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PDF LTC3729EG Data sheet ( Hoja de datos )
| Número de pieza | LTC3729EG | |
| Descripción | 550kHz/ PolyPhase/ High Efficiency/ Synchronous Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
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LTC3729
FEATURES
550kHz, PolyPhase,
High Efficiency, Synchronous
Step-Down Switching Regulator
DESCRIPTIO
s Wide VIN Range: 4V to 36V Operation
s Reduces Required Input Capacitance and Power
Supply Induced Noise
s ±1% Output Voltage Accuracy
s Phase-Lockable Fixed Frequency: 250kHz to 550kHz
s True Remote Sensing Differential Amplifier
s PolyPhaseTM Extends from Two to Twelve Phases
s Reduces the Size and Value of Inductors
s Current Mode Control Ensures Current Sharing
s 1.1MHz Effective Switching Frequency (2-Phase)
s OPTI-LOOP® Compensation Reduces COUT
s Power Good Output Voltage Indicator
s Very Low Dropout Operation: 99% Duty Cycle
s Adjustable Soft-Start Current Ramping
s Internal Current Foldback Plus Shutdown Timer
s Overvoltage Soft-Latch Eliminates Nuisance Trips
s Available in 5mm × 5mm QFN
and 28-Lead SSOP Packages
U
APPLICATIO S
s Desktop Computers/Servers
s Large Memory Arrays
s DC Power Distribution Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
OPTI-LOOP is a registered trademark of Linear Technology Corporation.
PolyPhase is a trademark of Linear Technology Corporation.
The LTC®3729 is a multiple phase, synchronous step-
down current mode switching regulator controller that
drives N-channel external power MOSFET stages in a
phase-lockable fixed frequency architecture. The PolyPhase
controller drives its two output stages out of phase at
frequencies up to 550kHz to minimize the RMS ripple
currents in both input and output capacitors. The output
clock signal allows expansion for up to 12 evenly phased
controllers for systems requiring 15A to 200A of output
current. The multiple phase technique effectively multi-
plies the fundamental frequency by the number of chan-
nels used, improving transient response while operating
each channel at an optimum frequency for efficiency.
Thermal design is also simplified.
An internal differential amplifier provides true remote
sensing of the regulated supply’s positive and negative
output terminals as required for high current applications.
A RUN/SS pin provides both soft-start and optional timed,
short-circuit shutdown. Current foldback limits MOSFET
dissipation during short-circuit conditions when the
overcurrent latchoff is disabled. OPTI-LOOP compensa-
tion allows the transient response to be optimized over a
wide range of output capacitance and ESR values. The
LTC3729 includes a power good output pin that indicates
when the output is within ±7.5% of the designed set point.
TYPICAL APPLICATIO
3.3k
S
S
16k
0.1µF
S
0.1µF
1000pF
16k
VIN LTC3729 TG1
BOOST1
RUN/SS
SW1
BG1
PGOOD
ITH
PGND
SENSE1+
SENSE1–
TG2
SGND
BOOST2
SW2
VDIFFOUT
EAIN
VOS–
VOS+
BG2
INTVCC
SENSE2+
SENSE2–
10Ω
S
0.47µF
S
S
S
0.47µF
S
S
10µF
M1
0.002Ω
M2
×2
D1
L1
0.8µH
M3
0.002Ω
L2
M4 0.8µH
×2 D2
COUT: T510E108K004AS
D1, D2: UP5840
L1, L2: CEPH149-IROMC
M1, M3: IRF7811W
M2, M4: IRF7822
Figure 1. High Current Dual Phase Step-Down Converter
10µF
35V
CERAMIC
×4
VIN
5V TO 28V
VOUT
1.6V/40A
+ COUT
1000µF ×2
4V
3729 TA01
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1 page  
LTC3729
TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Input Voltage
and Mode
1000
800
600
ON
400
200
SHUTDOWN
0
0 5 10 15 20 25 30 35
INPUT VOLTAGE (V)
3729 G04
Internal 5V LDO Line Reg
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)
3729 G07
Maximum Current Sense Threshold
vs VRUN/SS (Soft-Start)
80
VSENSE(CM) = 1.6V
60
40
20
0
01 2 34 5 6
VRUN/SS (V)
3729 G10
EXTVCC Voltage Drop
250
200
150
100
50
0
0 10 20 30 40 50
CURRENT (mA)
3729 G05
Maximum Current Sense Threshold
vs Duty Factor
75
50
25
0
0 20 40 60 80 100
DUTY FACTOR (%)
3729 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
3729 G11
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
3729 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 (%)
3729 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)
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OPERATIO (Refer to Functional Diagram)
INTVCC/EXTVCC Power
Power for the top and bottom MOSFET drivers and most
of the IC circuitry is derived from INTVCC. When the
EXTVCC pin is left open, an internal 5V low dropout
regulator supplies INTVCC power. If the EXTVCC pin 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. An external Schottky
diode can be used to minimize the voltage drop from
EXTVCC to INTVCC in applications requiring greater than
the specified INTVCC current. Voltages up to 7V can be
applied to EXTVCC for additional gate drive capability.
