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PDF LT1107-5 Data sheet ( Hoja de datos )
| Número de pieza | LT1107-5 | |
| Descripción | Micropower DC/DC Converter Adjustable and Fixed 5V/ 12V | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT1107
Micropower
DC/DC Converter
Adjustable and Fixed 5V, 12V
FEATURES
s Operates at Supply Voltages from 2V to 30V
s Consumes Only 320µA Supply Current
s Works in Step-Up or Step-Down Mode
s Only Three 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 SO-8 Package
APPLICATI S
s Palmtop Computers
s 3V to 5V, 5V to 12V Converters
s 24V 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
DESCRIPTIO
The LT1107 is a versatile micropower DC/DC converter.
The device requires only three 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 LT1107 functions equally well in step-up, step-
down, or inverting applications.
The LT1107 is pin-for-pin compatible with the LT1111, but
has a duty cycle of 70%, resulting in increased output
current in many applications. The LT1107 can deliver
150mA at 5V from a 2AA cell input and 5V at 300mA from
24V in step-down mode. Quiescent current is just 320µA,
making the LT1107 ideal for power-conscious battery-
operated systems. The 63kHz oscillator is optimized to
work with surface mount inductors and capacitors.
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*
33µH MBRS120T3
2 × AA
ALKALINE CELLS
47Ω
ILIM VIN
+ SW1
47µF
LT1107-5
SENSE
GND SW2
5V
150mA
+
100µF
* SUMIDA CD54-330K
COILCRAFT DT3316-473
1107 TA01
Efficiency
82
80 VIN = 3V
78
76 VIN = 2V
VIN = 2.5V
74
72
70
68
66
64
1
10 100 400
LOAD CURRENT (mA)
1107 TA02
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT1107
Minimum/Maximum Frequency
vs ON Time, Step-Down Mode
100
90
0°C ≤ TA ≤ 70°C
80
70
60
50
40 –55°C ≤ TA ≤ 125°C
30
4 5 6 7 8 9 10 11 12 13 14
ON TIME (µs)
LT1107-5
Output Voltage
1107 G11
5.3
5.2
5.1
5.0
4.9
4.8
4.7
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
1107 G16
Minimum/Maximum Frequency
vs ON Time, Step-Up Mode
100
90
0°C ≤ TA ≤ 70°C
80
70
60
50 TA = 25°C
–55°C ≤ TA ≤ 125°C
40
30
6 7 8 9 10 11 12 13 14 15 16
ON TIME (µs)
LT1107-12
Output Voltage
1107 G12
12.20
12.15
12.10
12.05
12.00
11.95
11.90
11.85
11.80
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
1107 G17
Duty Cycle
Step-Down Mode
70
65
60
55
50
45
40
35
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
LT1107
Feedback Voltage
1107 G13
1.30
1.29
1.28
1.27
1.26
1.25
1.24
1.23
1.22
1.21
1.20
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
1107 G18
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.
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.
GND (Pin 5): Ground.
AO(Pin6): AuxiliaryGainBlock(GB)Output.Opencollector,
can sink 300µA.
SET (Pin 7): GB Input. GB is an op amp with positive input
connected to SET pin and negative input connected to
1.25V reference.
FB/SENSE (Pin 8): On the LT1107 (adjustable), this pin
goes to the comparator input. On the LT1107-5 and
LT1107-12, this pin goes to the internal application resistor
that sets output voltage.
5
5 Page 
LT1107
APPLICATI S I FOR ATIO
Figure 7 details current limit circuitry. Sense transistor A1,
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 is
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 suffi-
cient 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 800ns. The current trip point becomes less
accurate for switch ON times less than 3µs. Resistor
values programming switch ON time for 800ns or less will
cause spurious response in the switch circuitry although
the device will still maintain output regulation.
Using the Gain Block
The gain block (GB) on the LT1107 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 1.25V reference. The
positive input comes out on the SET pin.
Arrangement of the gain block as a low-battery detector is
straightforward. Figure 8 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 of R3 will change the trip point,
however.
Output ripple of the LT1107, normally 50mV at 5VOUT can
be reduced significantly by placing the gain block in front
of the FB input as shown in Figure 9. This effectively
reduces the comparator hysteresis by the gain of the gain
block. Output ripple can be reduced to just a few millivolts
using this technique. Ripple reduction works with step-
down or inverting modes as well. For this technique to be
effective, output capacitor C1 must be large, so that each
switching cycle increases VOUT by only a few millivolts.
1000µF is a good starting value. C1 should be a low ESR
type as well.
RLIM
(EXTERNAL)
VIN
Q3
DRIVER
OSCILLATOR
ILIM
R1
80Ω
(INTERNAL)
SW1
Q1
Q2
SW2
1107 F07
Figure 7. LT1107 Current Limit Circuitry
5V
VIN LT1107
47k
R1
1.25V
REF
–
AO
TO
VBAT SET +
PROCESSOR
GND
R2
R3
( )R1 =
VLB – 1.25V
35.1µA
VLB = BATTERY TRIP POINT
R2 = 33k
R3 = 1.6M
1107 F08
Figure 8. Setting Low-Battery Detector Trip Point
VBAT
L1
R3
270k
ILIM VIN
AO SW1
LT1107
FB
GND
SET
SW2
( )( )VOUT =
R2 + 1 1.25V
R1
D1
VOUT
+
R2 C1
R1
1107 F09
Figure 9. Output Ripple Reduction Using Gain Block
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 LT1107-5.PDF ] | |
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