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PDF LTC3854 Data sheet ( Hoja de datos )
| Número de pieza | LTC3854 | |
| Descripción | Wide VIN Range Synchronous Step-Down Controller | |
| Fabricantes | Linear Technology Corporation | |
| Logotipo |  |
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FEATURES
n Wide Operating VIN Range: 4.5V to 38V
n RSENSE or DCR Current Sensing
n ±1% 0.8V Reference Accuracy Over Temperature
n 400kHz Switching Frequency
n Dual N-channel MOSFET Synchronous Drive
n Very Low Dropout Operation: 97% Duty Cycle
n Starts Up Into Pre-Biased Output
n Adjustable Output Voltage Soft-Start
n Output Current Foldback Limiting
(Disabled During Soft-Start)
n Output Overvoltage Protection
n 5V LDO for External Gate Drive
n OPTI-LOOP® Compensation Minimizes COUT
n Low Shutdown IQ: 15μA
n Tiny 12-Pin 2mm × 3mm Thermally Enhanced DFN
and MSOP Packages
APPLICATIONS
n Automotive Systems
n Telecom Systems
n Industrial Equipment
n Distributed DC Power Systems
LTC3854
Small Footprint, Wide
VIN Range Synchronous
Step-Down Controller
DESCRIPTION
The LTC3854® is a high performance synchronous step-
down switching regulator controller that drives an all
N-channel synchronous power MOSFET stage.
The LTC3854 features a 400kHz constant frequency current
mode architecture. The LTC3854 operates from a 4.5V to
38V (40V absolute maximum) input voltage range and
regulates the output voltage from 0.8V to 5.5V.
The RUN/SS pin provides both soft-start and enable
features. OPTI-LOOP compensation allows the transient
response to be optimized over a wide range of output ca-
pacitance and ESR values. Current foldback limits MOSFET
dissipation during short circuit conditions. Current foldback
functions are disabled during soft-start.
The LTC3854 has a minimum on-time at 75ns, making it
well suited for high step-down ratios. The strong onboard
MOSFET drivers allow the use of high power external
MOSFETs to produce output currents up to 20A.
L, LT, LTC, LTM, Linear Technology, the Linear logo and OPTI-LOOP are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners. Protected by U.S. Patents, including 5705919, 6498466, 5408150, 6222231.
TYPICAL APPLICATION
High Efficiency Synchronous Step-Down Converter
0.1μF
2200pF
15k
8.06k
42.2k
VIN
TG
LTC3854
RUN/SS BOOST
ITH SW
100pF
INTVCC
FB
SENSE–
SENSE+
BG
GND
0.1μF
150μF
50V
VIN
6V TO 38V
4.7μF
6.0μH
AT 5.8mΩ DCR
0.1μF
10.2k
VOUT
5V
5A
330μF
s2
3854 TA01
Efficiency and Power Loss
vs Load Current
97.0
VIN = 6V
96.5
96.0
95.5
95.0
94.5
94.0
93.5
1.4
1.2
1.0
0.8
0.6
0.4
0.2
93.0 0
0.5 1.0 1.4 1.9 2.3 2.8 3.2 3.7 4.1 4.6 5
LOAD CURRENT (A)
3854 TA01b
3854f
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TYPICAL PERFORMANCE CHARACTERISTICS
LTC3854
Switching Waveforms at No Load
Switching Waveforms at High
Duty Cycle, No Load
VOUT = 5V
TG
BG
IL
1μs/DIV
3854 G06
VOUT = 2.5V
TG
BG
IL
2μs/DIV
3854 G07
Quiescent Current vs VIN
1.17
1.16
1.15
1.14
1.13
1.12
1.11
1.10
1.09
6
11 16 21 26 31
VIN (V)
36
3854 G08
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
60
58
56
54
52
50
48
46
44
42
40
01234567
COMMON MODE VOLTAGE (V)
3854 G11
Quiescent Current vs Temperature
1.30
1.25
1.20
1.15
1.10
1.05
1.00
0.95
0.90
–45 –25 –5 15 35 55 75
TEMPERATURE (°C)
95
3854 G09
Maximum Current Sense Threshold
vs Duty Cycle
65
60
55
50
45
40
35
0 20 40 60 80 100 120
DUTY CYCLE (%)
3854 G12
INTVCC vs Input Voltage
5.05
5.00
4.95
4.90
4.85
4.80
4
9 14 19 24 29 34 39
VIN (V)
3854 G10
Maximum Current Sense Voltage
vs Feedback Voltage
(Current Foldback)
60
50
40
30
20
10
0
0 0.05 0.15 0.25 0.35 0.45 0.55 0.65 0.75
FB (V)
3854 G13
3854f
5
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APPLICATIONS INFORMATION
VIN
CSS
CC
RC
RFB1
RFB2
VIN
RUN/SS
TG
ITH BOOST
CC2 LTC3854
SW
INTVCC
FB
SENSE–
BG
SENSE+
GND
M1
CB
CIN
DBOOST
CVINT
M2
INDUCTOR
L DCR
C1
R1
R2
LTC3854
VOUT
COUT
3854 F02
Figure 2. Buck Regulator Using DCR Current Sense
capacitor is equal to the voltage drop across the inductor
DCR • R2/(R1+R2). R2 may be used to scale the voltage
across the same terminals when the DCR is greater than
the target sense resistance. Check the manufacturer’s
datasheet for specifications regarding the inductor DCR, in
order to properly dimension the external filter components.
The DCR of the inductor can also be measured using a
precision RLC meter.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant-fre-
quency architectures by preventing subharmonic oscil-
lations at high duty cycles. It is accomplished internally
by adding a compensating ramp to the inductor current
signal. Normally, this results in a reduction of maximum
inductor peak current for high duty cycles. However, the
LTC3854 uses a novel scheme that allows the maximum
inductor peak current to remain unaffected throughout
all duty cycles.
Inductor Value Calculation
The inductor value has a direct effect on ripple current.
The inductor ripple current ΔIL decreases with higher
inductance or frequency and increases with higher VIN.
LMIN
=
ΔIL
1
• fSW
• VOUT
⎛
⎜1−
⎝
VOUT
VIN(MAX
)
⎞
⎟
⎠
Accepting larger values of ΔIL allows the use of low value
inductors, but results in a higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is IL = 0.4 • (IMAX). The maximum
ΔIL occurs at the maximum input voltage.
Option 1: DCR within desired range
R1•
C1=
L
DCR
(R2 not used)
Option 2: DCR > desired RSENSE
R1||R2 • C1 =
L
DCR
(at 20°C)
RSENSE
(EQ)
=
DCR(MAX)
•
R2
R1+ R2
3854f
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC3854.PDF ] | |
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