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PDF LT3437 Data sheet ( Hoja de datos )
| Número de pieza | LT3437 | |
| Descripción | High Voltage 500mA and 200kHz Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT3437www.DataSheet4U.com
High Voltage 500mA, 200kHz
Step-Down Switching Regulator
with 100µA Quiescent Current
FEATURES
■ Wide Input Range: 3.3V to 60V
■ Load Dump (Input Transient) Protection to 80V
■ 500mA Peak Switch Current
■ Burst Mode® Operation: 100µA Quiescent Current**
■ Low Shutdown Current: IQ < 1µA
■ Defeatable Burst Mode Operation
■ 200kHz Switching Frequency
■ Saturating Switch Design: 0.8Ω On-Resistance
■ Peak Switch Current Maintained Over
Full Duty Cycle Range*
■ 1.25V Feedback Reference Voltage
■ Easily Synchronizable
■ Soft-Start Capability
■ Small 10-Pin TheUrmally Enhanced DFN Package
APPLICATIO S
■ High Voltage Power Conversion
■ 14V and 42V Automotive Systems
■ Industrial Power Systems
■ Distributed Power Systems
■ Battery-Powered Systems
■ Powered Ethernet
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode
is a registered trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners. *Protected by U.S. Patents, including 6498466.
**See Burst Mode Operation section for conditions.
DESCRIPTIO
The LT®3437 is a 200kHz monolithic buck switching
regulator that accepts input voltages up to 80V. A high
efficiency 500mA, 0.8Ω switch is included on the die along
with all the necessary oscillator, control and logic cir-
cuitry. Current mode topology is used for fast transient
response and good loop stability.
Innovative design techniques along with a new high volt-
age process achieve high efficiency over a wide input
range. Efficiency is maintained over a wide output current
range by employing Burst Mode operation at low currents,
utilizing the output to bias the internal circuitry, and by
using a supply boost capacitor to fully saturate the power
switch. Burst Mode operation can be defeated by a logic
high signal on the SYNC pin which results in lower light
load ripple at the expense of light load efficiency. Patented
circuitry maintains peak switch current over the full duty
cycle range.* Shutdown reduces input supply current to
less than 1µA. External synchronization can be imple-
mented by driving the SYNC pin with logic-level inputs. A
single capacitor from the CSS pin to the output provides a
controlled output voltage ramp (soft-start).
The LT3437 is available in a low profile (0.75mm) 3mm ×
3mm 10-pin DFN package or a 16-Pin TSSOP Package,
both with exposed pad leadframes for low thermal
resistance.
TYPICAL APPLICATIO
14V to 3.3V Step-Down Converter with
100µA No Load Quiescent Current
VIN
4.5V TO 80V*
1500pF
25k
2.2µF
100V
CER
330pF
VIN BOOST
SHDN
SW
0.1µF 100µH BAS21
LT3437 0.1µF
10MQ100N
VC CSS
VBIAS
27pF
165k
SYNC
FB
GND 100k
VOUT
3.3V
400mA
100µF
6.3V
TANT
*FOR INPUT VOLTAGES ABOVE 60V RESTRICTIONS APPLY
3437 TA01
Supply Current vs Input Voltage
Input Voltage
Transient Response
200
180
160
140
120
100
80
60
40
20
0
0
10 20 30 40 50 60 70 80
INPUT VOLTAGE (V)
3437 TA02
VIN
20V/DIV
VIN
0V
VOUT
20mV/DIV
AC COUPLED
IOUT 250mA
LOAD DUMP
50ms/DIV
3437 TA03
COLD CRANK
3437fc
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT3437www.DataSheet4U.com
Efficiency and Power Loss
vs Load Current
100 VIN = 12V
90 VOUT = 3.3V
80 TA = 25°C
70 EFFICIENCY
60
50
40
30
20 POWER LOSS
10
0
0.1 1 10 100
LOAD CURRENT (mA)
500
450
400
350
300
250
200
150
100
50
0
1000
3437 G01
SHDN Threshold
1.50
1.45
1.40
1.35
1.30
1.25
1.20
1.15
1.10
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
3437 G04
FB Voltage vs Temperature
1.30
1.28
1.26
1.24
1.22
1.20
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
3437 G02
SHDN Pin Current
10
8
6
4
2
0
0 10 20 30 40 50 60
VSHDN (V)
3437 G05
Oscillator Frequency
vs Temperature
250
240
