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PDF LTC3727 Data sheet ( Hoja de datos )
| Número de pieza | LTC3727 | |
| Descripción | 2-Phase Synchronous Step-Down Switching Regulators | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC3727/LTC3727-1
High Efficiency, 2-Phase
Synchronous Step-Down Switching Regulators
FEATURES
■ Wide Output Voltage Range: 0.8V ≤ VOUT ≤ 14V
■ Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise
■ OPTI-LOOP® Compensation Minimizes COUT
■ ±1% Output Voltage Accuracy
■ Power Good Output Voltage Monitor
■ Phase-Lockable Fixed Frequency 250kHz to 550kHz
■ Latched Short-Circuit Shutdown (LTC3727 Only)
■ Dual N-Channel MOSFET Synchronous Drive
■ Wide VIN Range: 4V to 36V Operation
■ Very Low Dropout Operation: 99% Duty Cycle
■ Adjustable Soft-Start Current Ramping
■ Foldback Output Current Limiting
■ Output Overvoltage Protection
■ Low Shutdown IQ: 20μA
■ Selectable Constant Frequency or Burst Mode®
Operation
■ Small 28-Lead SSOP Package
■ LTC3727-1 Also Available in the 5mm × 5mm QFN
Package
U
APPLICATIO S
■ Telecom Systems
■ Automotive Systems
■ Battery-Operated Digital Devices
DESCRIPTIO
The LTC®3727/LTC3727-1 are high performance dual
step-down switching regulator controllers that drive all
N-channel synchronous power MOSFET stages. A con-
stant frequency current mode architecture allows phase-
lockable frequency of up to 550kHz. Power loss and noise
due to the ESR of the input capacitors are minimized by
operating the two controller output stages out of phase.
OPTI-LOOP compensation allows the transient response
to be optimized over a wide range of output capacitance and
ESR values. There is a precision 0.8V reference and a power
good output indicator. A wide 4V to 30V (36V maximum)
input supply range encompasses all battery chemistries.
A RUN/SS pin for each controller provides soft-start, and
on the LTC3727GN, optional timed, short-circuit shut-
down. Current foldback limits MOSFET heat dissipation
during short-circuit conditions when overcurrent latchoff
is disabled. Output overvoltage protection circuitry latches
on the bottom MOSFET until VOUT returns to normal. The
FCB mode pin can select among Burst Mode, constant
frequency mode and continuous inductor current mode or
regulate a secondary winding.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst
Mode 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
5481178, 5929620, 6177787, 6144194, 6100678, 5408150, 6580258, 6304066, 5705919.
TYPICAL APPLICATIO
8μH
0.015Ω
VOUT1
5V
5A
+
47μF
6V
SP
105k
1%
+
4.7μF
M1
0.1μF
M2
1000pF
220pF
20k
1% 15k
VIN PGOOD INTVCC
TG1 TG2
BOOST1
BOOST2
SW1
LTC3727/
BG1 LTC3727-1
SW2
BG2
PLLIN
SENSE1+
PGND
SENSE2+
SENSE1–
SENSE2–
VOSENSE1
VOSENSE2
ITH1 ITH2
RUN/SS1 SGND RUN/SS2
0.1μF
0.1μF
1μF
CERAMIC
0.1μF
M3
M4
VIN
22μF 18V TO 28V
50V
CERAMIC
15μH
1000pF
220pF
15k
280k
1%
20k
1%
0.015Ω
VOUT2
12V
4A
+ 56μF
15V
SP
M1, M2, M3, M4: FDS6680A
Figure 1. High Efficiency Dual 12V/5V Step-Down Converter
3727 F01
3727fc
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC3727/LTC3727-1
Supply Current vs Input Voltage
and Mode (Figure 13)
1000
800
BOTH
CONTROLLERS ON
600
400
200
0
0
SHUTDOWN
10 20
INPUT VOLTAGE (V)
30
3727 G04
Maximum Current Sense Threshold
vs Duty Factor
150
125
100
75
50
25
0
0 20 40 60 80 100
DUTY FACTOR (%)
3727 G07
Current Sense Threshold
vs ITH Voltage
150
125
100
75
50
25
0
–25
–50
0
0.5 1.0 1.5 2.0 2.5
VITH (V)
3727 G10
EXTVCC Voltage Drop
160
VEXTVCC = 8.5V
140
120
100
80
60
40
20
0
0 10 20 30
CURRENT (mA)
40 50
3727 G05
Maximum Current Sense Threshold
vs Percent of Nominal Output
Voltage (Foldback)
150
135
120
105
90
75
60
45
30
15
0
0 20 40 60 80 100
PERCENT OF NOMINAL OUTPUT VOLTAGE (%)
3727 G08
Load Regulation
0.0
–0.1
FCB = 0V
VIN = 15V
FIGURE 1
–0.2
–0.3
–0.4
0
1 2 34
LOAD CURRENT (A)
5
3727 G11
Internal 7.5V LDO Line Regulation
7.7
ILOAD = 1mA
7.6
7.5
7.4
7.3
7.2
7.1
7.0
6.9
6.8
0
5 10 15 20 25 30 35
INPUT VOLTAGE (V)
3727 G06
Maximum Current Sense Threshold
vs VRUN/SS (Soft-Start)
150
VSENSE(CM) = 1.6V
125
100
75
50
01 2 34 5 6
VRUN/SS (V)
3727 G09
VITH vs VRUN/SS
2.5
VOSENSE = 0.7V
2.0
1.5
1.0
0.5
0
012 34
VRUN/SS (V)
56
3727 G12
3727fc
5
5 Page 
U
OPERATIO (Refer to Functional Diagram)
Power Good (PGOOD) Pin
The PGOOD pin is connected to an open drain of an internal
MOSFET. The MOSFET turns on and pulls the pin low when
either output is not within ±7.5% of the nominal output
level as determined by the resistive feedback divider.
