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PDF LT8471 Data sheet ( Hoja de datos )
| Número de pieza | LT8471 | |
| Descripción | Dual Multitopology DC/DC Converters | |
| Fabricantes | Linear Technology Corporation | |
| Logotipo |  |
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LT8471
Dual Multitopology
DC/DC Converters with 2A
Switches and Synchronization
Features
n Dual 2A and One 500mA, 50V Internal Power
Switch Channels
n 2A Primary Channels Can Be Buck, Boost, SEPIC,
ZETA, Flyback or Inverting DC/DC Converter
n 500mA Skyhook Channel Efficiently Generates
Boosted Input Voltage
n Wide Input Voltage Range of 2.6V to 50V
n UVLO and OVLO Programmable on OV/UV Pin
n Soft-Start Programmable for Each Channel
n Fixed Frequency PWM (Set by RT Pin or
Synchronized to External Clock)
n Anti-Phase Switching Reduces Input Ripple
n 20-Lead TSSOP Package
Applications
n Dual Rail Power for Signal Chain.
n Buck/Buck, Buck/Boost, Boost/Boost, Boost/Invert,
Invert/Invert, Buck/Invert
Description
The LT®8471 is a dual PWM DC/DC converter containing
two internal 2A, 50V switches and an additional 500mA
switch to facilitate step-down and inverting conversion.
Each 2A channel can be independently configured as a
buck, boost, SEPIC, flyback or inverting converter. Ca-
pable of generating positive and negative outputs from a
single input rail, the LT8471 is ideal for many local power
supply designs.
The LT8471 has an adjustable oscillator, set by a resistor
placed from the RT pin to ground. Additionally, the LT8471
can be synchronized to an external clock. The free running
or synchronized switching frequency range of the part can
be set between 100kHz and 2MHz.
Additional features such as frequency foldback, soft-start,
and power good are integrated. The LT8471 is available
in a 20-lead TSSOP package.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
Typical Application
6V to 32V to ±5V, Dual DC/DC Converter
6V TO 32V
2.2µF
1µF
1µF
15µH
475k
100k
100k
0.1µF
VIN1
VIN2
SHOUT
LT8471
C3
C1
FB1
E1
OV/UV
PG1
PG2
SS1
SS2
SYNC
GND
C2
FB2
E2
RT
0.1µF
187k
2.2µF
10µH
2.2µF
316k
59k
59k
316k
10µH
8471 TA01a
VOUT1
5V
1.5A
47µF
×2
47µF
×2
VOUT2
–5V
0.65A
For more information www.linear.com/8471
Efficiency and Power Loss
Load from VOUT1 to VOUT2
90
4
80
70
EFFICIENCY
3
60
50 POWER LOSS
2
40
30
20
VCC = 6V
1
10 18V
32V
00
0 0.2 0.4 0.6 0.8 1.0
LOAD CURRENT (A)
8471 F10b
8471fb
1
1 page  
LT8471
Typical Performance Characteristics TA = 25°C, unless otherwise specified.
Primary Switch Current Limit
at 500kHz
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0 20 40 60 80 100
DUTY CYCLE (%)
8471 G01
CH1, CH2 Switch VCESAT
0.6
0.5
0.4
0.3
0.2
0.1
0
0 0.5 1 1.5 2 2.5
SWITCH CURRENT (A)
8471 G02
Skyhook Switch VCESAT
0.5
0.4
0.3
0.2
0.1
0
0 0.1 0.2 0.3 0.4 0.5 0.6
SWITCH CURRENT (A)
8471 G03
Switch Current Limit
at Minimum Duty Cycle
3.0
2.5
2.0
1.5
1.0
0.5
0
–50 –30 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
8471 G04
Skyhook Current Limit
vs Temperature
1.0
Feedback Voltages
805
–805
795 –795
0.8 + FB
785 – FB
–785
0.5
775 –775
0.3 765 –765
0
–50 –30 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
8471 G05
755
–50 –25
–755
0 25 50 75 100 125 150
TEMPERATURE (°C)
8471 G06
Oscillator Frequency
1.2
1.0 RT = 84.5k
0.8
0.6
0.4
0.2 RT = 732k
0
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
8471 G07
Switching Frequency
During Soft-Start
1.00
0.75
0.50
0.25
0
–0.8 –0.4 0 0.4 0.8
FB VOLTAGE (V)
8471 G08
Internal UVLO for VIN1 and VIN2
2.5
2.4
VIN1
2.3
VIN2
2.2
2.1
2.0
–50 –30 –10 10 30 50 70 90 110 130 150
TEMPERATURE (°C)
8471 G09
For more information www.linear.com/8471
8471fb
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5 Page 
LT8471
Applications Information
Internal Undervoltage Lockouts
The LT8471 monitors VIN1 and VIN2 supply voltages in case
either drops below a minimum operating level (typically
about 2.35V and 2.25V, respectively).
