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PDF LTC1474 Data sheet ( Hoja de datos )
| Número de pieza | LTC1474 | |
| Descripción | Low Quiescent Current High Efficiency Step-Down Converters | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s High Efficiency: Over 92% Possible
s Very Low Standby Current: 10µA Typ
s Available in Space Saving 8-Lead MSOP Package
s Internal 1.4Ω Power Switch (VIN = 10V)
s Wide VIN Range: 3V to 18V Operation
s Very Low Dropout Operation: 100% Duty Cycle
s Low-Battery Detector Functional During Shutdown
s Programmable Current Limit with Optional
Current Sense Resistor (10mA to 400mA Typ)
s Short-Circuit Protection
s Few External Components Required
s Active Low Micropower Shutdown: IQ = 6µA Typ
s Pushbutton On/Off (LTC1475 Only)
s 3.3V, 5V and Adjustable Output Versions
U
APPLICATIONS
s Cellular Telephones and Wireless Modems
s 4mA to 20mA Current Loop Step-Down Converter
s Portable Instruments
s Battery-Operated Digital Devices
s Battery Chargers
s Inverting Converters
s Intrinsic Safety Applications
LTC1474/LTC1475
Low Quiescent Current
High Efficiency Step-Down
Converters
DESCRIPTION
The LT C®1474/LTC1475 series are high efficiency step-
down converters with internal P-channel MOSFET power
switches that draw only 10µA typical DC supply current at
no load while maintaining output voltage. The LTC1474
uses logic-controlled shutdown while the LTC1475 fea-
tures pushbutton on/off.
The low supply current coupled with Burst ModeTM opera-
tion enables the LTC1474/LTC1475 to maintain high effi-
ciency over a wide range of loads. These features, along
with their capability of 100% duty cycle for low dropout
and wide input supply range, make the LTC1474/LTC1475
ideal for moderate current (up to 300mA) battery-powered
applications.
The peak switch current is user-programmable with an
optional sense resistor (defaults to 325mA minimum if not
used) providing a simple means for optimizing the design
for lower current applications. The peak current control
also provides short-circuit protection and excellent start-
up behavior. A low-battery detector that remains functional
in shutdown is provided .
The LTC1474/LTC1475 series availability in 8-lead MSOP
and SO packages and need for few additional components
provide for a minimum area solution.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATION
VIN
4V TO 18V
+ 10µF
25V
0.1µF
LOW BATTERY IN
7
6 VIN
SENSE
VFB
3
LBI
LTC1474-3.3
LBO
1
2
RUN SHDN
100k 8
RUN
GND
4
5
SW
L1 = SUMIDA CDRH74-101
LOW BATTERY OUT
L1 +
100µH
VOUT
3.3V AT 250mA
100µF
6.3V
D1
MBR0530
1474/75 F01
Figure 1. High Efficiency Step-Down Converter
LTC1474 Efficiency
100
90 VIN = 5V
VIN = 10V
80 VIN = 15V
70
60
50
0.03
L = 100µH
VOUT = 3.3V
RSENSE = 0Ω
0.3 3 30 300
LOAD CURRENT (mA)
1474/75 TA01
1
1 page  
LTC1474/LTC1475
PIN FUNCTIONS
VOUT/VFB (Pin 1): Feedback of Output Voltage. In the fixed
versions, an internal resistive divider divides the output
voltage down for comparison to the internal 1.23V refer-
ence. In the adjustable versions, this divider must be
implemented externally.
LBO (Pin 2): Open Drain Output of the Low Battery
Comparator. This pin will sink current when Pin 3 is below
1.23V.
LBI/OFF (Pin 3): Input to Low Battery Comparator. This
input is compared to the internal 1.23V reference. For the
LTC1475, a momentary ground on this pin puts regulator
in shutdown mode.
GND (Pin 4): Ground Pin.
SW (Pin 5): Drain of Internal PMOS Power Switch. Cath-
ode of Schottky diode must be closely connected to this
pin.
SENSE (Pin 6): Current Sense Input for Monitoring Switch
Current and Source of Internal PMOS Power Switch.
Maximum switch current is programmed with a resistor
between SENSE and VIN pins.
VIN (Pin 7): Main Supply Pin.
RUN/ON (Pin 8): On LTC1474, voltage level on this pin
controls shutdown/run mode (ground = shutdown, open/
high = run). On LTC1475, a momentary ground on this pin
puts regulator in run mode. A 100k series resistor must be
used between Pin 8 and the switch or control voltage.
