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參數(shù)資料
| 型號(hào)： | LTCEMSE-1#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, PDSO16 |
| 封裝： | 0.150 INCH, LEAD FREE, PLASTIC, MSOP-16 |
| 文件頁數(shù)： | 2/28頁 |
| 文件大?。?/td> | 337K |
| 代理商： | LTCEMSE-1#PBF |
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LTC3851-1
10
38511f
OPERATION
Main Control Loop
The LTC3851-1 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 amplier 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 oating 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 LTC3851-1 can be shut down using the RUN pin. Pull-
ing this pin below 1.1V disables the controller and most
of the internal circuitry, including the INTVCC regulator.
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 LTC3851-1 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 nal
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 LTC3851-1 can be enabled to enter high efciency
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, oat
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 amplier, 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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