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PDF LTC3869-2 Data sheet ( Hoja de datos )
| Número de pieza | LTC3869-2 | |
| Descripción | 2-Phase Synchronous Step-Down DC/DC Controllers | |
| Fabricantes | Linear Technology | |
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FEATURES
n Dual, 180° Phased Controllers Reduce Required
Input Capacitance and Power Supply Induced Noise
n Accurate Multiphase Current Matching
n RSENSE or DCR Current Sensing
n ±0.75% 0.6V Output Voltage Accuracy
n Phase-Lockable Fixed Frequency 250kHz to 780kHz
n High Efficiency: Up to 95%
n Dual N-channel MOSFET Synchronous Drive
n Wide VIN Range: 4V to 38V (40V Max) Operation
n Wide VOUT Range: 0.6V to 12.5V Operation
n Adjustable Soft-Start Current Ramping or Tracking
n Foldback Output Current Limiting
n Output Overvoltage Protection
n Power Good Output Voltage Monitor
n 5V Low Dropout Regulator
n Small 28-Lead QFN and Narrow SSOP Packages
APPLICATIONS
n Server Systems
n Telecom Systems
n Industrial and Medical Instruments
n High Power Battery-Operated Devices
n DC Power Distribution Systems
LTC3869/LTC3869-2
Dual, 2-Phase
Synchronous Step-Down
DC/DC Controllers
DESCRIPTION
The LTC®3869 is a high performance dual synchronous
step-down switching regulator controller that drives all
N-channel synchronous power MOSFET stages. A constant
frequency current mode architecture allows a phase-
lockable frequency of up to 780kHz. 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 LTC3869 features a precision 0.6V
reference and a power good output indicator. A wide 4V
to 38V input supply range encompasses most battery
chemistries. Independent TK/SS pins for each controller
ramp the output voltage during start-up. Current foldback
limits MOSFET heat dissipation during short-circuit condi-
tions. The MODE/PLLIN pin selects among Burst Mode®
operation, pulse-skipping mode, or continuous inductor
current mode and allows the IC to be synchronized to an
external clock.
TheLTC3869ispincompatiblewithLTC3850andis available
in both low profile 28-lead QFN and narrow SSOP packages.
L, LT, LTC, LTM, Linear Technology, the Linear logo, OPTI-LOOP, Burst Mode and PolyPhase
are registered trademarks and No RSENSE is a trademark of Linear Technology Corporation. All
other trademarks are the property of their respective owners. Protected by U.S. Patents including
5481178, 5705919, 5929620, 6100678, 6144194, 6177787, 6580258, 6498466, 6611131.
TYPICAL APPLICATION
3.2µH
VOUT1
5V
5A
+
147k
47µF
High Efficiency Dual 5V/3.3V Step-Down Converter
+
4.7µF
0.1µF
fIN
500kHz
470pF
20k
15k
VIN PGOOD INTVCC
TG1 TG2
BOOST1
SW1
BG1
LTC3869
BOOST2
SW2
BG2
MODE/PLLIN
ILIM
SENSE1+
RUN1
SENSE1–
VFB1
ITH1
TK/SS1
SGND
PGND
FREQ
SENSE2+
RUN2
SENSE2–
VFB2
ITH2
TK/SS2
0.1µF
0.1µF
1µF
0.1µF
22µF
VIN
7V TO
24V
2.2µH
122k
470pF
15k
90.9k
20k
+
VOUT2
3.3V
5A
56µF
3869 TA01
Efficiency and Power Loss
100
90
80
70
60
50
40
30
20
10
0
0.01
VIN = 12V, VOUT = 3.3V
VIN = 12V, VOUT = 5V
1300
1200
1100
EFFICIENCY
1000
900
800
700
600
POWER LOSS 500
0.1 1
LOAD CURRENT (A)
400
10
3869 TA01b
3869f
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LTC3869/LTC3869-2
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Efficiency vs Output Current
and Mode
100
90 Burst Mode
80 OPERATION
70
60 DCM
50
VIN = 12V
VOUT = 1.8V
40 CCM
30
20
10
0
0.01
CIRCUIT OF FIGURE 16
0.1 1 10
LOAD CURRENT (A)
100
3869 G01
Efficiency vs Output Current
and Mode
100
90
80
Burst Mode
OPERATION
70
60
50 DCM
VIN = 12V
VOUT = 1.2V
40 CCM
30
20
10
0
0.01
CIRCUIT OF FIGURE 16
0.1 1 10
LOAD CURRENT (A)
100
3869 G02
Full Load Efficiency and Power
Loss vs Input Voltage
90
1.8V
EFFICIENCY
1.2V
85
5
4
1.8V
80 POWER LOSS
1.2V
3
75 CIRCUIT OF FIGURE 16
5 10
15
INPUT VOLTAGE (V)
2
20
3869 G03
Load Step
(Burst Mode Operation)
ILOAD
5A/DIV
300mA TO 5A
IL
5A/DIV
VOUT
100mV/DIV
AC-COUPLED
VIN = 12V
VOUT = 1.8V
50µs/DIV
Load Step
(Forced Continuous Mode)
ILOAD
5A/DIV
300mA TO 5A
Load Step
(Pulse-Skipping Mode)
ILOAD
5A/DIV
300mA TO 5A
IL
5A/DIV
IL
5A/DIV
VOUT
100mV/DIV
AC-COUPLED
3869 G04
VIN = 12V
VOUT = 1.8V
50µs/DIV
VOUT
100mV/DIV
AC-COUPLED
3869 G05
VIN = 12V
VOUT = 1.8V
50µs/DIV
3869 G06
Inductor Current at Light Load
Prebiased Output at 2V
FORCED
CONTINUOUS
MODE
5A/DIV
Burst Mode
OPERATION
5A/DIV
PULSE-
SKIPPING
MODE
5A/DIV
VIN = 12V
VOUT = 1.8V
ILOAD = 400mA
1µs/DIV
VOUT
2V/DIV
VFB
500mV/DIV
TK/SS
500mV/DIV
3869 G07
VIN = 12V
VOUT = 3.3V
2ms/DIV
