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PDF LT1375IN8 Data sheet ( Hoja de datos )
| Número de pieza | LT1375IN8 | |
| Descripción | 1.5A/ 500kHz Step-Down Switching Regulators | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT1375/LT1376
1.5A, 500kHz Step-Down
Switching Regulators
FEATURES
s Constant 500kHz Switching Frequency
s Easily Synchronizable
s Uses All Surface Mount Components
s Inductor Size Reduced to 5µH
s Saturating Switch Design: 0.4Ω
s Effective Supply Current: 2.5mA
s Shutdown Current: 20µA
s Cycle-by-Cycle Current Limiting
U
APPLICATIO S
s Portable Computers
s Battery-Powered Systems
s Battery Charger
s Distributed Power
DESCRIPTIO
The LT®1375/LT1376 are 500kHz monolithic buck mode
switching regulators. A 1.5A switch is included on the die
along with all the necessary oscillator, control and logic
circuitry. High switching frequency allows a considerable
reduction in the size of external components. The topology
is current mode for fast transient response and good loop
stability. Both fixed output voltage and adjustable parts are
available.
A special high speed bipolar process and new design
techniques achieve high efficiency at high switching fre-
quency. Efficiency is maintained over a wide output cur-
rent range by using the output to bias the circuitry and by
utilizing a supply boost capacitor to saturate the power
switch. A shutdown signal will reduce supply current to
20µA on both parts. The LT1375 can be externally syn-
chronized from 550kHz to 1MHz with logic level inputs.
The LT1375/LT1376 fit into standard 8-pin PDIP and SO
packages, as well as a fused lead 16-pin SO with much
lower thermal resistance. Full cycle-by-cycle short-cir-
cuit protection and thermal shutdown are provided.
Standard surface mount external parts are used, includ-
ing the inductor and capacitors.
For low input voltage applications with 3.3V output, see
LT1507. This is a functionally identical part that can
operate with input voltages between 4.5V and 12V.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
5V Buck Converter
INPUT
6V† TO 25V C3* +
10µF TO
50µF
DEFAULT
= ON
BOOST
VIN VSW
LT1376-5 BIAS
SHDN
GND
FB
VC
CC
3.3nF
D2
1N914
C2
0.1µF
L1**
5µH
OUTPUT**
5V, 1.25A
D2
1N5818
+ C1
100µF, 10V
SOLID
TANTALUM
* RIPPLE CURRENT ≥ IOUT/2
** INCREASE L1 TO 10µH FOR LOAD CURRENTS ABOVE 0.6A AND TO 20µH ABOVE 1A
† FOR INPUT VOLTAGE BELOW 7.5V, SOME RESTRICTIONS MAY APPLY.
SEE APPLICATIONS INFORMATION.
1375/76 TA01
Efficiency vs Load Current
100
VOUT = 5V
VIN = 10V
90 L = 10µH
80
70
60
50
0
0.25 0.50 0.75 1.00
LOAD CURRENT (A)
1.25
1375/76 TA02
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
LT1375/LT1376
Shutdown Pin Bias Current
500
CURRENT REQUIRED TO FORCE SHUTDOWN
400 (FLOWS OUT OF PIN). AFTER SHUTDOWN,
CURRENT DROPS TO A FEW µA
300
200
8
AT 2.38V STANDBY THRESHOLD
(CURRENT FLOWS OUT OF PIN)
4
0
–50 –25 0 25 50 75
TEMPERATURE (°C)
100 125
1375/76 G04
Shutdown Supply Current
150
125
100
VIN = 25V
75
50 VIN = 10V
25
0
0 0.1 0.2 0.3 0.4 0.5
SHUTDOWN VOLTAGE (V)
1375/76 G07
Frequency Foldback
500
400
SWITCHING
FREQUENCY
300
200
100
0
0
FEEDBACK PIN
CURRENT
0.5 1.0 1.5 2.0
FEEDBACK PIN VOLTAGE (V)
2.5
1375/76 G10
Standby and Shutdown Thresholds
2.40
STANDBY
2.36
2.32
0.8
START-UP
0.4
SHUTDOWN
0
–50 –25 0 25 50 75 100 125
JUNCTION TEMPERATURE (°C)
1375/76 G05
Shutdown Supply Current
30
VSHUTDOWN = 0V
25
20
15
10
5
0
05
10 15 20 25
INPUT VOLTAGE (V)
1375/76 G06
Error Amplifier Transconductance
2500
Error Amplifier Transconductance
3000
200
2000
2500
PHASE
150
1500
1000
2000
GAIN
( )1500 VFB 2 • 10–3
ROUT
200k
VC
COUT
12pF
100
50
500
0
–50 –25 0 25 50 75 100 125
JUNCTION TEMPERATURE (°C)
1375/76 G02
1000 ERROR AMPLIFIER EQUIVALENT CIRCUIT
0
RLOAD = 50Ω
500
100 1k 10k
100k
FREQUENCY (Hz)
–50
1M 10M
1375/76 G03
Switching Frequency
600
550
500
450
400
–50 –25 0 25 50 75 100 125
JUNCTION TEMPERATURE (°C)
1375/76 G11
LT1376 Minimum Input Voltage
with 5V Output
8.5
MINIMUM INPUT VOLTAGE CAN BE
8.0 REDUCED BY ADDING A SMALL EXTERNAL
PNP. SEE APPLICATIONS INFORMATION
7.5
7.0 MINIMUM
VOLTAGE TO
6.5
START WITH
STANDARD
CIRCUIT
6.0
MINIMUM VOLTAGE
5.5 TO RUN WITH
STANDARD CIRCUIT
5.0
