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PDF LTC3851A-1 Data sheet ( Hoja de datos )
| Número de pieza | LTC3851A-1 | |
| Descripción | Synchronous Step-Down Switching Regulator Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Features
n Wide VIN Range: 4V to 38V Operation
n RSENSE or DCR Current Sensing
n ±1% Output Voltage Accuracy
n Power Good Output Voltage Monitor
n Phase-Lockable Fixed Frequency: 250kHz to 750kHz
n Dual N-Channel MOSFET Synchronous Drive
n Very Low Dropout Operation: 99% Duty Cycle
n Adjustable Output Voltage Soft-Start or Tracking
n Output Current Foldback Limiting
n Output Overvoltage Protection
n OPTI-LOOP® Compensation Minimizes COUT
n Selectable Continuous, Pulse-Skipping or
Burst Mode® Operation at Light Loads
n Low Shutdown IQ: 20µA
n VOUT Range: 0.8V to 5.5V
n Thermally Enhanced 16-Lead MSOP
or 3mm × 3mm QFN Package
Applications
n Automotive Systems
n Telecom Systems
n Industrial Equipment
n Distributed DC Power Systems
LTC3851A-1
Synchronous
Step-Down Switching
Regulator Controller
Description
The LTC®3851A-1 is a high performance synchronous
step-down switching regulator controller that drives
an all N-channel synchronous power MOSFET stage. A
constant frequency current mode architecture allows a
phase‑lockable frequency of up to 750kHz.
OPTI-LOOP compensation allows the transient response
to be optimized over a wide range of output capacitance
and ESR values. The LTC3851A-1 features a precision 0.8V
reference and a power good indicator. A wide 4V to 38V
(40V absolute maximum) input supply range encompasses
most battery configurations and intermediate bus voltages.
The TK/SS pin ramps the output voltage during start‑up and
shutdown with coincident or ratiometric tracking. Current
foldback limits MOSFET heat dissipation during short-
circuit conditions. The MODE/PLLIN pin selects among
Burst Mode operation, pulse-skipping mode or continu-
ous inductor current mode at light loads and allows the IC
to be synchronized to an external clock. The LTC3851A-1
contains an improved PLL compared to the LTC3851-1.
The LTC3851A-1 is identical to the LTC3851A except that
the ILIM pin is replaced by PGOOD.
L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, Linear Technology and the Linear logo are registered
trademarks and No RSENSE, UltraFast are trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Protected by U.S. Patents including
5408150, 5481178, 5705919, 5929620, 6304066, 6498466, 6580258, 6611131.
Typical Application
High Efficiency Synchronous Step-Down Converter
INTVCC
82.5k
100k
0.1µF
0.1µF
PGO0D
VIN
FREQ/PLLFLTR TG
RUN SW
LTC3851A-1
TK/SS BOOST
2200pF
15k
INTVCC
ITH
330pF
BG
GND
SENSE+
MODE/PLLIN
SENSE–
VFB
0.1µF
4.7µF
0.68µH
3.01k
22µF
VIN
4.5V TO 38V
VOUT
3.3V
15A
330µF
×2
0.047µF 30.1k
154k
48.7k
3851A1 TA01a
Efficiency and Power Loss
vs Load Current
100
95
VVIONU=T
12V
= 3.3V
90
EFFICIENCY
10000
85 1000
80
75 POWER LOSS
70
65 100
60
55
50
10
100
1000
10000
LOAD CURRENT (mA)
10
100000
3851A1 TA01b
3851a1fa
1
1 page  
Typical Performance Characteristics
LTC3851A-1
Efficiency vs Output Current
and Mode
100
90
VVIONU=T
12V
= 1.5V
80 BURST
70
60
PULSE
SKIP
50
40 CCM
30
20
10
0
10
100
1000
10000 100000
LOAD CURRENT (mA)
3851A1 G01
Efficiency vs Output Current
and Mode
100
90
80
BURST
70
60 PULSE
50 SKIP
40
CCM
30
20
10
0
10
VVIONU=T
12V
= 5V
100
1000
10000 100000
LOAD CURRENT (mA)
3851A1 G03
Load Step
(Burst Mode Operation)
ILOAD
5A/DIV
0.2A TO 7.5A
IL
5A/DIV
VOUT
100mV/DIV
AC-COUPLED
VOUT = 1.5V
100µs/DIV
VIN = 12V
FIGURE 11 CIRCUIT
3851A1 G05
Efficiency vs Output Current
and Mode
100
90 BURST
80
70
60 PULSE
50 SKIP
40
CCM
30
20
10
0
10
VVIONU=T
12V
= 3.3V
FIGURE 11 CIRCUIT
100
1000
10000
LOAD CURRENT (mA)
100000
3851A1 G02
Efficiency and Power Loss
vs Input Voltage
100
95
EFFICIENCY,
IOUT = 5A
90
POWER LOSS,
IOUT = 5A
10000
85
80
75
70
4
EFFICIENCY,
IOUT = 0.5A
1000
POWER LOSS,
IOUT = 0.5A
8 12 16
VIN = 12V
VOUT = 3.3V
FIGURE 11 CIRCUIT
100
20 24 28 32
INPUT VOLTAGE (V)
3851A1 G04
Load Step
(Forced Continuous Mode)
ILOAD
5A/DIV
0.2A TO 7.5A
IL
5A/DIV
VOUT
100mV/DIV
AC-COUPLED
VOUT = 1.5V
100µs/DIV
VIN = 12V
FIGURE 11 CIRCUIT
3851A1 G06
3851a1fa
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LTC3851A-1
Operation
Main Control Loop
The LTC3851A-1 is a constant frequency, current mode
step-down controller. During normal operation, the top
MOSFET is turned on when the clock sets the RS latch,
and is turned off when the main current comparator, ICMP ,
resets the RS latch. The peak inductor current at which
ICMP resets the RS latch is controlled by the voltage on
the ITH pin, which is the output of the error amplifier, EA.