Coincident Tracking
3869 G08
RUN
2V/DIV
VOUT1
VOUT1
VOUT2
1V/DIV
5ms/DIV
VOUT1 = 1.8V, 1.5Ω LOAD
VOUT2 = 1.2V, 1Ω LOAD
VOUT2
3869 G09
3869f
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LTC3869/LTC3869-2
OPERATION
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping, or Continuous Conduction)
The LTC3869 can be enabled to enter high efficiency Burst
Mode operation, constant-frequency pulse-skipping mode,
or forced continuous conduction mode. To select forced
continuous operation, tie the MODE/PLLIN pin to a DC
voltage below 0.6V (e.g., SGND). To select pulse-skipping
mode of operation, tie the MODE/PLLIN pin to INTVCC. To
select Burst Mode operation, float the MODE/PLLIN pin.
When a controller is enabled for Burst Mode operation,
the peak current in the inductor is set to approximately
one-third of the maximum sense voltage even though the
voltage on the ITH pin indicates a lower value. If the average
inductor current is higher than the load current, the error
amplifier EA will decrease the voltage on the ITH pin. When
the ITH voltage drops below 0.5V, the internal sleep signal
goes high (enabling sleep mode) and the top MOSFET is
turned off immediately, but the bottom MOSFET is turned
off when the inductor current reaches zero.
In sleep mode, the load current is supplied by the output
capacitor. As the output voltage decreases, the EA’s output
begins to rise. When the output voltage drops enough, the
sleep signal goes low, and the controller resumes normal
operation by turning on the top external MOSFET on the
next cycle of the internal oscillator. When a controller is
enabled for Burst Mode operation, the inductor current is
not allowed to reverse. The reverse current comparator
(IREV) turns off the bottom external MOSFET just before
the inductor current reaches zero, preventing it from
reversing and going negative. Thus, the controller oper-
ates in discontinuous operation. In forced continuous
operation, the inductor current is allowed to reverse at
light loads or under large transient conditions. The peak
inductor current is determined by the voltage on the ITH
pin. In this mode, the efficiency at light loads is lower than
in Burst Mode operation. However, continuous mode has
the advantages of lower output ripple and less interference
with audio circuitry.
When the MODE/PLLIN pin is connected to INTVCC, the
LTC3869 operates in PWM pulse-skipping mode at light
loads. At very light loads, the current comparator ICMP may
remain tripped for several cycles and force the external top
MOSFET to stay off for the same number of cycles (i.e.,
skipping pulses). The inductor current is not allowed to
reverse (discontinuous operation). This mode, like forced
continuous operation, exhibits low output ripple as well as
low audio noise and reduced RF interference as compared
to Burst Mode operation. It provides higher low current
efficiency than forced continuous mode, but not nearly as
high as Burst Mode operation.
Single Output Multiphase Operation
The LTC3869 can be used for single output multiphase
converters by making these connections
• Tie all of the ITH pins together.
• Tie all of the VFB pins together.
• Tie all of the TK/SS pins together.
• Tie all of the RUN pins together.
LTC3869 has excellent current matching performance
between channels to ensure that there are equal thermal
stress for both channels.
Frequency Selection and Phase-Locked Loop
(FREQ and MODE/PLLIN Pins)
The selection of switching frequency is a trade-off between
efficiency and component size. Low frequency operation
increases efficiency by reducing MOSFET switching losses,
but requires larger inductance and/or capacitance to main-
tain low output ripple voltage. The switching frequency of
the LTC3869 controller can be selected using the FREQ pin.
If the MODE/PLLIN pin is not being driven by an external
clock source, the FREQ pin can be used to program the
controller’s operating frequency from 250kHz to 780kHz.
There is a precision 10µA current flowing out of the FREQ
pin, so the user can program the controller’s switching
frequency with a single resistor to SGND. A curve is
provided later in the application section showing the
relationship between the voltage on the FREQ pin and
switching frequency.
3869f
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| PDF Descargar | [ Datasheet LTC3869-2.PDF ] | |
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