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PDF LT1949-1 Data sheet ( Hoja de datos )
| Número de pieza | LT1949-1 | |
| Descripción | 1A Switch PWM DC/DC Converter | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s 1A, 0.5Ω, 30V Internal Switch
s 1.1MHz Fixed Frequency Operation
s Operates with VIN as Low as 1.5V
s Low-Battery Detector Stays Active in Shutdown
s Low VCESAT Switch: 410mV at 800mA
s Pin-for-Pin Compatible with the LT1317B
s Uses Ceramic Capacitors
s Small 8-Lead MSOP Package
U
APPLICATIO S
s LCD Bias Supplies
s GPS Receivers
s Battery Backup
s Portable Electronic Equipment
s Diagnostic Medical Instrumentation
Final Electrical Specifications
LT1949-1
1.1MHz, 1A Switch
PWM DC/DC Converter
June 2000
DESCRIPTIO
The LT®1949-1 is a fixed frequency step-up DC/DC con-
verter with a 1A, 0.5Ω internal switch. Capable of gener-
ating 10V at 175mA from a 3.3V input, the LT1949-1 is
ideal for generating bias voltages for large screen LCD
panels. Constant frequency 1.1MHz operation results in a
low noise output that is easy to filter and the 30V switch
rating allows output voltage up to 28V using a single
inductor. The high switching frequency allows the use of
ceramic output capacitors. An external compensation pin
gives the user flexibility in optimizing loop compensation,
allowing small, low ESR ceramic capacitors to be used at
the output. The 8-lead MSOP package ensures a low
profile overall solution.
The LT1949-1 includes a low-battery detector that stays
alive when the device goes into shutdown. Quiescent
current in shutdown is 50µA, while operating current is
8mA.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
VIN
3.3V
C1
3.3µF
CERAMIC
L1
4.7µH
VIN SW
LT1949-1
SHUTDOWN SHDN
FB
VC GND
84.5k
50pF
C1, C2: TAIYO YUDEN LMK325BJ335MD
D1: MBRM120LT3
L1: SUMIDA CLQ4D10-4R7
D1
R1
196k
1%
3pF
VOUT
10V
175mA
R2
28k
1%
C2
3.3µF
CERAMIC
1949-1 F01
Figure 1. 3.3V to 10V/175mA DC/DC Converter
90
VOUT = 10V
80
70
3.6VIN 4.2VIN 3VIN
60
50
40
30
20
5
10 50 100
LOAD CURRENT (mA)
300
1949-1 F02
Figure 2. 3.3V to 10V Converter Efficiency
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.
1
1 page  
LT1949-1
BLOCK DIAGRA
1.24V
REFERENCE
FB
2
VOUT
R1
(EXTERNAL)
FB
R2
(EXTERNAL)
+
gm
–
ERROR
AMPLIFIER
RAMP
GENERATOR
1.1MHz
OSCILLATOR
VC
1
+
BIAS –
LBI
7+
200mV
ENABLE
A1
COMPARATOR
– A4
SHUTDOWN
LBO
8
SHDN
3
–
FF
+
Σ
+
RQ
S
+ A2
COMPARATOR
DRIVER
SW
5
Q3
+
A=2
–
0.06Ω
4
1949-1 BD
GND
Figure 3. LT1949-1 Block Diagram
U
OPERATIO
The LT1949-1 is a current mode, fixed frequency step-up
DC/DC converter with an internal 1A NPN power transis-
tor. Operation can best be understood by referring to the
Block Diagram.
At the beginning of each oscillator cycle, the flip-flop is set
and the switch is turned on. Current in the switch ramps
up until the voltage at A2’s positive input reaches the VC
pin voltage, causing A2’s output to change state and the
switch to be turned off. The signal at A2’s positive input is
a summation of a signal representing switch current and
a ramp generator (introduced to avoid subharmonic oscil-
lations at duty factors greater than 50%). If the load
increases, VOUT (and FB) will drop slightly and the error
amplifier will drive VC to a higher voltage, causing current
in the switch to increase. In this way, the error amplifier
drives the VC pin to the voltage necessary to satisfy the
load. Frequency compensation is provided by an external
series RC network connected between the VC pin and
ground.
Layout Hints
The LT1949-1 switches current at high speed, mandating
careful attention to layout for proper performance. You
will not get advertised performance with careless layouts.
Figure 4 shows recommended component placement for
a boost (step-up) converter. Follow this closely in your PC
layout. Note the direct path of the switching loops. Input
capacitor C1 must be placed close (< 5mm) to the IC
package. As little as 10mm of wire or PC trace from CIN to
VIN will cause problems such as inability to regulate or
oscillation.
The ground terminal of output capacitor C2 should tie
close to Pin 4 of the LT1949-1. Doing this reduces dI/dt in
the ground copper which keeps high frequency spikes to
a minimum. The DC/DC converter ground should tie to the
PC board ground plane at one place only, to avoid intro-
ducing dI/dt in the ground plane.
5
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| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet LT1949-1.PDF ] | |
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