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PDF LT3958 Data sheet ( Hoja de datos )
| Número de pieza | LT3958 | |
| Descripción | SEPIC and Inverting Converter | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
n Wide Input Voltage Range: 5V to 80V
n Single Feedback Pin for Positive or Negative
Output Voltage
n Internal 3.3A/84V Power Switch
n Current Mode Control Provides Excellent Transient
Response
n Programmable Operating Frequency (100kHz to
1MHz) with One External Resistor
n Synchronizeable to an External Clock
n Low Shutdown Current < 1μA
n Internal 7.2V Low Dropout Voltage Regulator
n Programmable Input Undervoltage Lockout with
Hysteresis
n Programmable Soft-Start
n Thermally Enhanced QFN (5mm × 6mm) Package
APPLICATIONS
n Automotive
n Telecom
n Industrial
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LT3958
High Input Voltage,
Boost, Flyback, SEPIC and
Inverting Converter
DESCRIPTION
The LT®3958 is a wide input range, current mode, DC/DC
converter which is capable of generating either positive
or negative output voltages. It can be configured as either
a boost, flyback, SEPIC or inverting converter. It features
an internal low side N-channel power MOSFET rated for
84V at 3.3A and driven from an internal regulated 7.2V
supply. The fixed frequency, current-mode architecture
results in stable operation over a wide range of supply
and output voltages.
The operating frequency of LT3958 can be set with an
external resistor over a 100kHz to 1MHz range, and can
be synchronized to an external clock using the SYNC pin.
A minimum operating supply voltage of 5V, and a low
shutdown quiescent current of less than 1μA, make the
LT3958 ideally suited for battery-powered systems.
The LT3958 features soft-start and frequency foldback
functions to limit inductor current during start-up.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
No RSENSE and ThinSOT are trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Patents pending.
TYPICAL APPLICATION
VIN
12V TO 40V
High Efficiency Boost Converter
33μH
4.7μF 392k
VIN SW
EN/UVLO
GND
53.6k
LT3958
SGND
SENSE1
SYNC
RT SS
SENSE2
FBX
VC INTVCC
41.2k
300kHz
0.33μF 10k
4.7μF
10nF
4.7μF
s2
VOUT
48V
0.5A
464k
15.8k
3958 TA01a
Efficiency vs Output Current
100
VIN = 24V
95
90
85
80
75
70
0
100 200 300 400 500
OUTPUT CURRENT (mA)
3958 TA01b
3958f
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LT3958
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Dynamic Quiescent Current
vs Switching Frequency
12
10
8
6
4
2
0
100 200 300 400 500 600 700 800 900 1000
SWITCHING FREQUENCY (kHz)
3958 G04
Switching Frequency
vs Temperature
325
RT = 41.2k
320
315
310
305
300
295
290
285
280
275
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3958 G07
EN/UVLO Threshold
vs Temperature
1.28
RT vs Switching Frequency
1000
100
10
0 100 200 300 400 500 600 700 800 900 1000
SWITCHING FREQUENCY (kHz)
3958 G05
SW Pin Current Limit
vs Temperature
4.4
4.3
4.2
4.1
4.0
3.9
3.8
3.7
3.6
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3958 G08
EN/UVLO Current vs Voltage
50
1.26
EN/UVLO RISING
1.24
1.22 EN/UVLO FALLING
40
30
20
1.20 10
1.18
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3958 G10
0
0 20 40 60 80
EN/UVLO VOLTAGE (V)
3958 G11
Normalized Switching
Frequency vs FBX
120
100
80
60
40
20
0
–0.8 –0.4
0 0.4 0.8
FBX VOLTAGE (V)
1.2 1.6
3958 G06
SW Pin Current Limit
vs Duty Cycle
4.4
4.3
4.2
4.1
4.0
3.9
3.8
3.7
3.6
0
20 40 60
DUTY CYCLE (%)
80 100
3958 G09
EN/UVLO Hysteresis Current
vs Temperature
2.4
2.2
2.0
1.8
1.6
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3958 G12
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LT3958
APPLICATIONS INFORMATION
Operating Frequency and Synchronization
The choice of operating frequency may be determined by
on-chip power dissipation (a low switching frequency may
be required to ensure IC junction temperature does not
exceed 125°C), otherwise it is a trade-off between efficiency
and component size. Low frequency operation improves
efficiency by reducing gate drive current and MOSFET
and diode switching losses. However, lower frequency
operation requires a physically larger inductor. Switching
frequency also has implications for loop compensation.
The LT3958 uses a constant-frequency architecture that
can be programmed over a 100kHz to 1000kHz range
with a single external resistor from the RT pin to ground,
as shown in Figure 1. The RT pin must have an external
resistor to SGND for proper operation of the LT3958.
A table for selecting the value of RT for a given operating
frequency is shown in Table 1.
Table 1. Timing Resistor (RT) Value
SWITCHING FREQUENCY (kHz)
100
200
300
400
500
600
700
800
900
1000
RT (kΩ)
140
63.4
41.2
30.9
24.3
19.6
16.5
14
12.1
10.5
The operating frequency of the LT3958 can be synchro-
nized to an external clock source. By providing a digital
clock signal into the SYNC pin, the LT3958 will operate
at the SYNC clock frequency. The LT3958 detects the ris-
ing edge of each Sync clock cycle. If this feature is used,
an RT resistor should be chosen to program a switching
frequency 20% slower than SYNC pulse frequency. Tie
the SYNC pin to SGND if this feature is not used. It is
recommended that the Sync input clock has a minimum
pulse width of 200ns.
Duty Cycle Consideration
Switching duty cycle is a key variable defining converter
operation. As such, its limits must be considered. Minimum
on-time is the smallest time duration that the LT3958 is
capable of turning on the power MOSFET. This time is
generally about 250ns (typical) (see Minimum On-Time
in the Electrical Characteristics table). In each switching
cycle, the LT3958 keeps the power switch off for at least
200ns (typical) (see Minimum Off-Time in the Electrical
Characteristics table).
The minimum on-time, minimum off-time and the switching
frequency define the minimum and maximum switching
duty cycles a converter is able to generate:
Minimum duty cycle = minimum on-time • frequency
Maximum duty cycle = 1 – (minimum off-time • frequency)
Programming the Output Voltage
The output voltage VOUT is set by a resistor divider, as
shown in Figure 1. The positive and negative VOUT are set
by the following equations:
VOUT,
POSITIVE
=
1.6V
•
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
VOUT,
NEGATIVE
=
–0.8V
•
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
The resistors R1 and R2 are typically chosen so that
the error caused by the current flowing into the FBX pin
during normal operation is less than 1% (this translates
to a maximum value of R1 at about 158k).
3958f
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11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LT3958.PDF ] | |
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