W
FUNCTIONAL DIAGRA
LBI/OFF
1µA
100mV
–
×C
ON +
LTC1474: RUN 8
LTC1475: ON
LBO 2
1-SHOT
4.75µs
TRIGGER
OUT
STRETCH WAKEUP
–
V
ON +
+
LB
–
READY
3 LTC1474: LBI
LTC1475: LBI/OFF
× CONNECTION NOT PRESENT IN LTC1474 SERIES
CONNECTION PRESENT IN LTC1474 SERIES ONLY
1.23V
1.23V
REFERENCE
VIN
7
RSENSE
(OPTIONAL)
VIN
+
VCC
6
SENSE
5Ω
1× 20×
SW
5
3M
(5V VERSION)
1.68M
(3.3V VERSION)
VOUT/VFB
1
VOUT
+
4 GND
1M
OUTPUT DIVIDER IS
IMPLEMENTED EXTERNALLY IN
ADJUSTABLE VERSIONS
1474/75 FD
5
5 Page 
LTC1474/LTC1475
APPLICATIONS INFORMATION
LTC1475 Pushbutton On/Off and
Microprocessor Interface
The LTC1475 provides pushbutton control of power on/off
for use with handheld products. A momentary ground on
the ON pin sets an internal S/R latch to run mode while a
momentary ground on the LBI/OFF pin resets the latch to
shutdown mode. See Figure 6 for a comparsion of on/off
operation of the LTC1474 and LTC1475 and Figure 7 for a
comparison of the circuit implementations.
In the LTC1475, the LBI/OFF pin has a dual function as
both the shutdown control pin and the low battery com-
parator input. Since the “OFF” pushbutton is normally
open, it does not affect the normal operation of the low
battery comparator. In the unpressed state, the LBI/OFF
input is the divided down input voltage from the resistive
divider to the internal low battery comparator and will
normally be above or just below the trip threshold of
1.23V. When shutdown mode is desired, the LBI/OFF pin
is pulled below the 0.7V threshold to invoke shutdown.
RUN
LTC1474
MODE
ON
LBI/OFF
LTC1475
MODE
RUN
SHUTDOWN
RUN
ON OVERRIDES LBI/OFF
WHILE ON IS LOW
RUN SHUTDOWN
RUN
1474/75 F06
Figure 6. Comparison of LTC1474 and LTC1475
Run/Shutdown Operation
The ON pin has precedence over the LBI/OFF pin. As seen
in Figure 6, if both pins are grounded simultaneously, run
mode wins.
Figure 18 in the Typical Applications section shows an
example for the use of the LTC1475 to control on/off of a
microcontroller with a single pushbutton. With both the
microcontroller and LTC1475 off, depressing the
pushbutton grounds the LTC1475 ON pin and starts up the
LTC1475 regulator which then powers up the microcon-
troller. When the pushbutton is depressed a second time,
the depressed switch state is detected by the microcon-
troller through its input. The microcontroller then pulls the
LBI/OFF pin low with the connection to one of its ouputs.
With the LBI/OFF pin low, the LTC1475 powers down
turning the microcontroller off. Note that since the I/O pins
of most microcontrollers have a reversed bias diode
between input and supply, a blocking diode with less than
1µA leakage is necessary to prevent the powered down
microcontroller from pulling down on the ON pin.
Figure 19 in the Typical Applications section shows how to
use the low battery comparator to provide a low battery
lockout on the “ON” switch. The LBO output disconnects
the pushbutton from the ON pin when the comparator has
tripped, preventing the LTC1475 from attempting to start
up again until VIN is increased.
100k
RUN
RUN LTC1474
ON
100k
VIN
ON
LTC1475
LBI/OFF
OFF
1474/75 F07
Figure 7. Simplified Implementation of
LTC1474 and LTC1475 On/Off
Absolute Maximum Ratings and Latchup Prevention
The absolute maximum ratings specify that SW (Pin 5) can
never exceed VIN (Pin 7) by more than 0.3V. Normally this
situation should never occur. It could, however, if the
output is held up while the supply is pulled down. A
condition where this could potentially occur is when a
battery is supplying power to an LTC1474 or LTC1475
regulator and also to one or more loads in parallel with the
the regulator’s VIN. If the battery is disconnected while the
LTC1474 or LTC1475 regulator is supplying a light load
and one of the parallel circuits is a heavy load, the input
capacitor of the LTC1474 or LTC1475 regulator could be
pulled down faster than the output capacitor, causing the
absolute maximum ratings to be exceeded. The result is
often a latchup which can be destructive if VIN is reapplied.
Battery disconnect is possible as a result of mechanical
stress, bad battery contacts or use of a lithium-ion battery
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC1474.PDF ] | |
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