300mA is to be generated
from a 12V to 24V input. Recalling Equation (10),
IPEAK =
(
)
2 300mA 5 + 0.5
12 – 1.5 + 0.5 = 500mA
0.60
(12)
Next, inductor value is calculated using Equation (11)
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 ILIM pin is the key to successful
operation over varying inputs.
IPEAK =
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.
Once IPEAK is known, inductor value can be derived from
Substituting IPEAK into Equation 04 results in
EL =
where DC = duty cycle (0.60)
VSW = switch drop in step-down mode
VD = diode drop (0.5V for a 1N5818)
IOUT = output current
VOUT = output voltage
VIN = minimum input voltage
(10)
L=
12 – 1.5 – 5
36µs = 396µH
500mA
(13)
Use the next lowest standard value (330µH).
Then pick RLIM from the curve. For IPEAK = 500mA,
RLIM = 220Ω.
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,
PL = (VOUT+ VD)(IOUT)
(14)
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LT1108
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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
()
IL t =
VL
R'
–R't
–
1
e
L
The usual step-up configuration for the LT1108 is shown in
Figure 1. The LT1108 first pulls SW1 low causing VIN –
VCESAT to appear across L1. A current then builds up in L1.
At the end of the switch-ON time the current in L1 is
IPEAK =
(15)
VIN
L
t ON *
(20)
D1
L1
where: R' = 0.65Ω + DCRL
VL = VIN – 0.75V
V IN
V OUT
R3
As an example, suppose – 5V at 100mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
PL = (– 5V+ 0.5V)(100mA) = 550mW.
I LIM
R2
V IN
SW1
LT1108
+
C1
FB
(16)
GND
R1
SW2
Energy required from the inductor is
550mW
PL
=
= 28.9µJ
19kHz
fOSC
Figure 1. Step-Up Mode Hookup
Picking an inductor value of 220µH with 0.3Ω DCR results
in a peak switch current of
IPEAK =
(4.5V – 0.75V) 1 – e –0.95Ω × 36µs
220µH
(0.65Ω + 0.3Ω)
(18)
= 568mA
Substituting IPEAK into Equation (04) results in
EL =
(
)(
)
1
220µH 0.568 A 2 = 35.5µJ
2
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
inductor current flows through D1 into C1, increasing VOUT.
This action is repeated as needed by the LT1108 to keep VFB
at the internal reference voltage of 1.245V. R1 and R2 set
the output voltage according to the formula
R2
VOUT = 1 + 1.245 V
R1
(
)
(21)
(19)
Since 35.5µJ > 28.9µJ, the 220µH inductor will work.
Finally, RLIM should be selected by looking at the Switch
Current vs RLIM curve. In this example, RLIM = 150Ω.
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.
8
LT1108 • F01
(17)
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 VIN – VSW. This
puts a voltage across L1 equal to VIN – VSW – VOUT, causing
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.
LT1108
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VIN − VSW − VOUT
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HIGHER CURRENT STEP-DOWN OPERATION
t ON
(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. 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 determined by
R2
VOUT = 1 + 1.245 V
R1
(
)
(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).
R3
100Ω
C2
ILIM
VIN
VSW = VR1 + VQ1SAT ≈ 1.0 V
(24)
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.
R1
0.15Ω
VIN
30V
MAX
Q1
ZETEX ZTX749
L1
VOUT
R2
100Ω
R6
100Ω
VIN
+
D1
1N5821
R3
330Ω
IL
SW1
C2
+
C1
LT1108
R4
FB
GND
SW2
R5
(
R4
VOUT = 1.245V 1 + R5
)
LT1108 • F03
Figure 3. Q1 Permits Higher Current Switching
The LT1108 Functions as Controller
VIN
+
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:
SW1
INVERTING CONFIGURATIONS
FB
LT1108
L1
VOUT
SW2
GND
D1
1N5818
R2
+
C1
R1
LT1108 • F02
Figure 2. Step-Down Mode Hookup
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 VOUTshould be less than 6.2V. More negative output
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
W
U
U
UO
APPLICATI
S I FOR ATIO
provided by the PNP transistor, supplies proper polarity
feedback information to the regulator.
VIN
VOUT + VDIODE
1
<
.
VIN − VSW
1 − DC
R3
ILIM
VIN
Another situation where the ILIM feature is useful occurs
when the device goes into continuous mode operation. This
occurs in step-up mode when
SW1
(25)
FB
+
C2
LT1108
L1
SW2
GND
D1
1N5818
R1
+
C1
R2
–VOUT
LT1108 • F04
Figure 4. Positive-to-Negative Converter
D1
L1
VOUT
+
C1
VIN
SW1
ILIM
+
C2
R1
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 ILIM feature, however, the
switch current turns off at a programmed level as shown in
Figure 7, keeping output ripple to a minimum.