230
220
210
200
190
180
170
160
150
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
3437 G03
Shutdown Supply Current
vs Temperature
22
20
18
16
14
12
10
8
6
4
2
0
–50 –25
VVIN = 80V
VVIN = 60V
VVIN = 12V
0 25 50 75 100 125 150
TEMPERATURE (°C)
3437 G06
Input Current vs Temperature
600
500 RUN MODE
400
300
200 SLEEP MODE VBIAS = 0V
100
SLEEP MODE VBIAS = 5V
0
–50 –30 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
3437 G07
Bias Current vs Temperature
800
700 RUN MODE
600
500
400
300
200
SLEEP MODE
100
0
–50 –30 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
3437 G08
Switch Peak Current Limit
vs Temperature
800
700
600
500
400
300
200
100
0
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
3437 G09
3437fc
5
5 Page 
LT3437www.DataSheet4U.com
APPLICATIO S I FOR ATIO
FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT3437 is used to set output
voltage and provide several overload protection features.
The first part of this section deals with selecting resistors
to set output voltage, and the remaining part talks about
frequency foldback and soft-start features. Please read
both parts before committing to a final design.
Referring to Figure 2, the output voltage is determined by
a voltage divider from VOUT to ground which generates
1.25V at the FB pin. Since the output divider is a load on the
output, care must be taken when choosing the resistor
divider values. For light load applications the resistor
values should be as large as possible to achieve peak
efficiency in Burst Mode operation. Extremely large values
for resistor R1 will cause an output voltage error due to the
50nA FB pin input current. The suggested value for the
output divider resistor (see Figure 2) from FB to ground
(R2) is 100k or less. A formula for R1 is shown below. A
table of standard 1% values is shown in Table 1 for
common output voltages.
R1= R2 • VOUT – 1.25
1.25 + R2 • 50nA
LT3437
SOFT-START
SW 1
CSS
5
C1
VOUT
200kHz
OSCILLATOR
FOLDBACK
DETECT
R1
–
ERROR
AMP
+
FB
8
1.25V
R2
VC
7
3437 F02
Figure 2. Feedback Network
Table 1
OUTPUT
VOLTAGE
(V)
2.5
3
3.3
5
6
8
10
12
R2
(kΩ, 1%)
100
100
100
100
100
100
100
100
R1
NEAREST (1%)
(kΩ)
100
140
165
300
383
536
698
866
OUTPUT
ERROR
(%)
0
0
0.38
0
0.63
– 0.63
– 0.25
0.63
More Than Just Voltage Feedback
The FB pin is used for more than just output voltage
sensing. It also reduces switching frequency and con-
trols the soft-start voltage ramp rate when output voltage
is below the regulated level (see the Frequency Foldback
and Soft-Start Current graphs in Typical Performance
Characteristics).
Frequency foldback is done to control power dissipation in
both the IC and in the external diode and inductor during
short-circuit conditions. A shorted output requires the
switching regulator to operate at very low duty cycles. As
a result, the average current through the diode and induc-
tor is equal to the short-circuit current limit of the switch
(typically 500mA for the LT3437). Minimum switch on
time limitations would prevent the switcher from attaining
a sufficiently low duty cycle if switching frequency were
maintained at 200kHz, so frequency is reduced by about
10:1 when the FB pin voltage drops below 0.4V (see
Frequency Foldback graph). As the feedback voltage rises,
the switching frequency increases to 200kHz with 0.95V
on the FB pin. During frequency foldback, external syn-
chronization is disabled to prevent interference with fold-
back operation. Frequency foldback does not affect opera-
tion during normal load conditions.
In addition to lowering switching frequency, the soft-start
ramp rate is also affected by the feedback voltage. Large
capacitive loads or high input voltages can cause a
3437fc
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LT3437.PDF ] | |
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