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PDF LT1108-12 Data sheet ( Hoja de datos )
| Número de pieza | LT1108-12 | |
| Descripción | Micropower DC/DC Converter Adjustable and Fixed 5V/ 12V | |
| Fabricantes | Linear Technology | |
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FEATURES
s Operates at Supply Voltages from 2V to 30V
s Consumes Only 110µA Supply Current
s Works in Step-Up or Step-Down Mode
s Only Four External Components Required
s Low Battery Detector Comparator On-Chip
s User Adjustable Current Limit
s Internal 1A Power Switch
s Fixed or Adjustable Output Voltage Versions
s Space Saving 8-Pin MiniDIP or S8 Package
APPLICATI S
s Palmtop Computers
s 3V to 5V, 5V to 12V Converters
s 9V to 5V, 12V to 5V Converters
s LCD Bias Generators
s Peripherals and Add-On Cards
s Battery Backup Supplies
s Cellular Telephones
s Portable Instruments
LT1108
Micropower
DC/DC Converter
Adjustable and Fixed 5V, 12V
DESCRIPTIO
The LT1108 is a versatile micropower DC/DC converter.
The device requires only four external components to
deliver a fixed output of 5V or 12V. Supply voltage ranges
from 2V to 12V in step-up mode and to 30V in step-down
mode. The LT1108 functions equally well in step-up, step-
down, or inverting applications.
The LT1108 is pin-for-pin compatible with the LT1173, but
has a duty cycle of 70%, resulting in increased output
current in many applications. The LT1108 can deliver
150mA at 5V from a 2 AA cell input and 5V at 300mA from
9V in step-down mode. Quiescent current is just 110µA,
making the LT1108 ideal for power conscious battery-
operated systems.
Switch current limit can be programmed with a single
resistor. An auxiliary gain block can be configured as a low
battery detector, linear post regulator, undervoltage lock-
out circuit, or error amplifier.
TYPICAL APPLICATI
Palmtop Computer Logic Supply
L1*
100µH
1N5817
2 × AA
CELLS
+
100µF
47Ω
ILIM VIN
SW1
LT1108-5
SENSE
GND SW2
5V
150mA
+
AVX
TPS
330µF
6.3V
*L1 = GOWANDA GA20-103K
COILTRONICS CTX100-4
SUMIDA CD105-101K
LT1108 • TA01
Efficiency
84
82
VIN = 3V
80 VIN = 2.5V
78 VIN = 2V
76
74
72
70
1
10 100
LOAD CURRENT (mA)
LT1108 • TA02
1
1 page  
LT1108
1 OPERATI
The LT1108 is a gated oscillator switcher. This type
architecture has very low supply current because the
switch is cycled when the feedback pin voltage drops
below the reference voltage. Circuit operation can best be
understood by referring to the LT1108 block diagram.
Comparator A1 compares the feedback (FB) pin voltage
with the 1.245V reference signal. When FB drops below
1.245V, A1 switches on the 19kHz oscillator. The driver
amplifier boosts the signal level to drive the output NPN
power switch. The switch cycling action raises the output
voltage and FB pin voltage. When the FB voltage is suffi-
cient to trip A1, the oscillator is gated off. A small amount
of hysteresis built into A1 ensures loop stability without
external frequency compensation. When the comparator
output is low, the oscillator and all high current circuitry
is turned off, lowering device quiescent current to just
110µA.
The oscillator is set internally for 36µs ON-time and 17µs
OFF-time, allowing continuous mode operation in many
cases such as 2V to 5V converters. Continuous mode
greatly increases available output power.
Gain block A2 can serve as a low battery detector. The
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.
BLOCK DIAGRA S
LT1108
LT1108-5/LT1108-12
SET
VIN
1.245V
REFERENCE
GND
FB
A2 A0
GAIN BLOCK/
ERROR AMP
ILIM SW1
A1 OSCILLATOR
COMPARATOR
DRIVER
LT1108 • BD SW2
SET
VIN
1.245V
REFERENCE
A2 A0
GAIN BLOCK/
ERROR AMP
ILIM SW1
A1 OSCILLATOR
DRIVER
COMPARATOR
SW2LT1108-5 • BD
R2
R1 753k
LT1108-5: R1 = 250k
GND
SENSE
LT1108-12: R1 = 87.4k
5
5 Page 
LT1108
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 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.
RLIM
(EXTERNAL)
VIN
Q3
OSCILLATOR
DRIVER
ILIM
R1
80Ω
(INTERNAL)
SW1
Q1 Q2
SW2
LT1108 • F08
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.
R1
VBAT
V IN
LT1108
1.245V
REF
–
SET +
5V
47k
AO TO
PROCESSOR
R2 GND
R3
R1 = VLB – 1.25V
35.1µA
VLB = BATTERY TRIP POINT
R2 = 33k
R3 = 1.6M
LT1108 • F09
Figure 9. Setting Low Battery Detector Trip Point
Table 1. Inductor Manufacturers
MANUFACTURER
Coiltronics International
984 S.W. 13th Court
Pompano Beach, FL 33069
305-781-8900
Sumida Electric Co. USA
708-956-0666
PART NUMBERS
OCTA-PACTM
Series
CD54
CDR74
CDR105
Table 2. Capacitor Manufacturers
MANUFACTURER
Sanyo Video Components
1201 Sanyo Avenue
San Diego, CA 92073
619-661-6322
Nichicon America Corporation
927 East State Parkway
Schaumberg, IL 60173
708-843-7500
AVX Corporation
Myrtle Beach, SC
803-946-0690
PART NUMBERS
OS-CON Series
PL Series
TPS Series
Table 3. Transistor Manufacturers
MANUFACTURER
Zetex Inc.
87 Modular Avenue
Commack, NY 11725
516-543-7100
PART NUMBERS
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
11
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet LT1108-12.PDF ] | |
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