The VFB pin receives the voltage feedback signal, which is
compared to the internal reference voltage by the EA. When
the load current increases, it causes a slight decrease in
VFB relative to the 0.8V reference, which in turn causes the
ITH voltage to increase until the average inductor current
matches the new load current. After the top MOSFET has
turned off, the bottom MOSFET is turned on until either
the inductor current starts to reverse, as indicated by the
reverse current comparator, IREV, or the beginning of the
next cycle.
INTVCC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTVCC pin. An
internal 5V low dropout linear regulator supplies INTVCC
power from VIN.
The top MOSFET driver is biased from the floating boot
strap capacitor, CB, which normally recharges during each
off cycle through an external diode when the top MOSFET
turns off. If the input voltage, VIN, decreases to a voltage
close to VOUT, the loop may enter dropout and attempt
to turn on the top MOSFET continuously. The dropout
detector detects this and forces the top MOSFET off for
about 1/10 of the clock period every tenth cycle to allow
CB to recharge. However, it is recommended that there is
always a load present during the drop-out transition to
ensure CB is recharged.
Shutdown and Start-Up (RUN and TK/SS)
The LTC3851A-1 can be shut down using the RUN pin.
Pulling this pin below 1.1V disables the controller and
most of the internal circuitry, including the INTVCC regula-
tor. Releasing the RUN pin allows an internal 2µA current
to pull up the pin and enable that controller. Alternatively,
the RUN pin may be externally pulled up or driven directly
by logic. Be careful not to exceed the absolute maximum
rating of 6V on this pin.
The start-up of the controller’s output voltage, VOUT , is
controlled by the voltage on the TK/SS pin. When the
voltage on the TK/SS pin is less than the 0.8V internal
reference, the LTC3851A-1 regulates the VFB voltage to
the TK/SS pin voltage instead of the 0.8V reference. This
allows the TK/SS pin to be used to program a soft-start
by connecting an external capacitor from the TK/SS pin to
GND. An internal 1µA pull-up current charges this capacitor
creating a voltage ramp on the TK/SS pin. As the TK/SS
voltage rises linearly from 0V to 0.8V (and beyond), the
output voltage VOUT rises smoothly from zero to its final
value. Alternatively, the TK/SS pin can be used to cause
the start-up of VOUT to track another supply. Typically,
this requires connecting to the TK/SS pin an external
resistor divider from the other supply to ground (see the
Applications Information section). When the RUN pin
is pulled low to disable the controller, or when INTVCC
drops below its undervoltage lockout threshold of 3.2V,
the TK/SS pin is pulled low by an internal MOSFET. When
in undervoltage lockout, the controller is disabled and the
external MOSFETs are held off.
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Continuous Conduction)
The LTC3851A-1 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
INTVCC. To select pulse-skipping mode of operation, float
the MODE/PLLIN pin or tie it to GND. To select Burst Mode
operation, tie MODE/PLLIN to INTVCC through a resistor
no less than 50k, but no greater than 250k.
When the controller is enabled for Burst Mode operation,
the peak current in the inductor is set to approximately
one-forth 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 0.4V, the internal
sleep signal goes high (enabling “sleep” mode) and both
external MOSFETs are turned off.
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11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC3851A-1.PDF ] | |
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