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PDF LT1956 Data sheet ( Hoja de datos )
| Número de pieza | LT1956 | |
| Descripción | High Voltage/ 1.5A/ 500kHz Step-Down Switching Regulators | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s Wide Input Range: 5.5V to 60V
s 1.5A Peak Switch Current
s Small 16-Pin SSOP or Thermally Enhanced
TSSOP Package
s Saturating Switch Design: 0.2Ω
s Peak Switch Current Maintained Over
Full Duty Cycle Range
s Constant 500kHz Switching Frequency
s Effective Supply Current: 2.5mA
s Shutdown Current: 25µA
s 1.2V Feedback Reference (LT1956)
s 5V Fixed Output (LT1956-5)
s Easily Synchronizable
s Cycle-by-Cycle Current Limiting
U
APPLICATIO S
s High Voltage, Industrial and Automotive
s Portable Computers
s Battery-Powered Systems
s Battery Chargers
s Distributed Power Systems
LT1956/LT1956-5
High Voltage, 1.5A,
500kHz Step-Down
Switching Regulators
DESCRIPTIO
The LT®1956/LT1956-5 are 500kHz monolithic buck
switching regulators with an input voltage capability up to
60V. A high efficiency 1.5A, 0.2Ω switch is included on the
die along with all the necessary oscillator, control and logic
circuitry. A current mode architecture provides fast tran-
sient response and good loop stability.
Special design techniques and a new high voltage process
achieve high efficiency over a wide input range. Efficiency
is maintained over a wide output current range by using the
output to bias the circuitry and by utilizing a supply boost
capacitor to saturate the power switch. Patented circuitry
maintains peak switch current over the full duty cycle
range*. A shutdown pin reduces supply current to 25µA and
the device can be externally synchronized from 580kHz to
700kHz with a logic level input.
The LT1956/LT1956-5 are available in fused-lead 16-pin
SSOP and thermally enhanced TSSOP packages.
, LTC and LT are registered trademarks of Linear Technology Corporation.
*U.S. PATENT NO. 6,498,466
TYPICAL APPLICATIO
5V Buck Converter
MMSD914TI
VIN
12V
(TRANSIENTS
TO 60V)
2.2µF†
100V
CERAMIC
6
BOOST
42
VIN SW
LT1956-5
15
SHDN
10
BIAS
14
SYNC
12
FB
GND VC
1, 8, 9, 16 11
220pF
4.7k
0.1µF
10µH
10MQ060N
VOUT
5V
1A
22µF
6.3V
CERAMIC
4700pF
†UNITED CHEMI-CON THCS50EZA225ZT
1956 TA01
Efficiency vs Load Current
100
VIN = 12V
L = 18µH
90
80
VOUT = 5V
VOUT = 3.3V
70
60
50
0 0.25 0.50 0.75 1.00 1.25
LOAD CURRENT (A)
1956 TA02
1956f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
Switching Frequency
575
550
525
500
475
450
425
–50
–25 0 25 50 75 100 125
JUNCTION TEMPERATURE (°C)
1956 G10
Minimum Input Voltage with 5V
Output
7.5
VOUT = 5V
L = 18µH
7.0
MINIMUM INPUT
6.5 VOLTAGE TO START
6.0
MINIMUM INPUT
VOLTAGE TO RUN
5.5
5.0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
LOAD CURRENT (A)
1956 G11
VC Pin Shutdown Threshold
2.1
1.9
1.7
1.5
1.3
1.1
0.9
0.7
–50 –25 0 25 50 75 100 125
JUNCTION TEMPERATURE (°C)
1956 G13
Switch Voltage Drop
450
400
TJ = 125°C
350
300
TJ = 25°C
250
200
150
TJ = –40°C
100
50
0
0 0.5 1 1.5
SWITCH CURRENT (A)
1766 G14
LT1956/LT1956-5
BOOST Pin Current
45
40
35
30
25
20
15
10
5
0
0 0.5 1
SWITCH CURRENT (A)
Switch Minimum ON Time
vs Temperature
600
1.5
1956 G12
500
400
300
200
100
0
–50 –25 0 25 50 75 100 125
JUNCTION TEMPERATURE (°C)
1956 G15
1956f
5
5 Page 
LT1956/LT1956-5
APPLICATIO S I FOR ATIO
iron cores are forgiving because they saturate softly,
whereas ferrite cores saturate abruptly. Other core mate-
rials fall somewhere in between. The following formula
assumes continuous mode of operation, but errs only
slightly on the high side for discontinuous mode, so it can
be used for all conditions.
( )IPEAK
= IOUT
+ ILP-P
2
= IOUT
+
VOUT VIN – VOUT
2 • VIN • f • L
EMI
Decide if the design can tolerate an “open” core geometry
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. 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 radiation will be a problem.
Additional Considerations
After making an initial choice, consider additional factors
such as core losses and second sourcing, etc. Use the
experts in Linear Technology’s Applications department if
you feel uncertain about the final choice. They have
experience with a wide range of inductor types and can tell
you about the latest developments in low profile, surface
mounting, etc.
MAXIMUM OUTPUT LOAD CURRENT
Maximum load current for a buck converter is limited by
the maximum switch current rating (IP). The current rating
for the LT1956 is 1.5A. Unlike most current mode convert-
ers, the LT1956 maximum switch current limit does not
fall off at high duty cycles. Most current mode converters
suffer a drop off of peak switch current for duty cycles
above 50%. This is due to the effects of slope compensa-
tion required to prevent subharmonic oscillations in cur-
rent mode converters. (For detailed analysis, see Applica-
tion Note 19.)
The LT1956 is able to maintain peak switch current limit
over the full duty cycle range by using patented circuitry to
cancel the effects of slope compensation on peak switch
current without affecting the frequency compensation it
provides.
Maximum load current would be equal to maximum
switch current for an infinitely large inductor, but with
finite inductor size, maximum load current is reduced by
one half of peak-to-peak inductor current (ILP-P). The
following formula assumes continuous mode operation,
implying that the term on the right is less than one half
of␣ IP.
IOUT(MAX) Continuous Mode
= IP
–
ILP-P
2
= IP
–
(VOUT
+ VF )(VIN – VOUT
(2)(VIN)(f)(L)
– VF )
For VOUT = 5V, VIN(MAX) = 8V, VF(DI) = 0.63V, f = 500kHz
and L = 10µH:
( )( )IOUT(MAX)
=
1.5
–
(5 +
(2)(8)
0.63)(8 –
500 •103
5 – 0.63)
10 •10–6
= 1.5 – 0.17 = 1.33A
Note that there is less load current available at the higher
input voltage because inductor ripple current increases. At
VIN = 15V and using the same set of conditions:
( )( )IOUT(MAX)
=
1.5
–
(5 + 0.63)(15 –
(2)(15) 500 •103
5 – 0.63)
10 •10–6
= 1.5 – 0.35 = 1.15A
To calculate peak switch current with a given set of
conditions, use:
ISW(PEAK)
=
IOUT
+
ILP-P
2
=
IOUT
+
(VOUT
+ VF )(VIN – VOUT
(2)(VIN)(f)(L)
–
VF )
Reduced Inductor Value and Discontinuous Mode
If the smallest inductor value is of the most importance to
a converter design, in order to reduce inductor size/cost,
discontinuous mode may yield the smallest inductor
1956f
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LT1956.PDF ] | |
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