When VIN1 is detected low, all power switches are de-
activated, and while sufficient VIN1 voltage persists, the
soft-start capacitors for both SS1 and SS2 are discharged.
After VIN1 is detected high, the channel 1 power switch is
re-enabled and SS1 begins charging.
When VIN2 is detected low, the channel 2 power switch is
deactivated, and while sufficient VIN1 voltage persists, the
soft-start capacitor for SS2 is discharged. After both VIN1
and VIN2 are detected high, the channel 2 power switch is
re-enabled and SS2 begins charging.
Driving SYNC high for an extended period of time effectively
stops the operating clock and prevents latches SR11 and
SR12 from becoming set (see the Block Diagram). As a
result, the switching operation of the LT8471 stops, and
all the power switches are turned off.
The duty cycle of the SYNC signal must be between 35%
and 65% for proper operation. Also, the frequency of the
SYNC signal must meet the following two criteria:
1. SYNC may not toggle outside the frequency range of
100kHz to 2MHz unless it is set low to enable the free-
running oscillator.
2. The SYNC frequency can always be higher than the
free-running oscillator frequency, fOSC, but should not
be less than 25% below fOSC.
Oscillator
The internal free-running oscillator can set the operating
frequency of the LT8471. When the SYNC pin is driven low
(< 0.4V), the frequency of operation is set by a resistor
from RT to ground. An internally trimmed timing capacitor
resides inside the IC. The oscillator frequency is calculated
using the following formula:
fOSC
=
85.5
RT + 1
where fOSC is in MHz and RT is in kΩ. Conversely, RT
(in kΩ) can be calculated from the desired frequency (in
MHz) using:
RT
=
85.5
fOSC
–1
Operating Frequency Selection
There are several considerations in selecting the operat-
ing frequency of the converter. The first is staying clear
of sensitive frequency bands, which cannot tolerate any
spectral noise. For example, in products incorporating RF
communications, the 455kHz IF frequency is sensitive to
any noise, therefore switching above 600kHz is desired.
Some communications have sensitivity to 1.1MHz, and in
that case, a 1.5MHz switching converter frequency may be
employed. The second consideration is the physical size
of the converter. As the operating frequency goes up, the
inductor and filter capacitors go down in value and size.
The trade-off is efficiency, since the switching losses due to
NPN base charge (see the Power and Thermal Calculation
section), Schottky diode charge, and other capacitive loss
terms increase proportionally with frequency.
Clock Synchronization
The operating frequency of the LT8471 can be synchro-
nized to an external clock source. To synchronize to the
external source, simply provide a digital clock signal into
the SYNC pin. The LT8471 will operate at the SYNC clock
frequency. The LT8471 will revert to the internal free-
running oscillator clock after SYNC is driven low for a few
free-running clock cycles.
Soft-Start
The LT8471 contains soft-start circuitry to limit peak
switch currents during start-up. High start-up current is
inherent in switching regulators since the feedback loop
is saturated due to VOUT being far from its final value. The
regulator tries to charge the output capacitor as quickly as
possible, which results in large peak currents.
The start-up current can be limited by connecting external
capacitors (typically 100nF to 1μF) to the SS1 and SS2 pins.
The capacitors are slowly charged to ~2.15V by internal
For more information www.linear.com/8471
8471fb
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| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LT8471.PDF ] | |
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