Differential Amplifier
This amplifier provides true differential output voltage
sensing. Sensing both VOUT+ and VOUT – benefits regula-
tion in high current applications and/or applications hav-
ing electrical interconnection losses.
Power Good (PGOOD)
The PGOOD pin is connected to the drain of an internal
MOSFET. The MOSFET turns on when the output is not
within ±7.5% of its nominal output level as determined by
LTC3729
the feedback divider. When the output is within ±7.5% of
its nominal value, the MOSFET is turned off within 10µs
and the PGOOD pin should be pulled up by an external
resistor to a source of up to 7V.
Short-Circuit Detection
The RUN/SS capacitor is used initially to limit the inrush
current from the input power source. Once the controllers
have been given time, as determined by the capacitor on
the RUN/SS pin, to charge up the output capacitors and
provide full load current, the RUN/SS capacitor is then
used as a short-circuit timeout circuit. If the output voltage
falls to less than 70% of its nominal output voltage the
RUN/SS capacitor begins discharging assuming that the
output is in a severe overcurrent and/or short-circuit
condition. If the condition 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 voltage
is recycled. This built-in latchoff can be overidden by
providing a >5µA pull-up current at a compliance of 5V to
the RUN/SS pin. This current shortens the soft-start
period but also prevents net discharge of the RUN/SS
capacitor during a severe overcurrent and/or short-circuit
condition. Foldback current limiting is activated when the
output voltage falls below 70% of its nominal level whether
or not the short-circuit latchoff circuit is enabled.
APPLICATIO S I FOR ATIO
The basic LTC3729 application circuit is shown in Figure␣ 1
on the first page. External component selection is driven
by the load requirement, and begins with the selection of
RSENSE1, 2. Once RSENSE1, 2 are known, L1 and L2 can be
chosen. Next, the power MOSFETs and D1 and D2 are
selected. The operating frequency and the inductor are
chosen based mainly on the amount of ripple current.
Finally, CIN is selected for its ability to handle the input
ripple current (that PolyPhase operation minimizes) and
COUT is chosen with low enough ESR to meet the output
ripple voltage and load step specifications (also minimized
with PolyPhase). The circuit shown in Figure␣ 1 can be
configured for operation up to an input voltage of 28V
(limited by the external MOSFETs).
RSENSE Selection For Output Current
RSENSE1, 2 are chosen based on the required output
current. The LTC3729 current comparator has a maxi-
mum threshold of 75mV/RSENSE and an input common
mode range of SGND to 1.1( INTVCC). The current com-
parator threshold sets the peak inductor current, yielding
a maximum average output current IMAX equal to the peak
value less half the peak-to-peak ripple current, ∆IL.
Allowing a margin for variations in the LTC3729 and
external component values yields:
RSENSE = (50mV/IMAX)N
where N = number of stages.
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11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC3729EG.PDF ] | |
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