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PDF LT8645S Data sheet ( Hoja de datos )
| Número de pieza | LT8645S | |
| Descripción | 8A Synchronous Step-Down Silent Switcher 2 | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT8645S
65V, 8A Synchronous
Step-Down Silent Switcher 2
with 2.5µA Quiescent Current
Features
nn Silent Switcher® 2 Architecture
nn Ultralow EMI/EMC Emissions on Any PCB
nn Eliminates PCB Layout Sensitivity
nn Internal Bypass Capacitors Reduce Radiated EMI
nn Optional Spread Spectrum Modulation
nn High Efficiency at High Frequency
nn Up to 95% Efficiency at 1MHz, 12VIN to 5VOUT
nn Up to 94% Efficiency at 2MHz, 12VIN to 5VOUT
nn Wide Input Voltage Range: 3.4V to 65V
nn Ultralow Quiescent Current Burst Mode® Operation
nn 2.5μA IQ Regulating 12VIN to 3.3VOUT
nn Output Ripple < 10mVP-P
nn Fast Minimum Switch On-Time: 40ns
nn Low Dropout Under All Conditions: 60mV at 1A
nn Safely Tolerates Inductor Saturation in Overload
nn Adjustable and Synchronizable: 200kHz to 2.2MHz
nn Peak Current Mode Operation
nn Output Soft-Start and Tracking
nn Small 32-Lead 6mm × 4mm LQFN Package
Applications
nn Automotive and Industrial Supplies
nn General Purpose Step-Down
nn GSM Power Supplies
Description
The LT®8645S synchronous step-down regulator features
second generation Silent Switcher architecture designed
to minimize EMI/EMC emissions while delivering high ef-
ficiency at high switching frequencies. This includes the
integration of bypass capacitors to optimize all the fast
current loops inside and make it easy to achieve advertised
EMI performance by eliminating layout sensitivity. This
performance makes the LT8645S ideal for noise sensitive
applications and environments.
The fast, clean, low-overshoot switching edges enable high
efficiency operation even at high switching frequencies,
leading to a small overall solution size. Peak current mode
control with a 40ns minimum on-time allows high step
down ratios even at high switching frequencies.
Burst Mode operation enables ultralow standby current
consumption, pulse-skipping mode allows full switching
frequency at lower output loads, or spread spectrum op-
eration can further reduce EMI/EMC emissions. Soft-start
and tracking functionality is accessed via the TR/SS pin,
and an accurate input voltage UVLO threshold can be set
using the EN/UV pin. A CLKOUT pin enables synchronizing
other regulators to the LT8645S.
L, LT, LTC, LTM, Linear Technology, the Linear logo, Silent Switcher and Burst Mode are
registered trademarks of Analog Devices, Inc. All other trademarks are the property of their
respective owners.
Typical Application
5V 8A Step-Down Converter
VIN
5.5V TO 65V
4.7µF
VIN SW
EN/UV
BIAS
LT8645S
2.2µH
2.2pF
RT FB
41.2k
GND
fSW = 1MHz
VOUT
5V
8A
1M
100µF
243k
8645S TA01a
12VIN to 5VOUT Efficiency
100 4.0
95 3.5
90 EFFICIENCY
3.0
85 2.5
80 2.0
75
POWER LOSS
70
1.5
1.0
65
60
1
1MHz, L = 2.2µH 0.5
2MHz, L = 1µH
0
2345678
LOAD CURRENT (A)
8645S TA01b
For more information www.linear.com/LT8645S
8645Sf
1
1 page  
Typical Performance Characteristics
LT8645S
Reference Voltage
979
977
975
973
971
969
967
965
963
961
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