0 10
100
LOAD CURRENT (mA)
1000
1375/76 G12
5
5 Page 
LT1375/LT1376
APPLICATIONS INFORMATION
Example: with L = 2µH, VOUT = 5V, and VIN(MAX) = 15V,
( ) ( )( ) ( )( )IOUT MAX =
1.5
2
500
•
103
2
•
10−6
15
2 5 15 − 5
= 338mA
The main reason for using such a tiny inductor is that it is
physically very small, but keep in mind that peak-to-peak
inductor current will be very high. This will increase output
ripple voltage. If the output capacitor has to be made larger
to reduce ripple voltage, the overall circuit could actually
wind up larger.
CHOOSING THE INDUCTOR AND OUTPUT CAPACITOR
For most applications the output inductor will fall in the
range of 3µH to 20µH. Lower values are chosen to reduce
physical size of the inductor. Higher values allow more
output current because they reduce peak current seen by
the LT1376 switch, which has a 1.5A limit. Higher values
also reduce output ripple voltage, and reduce core loss.
Graphs in the Typical Performance Characteristics section
show maximum output load current versus inductor size
and input voltage. A second graph shows core loss versus
inductor size for various core materials.
When choosing an inductor you might have to consider
maximum load current, core and copper losses, allowable
component height, output voltage ripple, EMI, fault cur-
rent in the inductor, saturation, and of course, cost. The
following procedure is suggested as a way of handling
these somewhat complicated and conflicting requirements.
1. Choose a value in microhenries from the graphs of
maximum load current and core loss. Choosing a small
inductor with lighter loads may result in discontinuous
mode of operation, but the LT1376 is designed to work
well in either mode. Keep in mind that lower core loss
means higher cost, at least for closed core geometries
like toroids. The core loss graphs show both absolute
loss and percent loss for a 5W output, so actual percent
losses must be calculated for each situation.
Assume that the average inductor current is equal to
load current and decide whether or not the inductor
must withstand continuous fault conditions. If maxi-
mum load current is 0.5A, for instance, a 0.5A inductor
may not survive a continuous 1.5A overload condition.
Dead shorts will actually be more gentle on the induc-
tor because the LT1376 has foldback current limiting.
2. Calculate peak inductor current at full load current to
ensure that the inductor will not saturate. Peak current
can be significantly higher than output current, espe-
cially with smaller inductors and lighter loads, so don’t
omit this step. Powdered iron cores are forgiving
because they saturate softly, whereas ferrite cores
saturate abruptly. Other core materials fall in between
somewhere. The following formula assumes continu-
ous mode of operation, but it errs only slightly on the
high side for discontinuous mode, so it can be used for
all conditions.
(()( )( ) )IPEAK
=
IOUT
+
VOUT
2
VIN
fL
− VOUT
VIN
VIN = Maximum input voltage
f = Switching frequency, 500kHz
3. Decide if the design can tolerate an “open” core geom-
etry like a rod or barrel, which have high magnetic field
radiation, or whether it needs a closed core like a toroid
to prevent EMI problems. One would not want an open
core next to a magnetic storage media, for instance!
This is a tough decision because the rods or barrels are
temptingly cheap and small and there are no helpful
guidelines to calculate when the magnetic field radia-
tion will be a problem.
4. Start shopping for an inductor (see representative
surface mount units in Table 2) which meets the re-
quirements of core shape, peak current (to avoid satu-
ration), average current (to limit heating), and fault
current (if the inductor gets too hot, wire insulation will
melt and cause turn-to-turn shorts). Keep in mind that
all good things like high efficiency, low profile, and high
temperature operation will increase cost, sometimes
dramatically. Get a quote on the cheapest unit first to
calibrate yourself on price, then ask for what you really
want.
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LT1375IN8.PDF ] | |
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