When both outputs meet the ±7.5% requirement, the
MOSFET is turned off within 10μs and the pin is allowed to
be pulled up by an external resistor to a source of up to 7V.
Foldback Current, Short-Circuit Detection
and Short-Circuit Latchoff (LTC3727 Only)
The RUN/SS capacitors are used initially to limit the inrush
current of each switching regulator. After the controller
has been started and been given adequate time to charge
up the output capacitors and provide full load current, the
RUN/SS capacitor is used in a short-circuit time-out
circuit. If the output voltage falls to less than 70% of its
nominal output voltage, the RUN/SS capacitor begins
discharging on the assumption that the output is in an
overcurrent and/or short-circuit condition. If the condi-
tion lasts for a long enough period as determined by the
size of the RUN/SS capacitor, the controller will be shut
down until the RUN/SS pin(s) voltage(s) are recycled.
This built-in latchoff can be overridden by providing a
>5μA pull-up at a compliance of 5V to the RUN/SS pin(s).
This current shortens the soft start period but also pre-
vents net discharge of the RUN/SS capacitor(s) during an
overcurrent and/or short-circuit condition. Foldback cur-
rent limiting is also activated when the output voltage falls
below 70% of its nominal level whether or not the short-
circuit latchoff circuit is enabled. Even if a short is present
and the short-circuit latchoff is not enabled, a safe, low
output current is provided due to internal current foldback
and actual power wasted is low due to the efficient nature
of the current mode switching regulator.
PART NUMBER
LTC3727
LTC3727-1
FUNCTION
With Latchoff Function Available
Latchoff Always Disabled
LTC3727/LTC3727-1
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC3727 dual high efficiency DC/DC controller brings
the considerable benefits of 2-phase operation to portable
applications. Notebook computers, PDAs, handheld ter-
minals and automotive electronics will all benefit from the
lower input filtering requirement, reduced electromag-
netic interference (EMI) and increased efficiency associ-
ated with 2-phase operation.
Why the need for 2-phase operation? Until recently, con-
stant-frequency dual switching regulators operated both
channels in phase (i.e., single-phase operation). This
means that both switches turned on at the same time,
causing current pulses of up to twice the amplitude of
those for one regulator to be drawn from the input capaci-
tor and battery. These large amplitude current pulses
increased the total RMS current flowing from the input
capacitor, requiring the use of more expensive input
capacitors and increasing both EMI and losses in the input
capacitor and battery.
With 2-phase operation, the two channels of the dual-
switching regulator are operated 180 degrees out of
phase. This effectively interleaves the current pulses
drawn by the switches, greatly reducing the overlap time
where they add together. The result is a significant reduc-
tion in total RMS input current, which in turn allows less
expensive input capacitors to be used, reduces shielding
requirements for EMI and improves real world operating
efficiency.
Figure 3 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the new
LTC3727 2-phase dual switching regulator. An actual
measurement of the RMS input current under these con-
ditions shows that 2-phase operation dropped the input
current from 2.53ARMS to 1.55ARMS. While this is an
impressive reduction in itself, remember that the power
losses are proportional to IRMS2, meaning that the actual
power wasted is reduced by a factor of 2.66. The reduced
input ripple voltage also means less power is lost in the
3727fc
11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3727.PDF ] | |
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