8645S G09
Line Regulation
0.20
0.15
0.10
0.05
0.00
–0.05
–0.10
5
VOUT = 5V
ILOAD = 2A
15 25 35 45 55 65
INPUT VOLTAGE (V)
8645S G12
EN Pin Thresholds
1.03
1.02
1.01 EN RISING
1.00
0.99
0.98
0.97 EN FALLING
0.96
0.95
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
8645S G10
No-Load Supply Current
4.5
VOUT = 3.3V
4.0
L = 4.7µH
IN-REGULATION
3.5
3.0
2.5
2.0
1.5
1.0
0
10 20 30 40 50 60
INPUT VOLTAGE (V)
8645S G13
Load Regulation
0.20
0.15
0.10
0.05
0
–0.05
VOUT = 5V
–0.10
VIN = 12V
012345678
LOAD CURRENT (A)
8645S G11
Top FET Current Limit
vs Duty Cycle
15.0
14.5
14.0
13.5
13.0
12.5
12.0
11.5
11.0
0.1
0.3 0.5 0.7 0.9
DUTY CYCLE
8645S G14
Top FET Current Limit
16
15
5% DC
14
13
12
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
8645S G15
Switch Drop
100
SWITCH CURRENT = 1A
80
60
40 TOP SWITCH
20 BOTTOM SWITCH
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
8645S G16
For more information www.linear.com/LT8645S
Switch Drop
350
300
250
200 TOP SWITCH
150
100
BOTTOM SWITCH
50
0
0 1234567
SWITCH CURRENT (A)
8645S G17
8645Sf
5
5 Page 
LT8645S
Pin Functions
TR/SS (Pin 29): Output Tracking and Soft-Start Pin. This
pin allows user control of output voltage ramp rate during
start-up. A TR/SS voltage below 0.97V forces the LT8645S
to regulate the FB pin to equal the TR/SS pin voltage. When
TR/SS is above 0.97V, the tracking function is disabled
and the internal reference resumes control of the error
amplifier. An internal 1.9μA pull-up current from INTVCC on
this pin allows a capacitor to program output voltage slew
rate. This pin is pulled to ground with an internal 200Ω
MOSFET during shutdown and fault conditions; use a series
resistor if driving from a low impedance output. This pin
may be left floating if the tracking function is not needed.
GND (Pin 30): Ground. Connect this pin to system ground
and to the ground plane.
PG (Pin 31): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within ±8% of the final regulation voltage, and there are
no fault conditions. PG is pulled low when EN/UV is below
1V, INTVCC has fallen too low, VIN is too low, or thermal
shutdown. PG is valid when VIN is above 3.4V.
FB (Pin 32): The LT8645S regulates the FB pin to 0.97V.
Connect the feedback resistor divider tap to this pin. Also,
connect a phase lead capacitor between FB and VOUT.
Typically, this capacitor is 1pF to 10pF.
Corner Pins: These pins are for mechanical support only
and can be tied anywhere on the PCB, typically ground
or N/C.
Block Diagram
VIN
CIN3
R3
OPT
R4
OPT
COPT1
VIN
4-6
1V
25 EN/UV
CIN1
20nF
+
–
INTERNAL 0.97V REF
SHDN
31 PG
VOUT
ERROR
±8% AMP
++–
C1 R1
R2 32 FB
CSS
OPT
29 TR/SS
RT 26 RT
SHDN
THERMAL SHDN
IVNINTVUCVCLUOVLO
1.9µA
INTVCC
–
+
SLOPE COMP
3.4V
REG
OSCILLATOR
200kHz TO 2.2MHz
VC BURST SWITCH
DETECT
LOGIC
AND
ANTI-
SHOOT
THROUGH
SHDN
THERMAL SHDN
VIN UVLO
28 SYNC/MODE
60k
600k
GND
30, 33-38
CIN2
20nF
VIN
21-23
13
COPT2
BIAS 1
CVCC
2.2µF
INTVCC 2
BST 11
CBST
M1 0.22µF
SW
12-16
M2
L
COUT
VOUT
GND
8-10
17-19
CLKOUT 27
8645s BD
For more information www.linear.com/LT8645S
8645Sf
11
11 Page | ||
| Páginas | Total 26 Páginas | |
| PDF Descargar | [ Datasheet LT8645S.PDF ] | |
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