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PDF LTC3728 Data sheet ( Hoja de datos )
| Número de pieza | LTC3728 | |
| Descripción | 2-Phase Synchronous Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
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LTC3728www.DataSheet4U.com
Dual, 550kHz, 2-Phase
Synchronous Step-Down Switching Regulator
FEATURES
DESCRIPTIO
■ Dual, 180° Phased Controllers Reduce Required
Input Capacitance and Power Supply Induced Noise
■ OPTI-LOOP® Compensation Minimizes COUT
■ ±1% Output Voltage Accuracy
■ Power Good Output Voltage Indicator
■ Phase-Lockable Fixed Frequency 250kHz to 550kHz
■ Dual N-Channel MOSFET Synchronous Drive
■ Wide VIN Range: 3.5V to 36V Operation
■ Very Low Dropout Operation: 99% Duty Cycle
■ Adjustable Soft-Start Current Ramping
■ Foldback Output Current Limiting
■ Latched Short-Circuit Shutdown with Defeat Option
■ Output Overvoltage Protection
■ Remote Output Voltage Sense
■ Low Shutdown IQ: 20µA
■ 5V and 3.3V Regulators
■ 3 Selectable Operating Modes: Constant Frequency,
Burst Mode® Operation and PWM
■ Available in 32-Pin 5mm × 5mm QFN and
28-Pin SSOP Packages
U
APPLICATIO S
■ Notebook and Palmtop Computers
■ Telecom Systems
■ Portable Instruments
■ Battery-Operated Digital Devices
■ DC Power Distribution Systems
The LTC®3728 is a dual high performance step-down
switching regulator controller that drives all N-channel
synchronous power MOSFET stages. A constant fre-
quency 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. The precision 0.8V reference and power good
output indicator are compatible with future microproces-
sor generations, and a wide 3.5V to 30V (36V maximum)
input supply range encompasses all battery chemistries.
A RUN/SS pin for each controller provides both soft-start
and optional timed, short-circuit shutdown. Current
foldback limits MOSFET 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. The LTC3728 includes a power good
output pin that indicates when both outputs are within
7.5% of their designed set point.
, 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
M1
L1
3.2µH
+
4.7µF
D3
VIN PGOOD INTVCC
TG1 TG2
D4
CB1, 0.1µF
fIN
500kHz
BOOST1
BOOST2
SW1
LTC3728
BG1
SW2
BG2
PLLIN
PGND
CB2, 0.1µF
1µF
VIN
CIN 5.2V TO 28V
22µF
M2 50V
L2
3.2µH
VOUT1
5V
5A
RSENSE1
0.01Ω
R2
105k
+
COUT1 1%
47µF
6V
SP
R1
20k
1%
1000pF
CC1
220pF
RC1
15k
SENSE1+
SENSE2+
SENSE1–
SENSE2–
VOSENSE1
ITH1
VOSENSE2
ITH2
RUN/SS1 SGND RUN/SS2
CSS1
0.1µF
CSS2
0.1µF
1000pF
CC2
220pF
RC2
15k
RSENSE2
0.01Ω
VOUT2
R4 3.3V
63.4k
5A
R3
20k
1%
1% COUT +
56µF
6V
SP
M1, M2: FDS6982S
Figure 1. High Efficiency Dual 5V/3.3V Step-Down Converter
3728 F01
3728fb
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Input Voltage
and Mode (Figure 13)
1000
800
600
BOTH
CONTROLLERS ON
400
200
SHUTDOWN
0
0 5 10 15 20 25 30 35
INPUT VOLTAGE (V)
3728 G04
Internal 5V LDO Line Regulation
5.1
ILOAD = 1mA
5.0
4.9
4.8
4.7
4.6
4.5
4.4
0
5 10 15 20 25 30 35
INPUT VOLTAGE (V)
3728 G07
Maximum Current Sense Threshold
vs VRUN/SS (Soft-Start)
80
VSENSE(CM) = 1.6V
60
40
20
0
01
2 34
VRUN/SS (V)
56
3728 G10
EXTVCC Voltage Drop
250
200
150
100
50
0
0 10 20 30 40 50
CURRENT (mA)
3728 G05
Maximum Current Sense Threshold
vs Duty Factor
75
50
25
0
0 20 40 60 80 100
DUTY FACTOR (%)
3728 G08
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
80
76
72
68
64
60
0
1 2 34
COMMON MODE VOLTAGE (V)
5
3728 G11
LTC3728www.DataSheet4U.com
INTVCC and EXTVCC Switch
Voltage vs Temperature
5.05
INTVCC VOLTAGE
5.00
4.95
4.90
4.85
4.80
EXTVCC SWITCHOVER THRESHOLD
4.75
4.70
– 50 – 25
0 25 50 75
TEMPERATURE (°C)
100 125
3728 G06
Maximum Current Sense Threshold
vs Percent of Nominal Output
Voltage (Foldback)
80
70
60
50
40
30
20
10
0
0 25 50 75 100
PERCENT ON NOMINAL OUTPUT VOLTAGE (%)
3728 G09
Current Sense Threshold
vs ITH Voltage
90
80
70
60
50
40
30
20
10
0
–10
–20
–30
0 0.5 1 1.5 2 2.5
VITH (V)
3728 G12
3728fb
5
5 Page 
U
OPERATIO (Refer to Functional Diagram)
INTVCC/EXTVCC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTVCC pin.
When the EXTVCC pin is left open, an internal 5V low
dropout linear regulator supplies INTVCC power. If EXTVCC
is taken above 4.7V, the 5V regulator is turned off and an
internal switch is turned on connecting EXTVCC to INTVCC.
This allows the INTVCC power to be derived from a high
efficiency external source such as the output of the regu-
lator itself or a secondary winding, as described in the
Applications Information section.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>7.5%) as well as other more serious condi-
tions that may overvoltage the output. In this case, the top
MOSFET is turned off and the bottom MOSFET is turned on
until the overvoltage condition is cleared.
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
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.
LTC3728www.DataSheet4U.com
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.
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC1628 and the LTC3728 dual high efficiency DC/DC
controllers bring the considerable benefits of 2-phase
operation to portable applications for the first time. Note-
book computers, PDAs, handheld terminals and automo-
tive electronics will all benefit from the lower input filtering
requirement, reduced electromagnetic interference (EMI)
and increased efficiency associated with 2-phase opera-
tion.
Why the need for 2-phase operation? Up until the 2-phase
family, constant-frequency dual switching regulators op-
erated both channels in phase (i.e., single-phase opera-
tion). 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
capacitor 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.
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11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3728.PDF ] | |
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