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PDF LT8494 Data sheet ( Hoja de datos )
| Número de pieza | LT8494 | |
| Descripción | SEPIC/Boost DC/DC Converter | |
| Fabricantes | Linear Technology Corporation | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT8494 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
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LT8494
SEPIC/Boost DC/DC
Converter with 2A, 70V Switch,
and 7µA Quiescent Current
FEATURES
nn Low Ripple Burst Mode® Operation:
7µA IQ at 12VIN to 5VOUT
Output Ripple (<10mV Typ.)
nn Dual Supply Pins:
Improves Efficiency
Reduces Minimum Supply Voltage to ~1V after
Start-Up to Extend Battery Life
nn Wide Input Voltage Range of ~1V to 60V (2.5V to
32V for Start-Up)
nn PG Functional for Input Supply Down to 1.3V
nn FMEA Fault Tolerant in TSSOP Package
nn Fixed Frequency PWM, SEPIC/BOOST/FLYBACK
Topologies
nn NPN Power Switch: 2A/70V
nn Programmable Switching Frequency: 250kHz to 1.5MHz
nn UVLO Programmable on SWEN Pin
nn Soft-Start Programmable with One Capacitor
nn Small 20-Lead QFN or 20-Lead TSSOP Packages
APPLICATIONS
nn Automotive ECU Power
nn Power for Portable Products
nn Industrial Supplies
L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective owners.
DESCRIPTION
TheLT®8494isanadjustablefrequency (250kHzto 1.5MHz)
monolithic switching regulator. Quiescent current can be
less than 7µA when operating and is ~0.3µA when SWEN
is low. The LT8494 can be configured as either a SEPIC,
boost or flyback converter.
The low ripple Burst Mode operation maintains high
efficiency at low output current while keeping output
ripple below 10mV. Dual supply pins (VIN and BIAS) allow
the part to automatically operate from the most efficient
supply. Input supply voltage can be up to 60V for SEPIC
topologies and up to 32V (with ride-through up to 60V)
for boost and flyback topologies. After start-up, battery
life is extended since the part can draw current from its
output (BIAS) even when VIN voltage drops below 2.5V.
Using a resistor divider on the SWEN pin provides a pro-
grammable undervoltage lockout (UVLO) for the converter.
A power good flag signals when VOUT reaches 92% of the
programmed output voltage.
Additional features such as frequency foldback and soft-
start are integrated. The LT8494 is available in 20-lead
QFN and 20-lead TSSOP packages with exposed pads
for low thermal resistance. Fault tolerance in the TSSOP
allows for adjacent pin shorts or an open without raising
the output voltage above its programmed value.
TYPICAL APPLICATION
VIN
3V TO 60V
450kHz, 5V Output SEPIC Converter
15µH •
2.2µF
15µH
VOUT
5V
0.35A (VIN = 3V)
0.6A (VIN = 5V)
1.0A (VIN > 12V)
4.7µF
VIN SW
BIAS
SWEN
PG LT8494
RT SS GND FB
169k
1µF
1M
316k
4.7pF
47µF
×2
8494 TA01a
No-Load Supply Current
20
15
10
5
0
0 12 24 36 48 60
INPUT VOLTAGE (V)
8494 TA01b
For more information www.linear.com/LT8494
90
85
80
75
70
65
60
0.0
Efficiency
VIN = 5V
VIN = 24V
VIN = 12V
0.2 0.4 0.6 0.8 1.0
LOAD CURRENT (A)
8494 TA01c
8494f
1
1 page  
LT8494
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise specified.