2N3906
LT1108
AO
GND
FB
SW2
IL
R2
( )
VOUT = R1 1.245V + 0.6V
R2
–VIN
LT1108 • F05
Figure 5. Negative-to-Positive Converter
SWITCH
ON
OFF
LT1108 • F06
USING THE ILIM PIN
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 RLIM” characteristic), the switch current
will be limited to 800mA, even if the input voltage increases.
10
Figure 6. No Current Limit Causes Large Inductor
Current Build-Up
IL
SWITCH
PROGRAMMED CURRENT LIMIT
ON
OFF
LT1108 • F07
Figure 7. Current Limit Keeps Inductor Current Under Control
LT1108
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APPLICATI
S I FOR ATIO
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
between ILIM and VIN sets the current limit. When sufficient
switch current flows to develop a VBE 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 programming switch-ON time for 2µs or less will cause spurious
response in the switch circuitry although the device will
still maintain output regulation.
5V
V IN
LT1108
47k
R1
VBAT
1.245V
REF
–
SET
+
AO
R2
TO
PROCESSOR
GND
R3
VLB – 1.25V
35.1µA
VLB = BATTERY TRIP POINT
R2 = 33k
R3 = 1.6M
R1 =
LT1108 • F09
Figure 9. Setting Low Battery Detector Trip Point
RLIM
(EXTERNAL)
ILIM
Table 1. Inductor Manufacturers
VIN
R1
80Ω
(INTERNAL)
Q3
SW1
DRIVER
OSCILLATOR
Q1
Q2
SW2
MANUFACTURER
PART NUMBERS
Coiltronics International
984 S.W. 13th Court
Pompano Beach, FL 33069
305-781-8900
OCTA-PACTM
Series
Sumida Electric Co. USA
708-956-0666
CD54
CDR74
CDR105
LT1108 • F08
Figure 8. LT1108 Current Limit Circuitry
Table 2. Capacitor Manufacturers
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.
MANUFACTURER
PART NUMBERS
Sanyo Video Components
1201 Sanyo Avenue
San Diego, CA 92073
619-661-6322
OS-CON Series
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
Table 3. Transistor Manufacturers
MANUFACTURER
PART NUMBERS
Zetex Inc.
87 Modular Avenue
Commack, NY 11725
516-543-7100
ZTX 749 (NPN)
ZTX 849 (NPN)
ZTX 949 (PNP)
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.
11
LT1108
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TYPICAL APPLICATI
S
5V to – 5V Converter
6.5V-20V to 5V Step-Down Converter
VIN
5V INPUT
VIN
6.5V
TO
20V
220Ω
ILIM
VIN
L1*
100µH
ZETEX
ZTX-949
0.22Ω
+
5VOUT
200mA AT 6.5VIN
500mA AT 8VIN
100Ω
100Ω
47µF
+
1N5818
SW1
+
33pF
330µF
VIN
LT1108-5
ILIM
220Ω
SW1
SENSE
GND
SW2
LT1108-5
L1*
300µH
MBRS130T3
SENSE
+
GND
330µF
LT1108 • TA04
SW2
* L1 = COILTRONICS CTX100-4
–5V OUTPUT
150mA
* L1 = COILTRONICS CTX300-4
LT1108 • TA03
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PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted.
N8 Package
8-Lead Plastic DIP
0.300 – 0.320
(7.620 – 8.128)
0.009 – 0.015
(0.229 – 0.381)
(
+0.025
0.325 –0.015
8.255
+0.635
–0.381
)
0.045 – 0.065
(1.143 – 1.651)
0.400
(10.160)
MAX
0.130 ± 0.005
(3.302 ± 0.127)
8
7
6
5
0.065
(1.651)
TYP
0.250 ± 0.010
(6.350 ± 0.254)
0.125
(3.175)
MIN
0.045 ± 0.015
(1.143 ± 0.381)
0.100 ± 0.010
(2.540 ± 0.254)
0.020
(0.508)
MIN
1
2
4
3
N8 0393
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
× 45°
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
7
6
5
0.004 – 0.010
(0.101 – 0.254)
0°– 8° TYP
0.016 – 0.050
0.406 – 1.270
8
0.053 – 0.069
(1.346 – 1.752)
0.014 – 0.019
(0.355 – 0.483)
0.050
(1.270)
BSC
0.228 – 0.244
(5.791 – 6.197)
0.150 – 0.157
(3.810 – 3.988)
SO8 0393
1
12
Linear Technology Corporation
2
3
4
LT/GP 0493 10K REV 0
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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