Transient Load Response, Load
Current is Stepped from 300mA
to 500mA
Switch Current Limit at 500kHz
3.0
Switch Current Limit at Minimum
Duty Cycle
3.0
VOUT
50mV/DIV
IL
0.5A/DIV
500µs/DIV
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
8494 G19
2.5
2.0
1.5
1.0
0.5
2.5
2.0
1.5
1.0
0.5
0.0
10 20 30 40 50 60 70 80 90
DUTY CYCLE (%)
8494 G06
0
–50 0
50 100 150
TEMPERATURE (°C)
8494 G07
Switch VCESAT
600
500
400
300
200
100
0
0 0.5 1.0 1.5 2.0
SWITCH CURRENT (A)
8494 G08
Feedback Voltage
1.23
1.22
1.21
1.20
1.19
1.18
–50
0 50 100
TEMPERATURE (°C)
150
8494 G09
Oscillator Frequency
1.5
RT = 68.1k
1.0
0.5
0.0
–50
RT = 324k
0 50 100
TEMPERATURE (°C)
150
8494 G11
Frequency Foldback
1200
1000
800
RT = 68.1k
600
400
RT = 324k
200
0
0 0.2 0.5 0.7 1.0 1.2
FB VOLTAGE (V)
8494 G12
Minimum Switch On-Time
140
120
100
80
60
40
20
0
–50 0
50 100 150
TEMPERATURE (°C)
8494 G13
For more information www.linear.com/LT8494
Minimum Switch Off-Time
180
160
140
120
100
80
60
40
20
0
–50 0
50 100
TEMPERATURE (°C)
150
8494 G14
8494f
5
5 Page 
LT8494
APPLICATIONS INFORMATION
where TP is the clock period and Minimum Switch Off-Time
(found in the Electrical Characteristics) is typically 70ns.
Conversely, the power NPNs (Q1 in the Block Diagram)
cannot remain off for 100% of each clock cycle, and will
turn on for a minimum time (Minimum Switch On-Time)
when in regulation. This Minimum Switch On-Time governs
the minimum allowable duty cycle given by:
DCMIN
=
Minimum
Switch
TP
On-Time
•
100%
where TP is the clock period and Minimum Switch On-Time
(found in the Electrical Characteristics) is typically 95ns.
The application should be designed such that the operating
duty cycle (DC) is between DCMIN and DCMAX. Normally,
DC rises with higher VOUT and lower VIN.
Duty cycle equations for both boost and SEPIC topologies
are given below, where VD is the diode forward voltage
drop and VCESAT is typically 340mV at 1.2A.
For the boost topology:
DC
≅
VOUT
VOUT +
– VIN + VD
VD – VCESAT
For the SEPIC topology:
DC
≅
VIN
+
VOUT
VOUT +
+ VD
VD –
VCESAT
The LT8494 can be used in configurations where the duty
cycle is higher than DCMAX, but it must be operated in the
discontinuous conduction mode or Burst Mode operation
so that the effective duty cycle is reduced.
Setting the Switching Frequency
The LT8494 uses a constant frequency PWM architecture
that can be programmed to switch from 250kHz to 1.5MHz
by using a resistor tied from the RT pin to ground. Table 1
shows the necessary RT values for various switching
frequencies.
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz)
RT VALUE (kΩ)
0.25 324
0.4 196
0.6 124
0.8 88.7
1.0 68.1
1.2 54.9
1.4 45.3
1.5 41.2
Inductor Selection
General Guidelines: The high frequency operation of the
LT8494 allows for the use of small surface mount inductors.
For high efficiency, choose inductors with high frequency
core material, such as ferrite, to reduce core losses. To
improve efficiency, choose inductors with more volume
for a given inductance. The inductor should have low DCR
(copper wire resistance) to reduce I2R losses, and must be
able to handle the peak inductor current without saturat-
ing. Note that in some applications, the current handling
requirements of the inductor can be lower, such as in the
SEPIC topology when using uncoupled inductors, where
each inductor only carries a fraction of the total switch
current. Molded chokes or chip inductors usually do not
have enough core area to support peak inductor currents
in the 2A to 3A range. To minimize radiated noise, use a
toroidal or shielded inductor. Note that the inductance of
shielded types will drop more as current increases, and
will saturate more easily.
Minimum Inductance: Although there can be a trade-off
with efficiency, it is often desirable to minimize board
space by choosing smaller inductors. When choosing
an inductor, there are two conditions that limit the mini-
mum inductance; (1) providing adequate load current,
and (2) avoidance of subharmonic oscillation. Choose
an inductance that is high enough to meet both of these
requirements.
For more information www.linear.com/LT8494
8494f
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT8494.PDF ] | |
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