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参数资料
| 型号： | LTC3856EUH#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 12/40页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 32WFQFN |
| 标准包装： | 73 |
| 系列： | PolyPhase® |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 850kHz |
| 占空比： | 94% |
| 电源电压： | 4.5 V ~ 38 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 32-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3856
OPERATION (Refer to Functional Diagram)
Main Control Loop
The LTC3856 uses a constant-frequency, current mode
step-down architecture. During normal operation, each
top MOSFET is turned on each cycle when the oscillator
sets the RS latch, and turned off when the main current
comparator, I CMP , resets each RS latch. The peak inductor
current at which I CMP resets the RS latch is controlled
by the voltage on the I TH pin, which is the output of the
error amplifier, EA. The V FB pin receives a portion of
output voltage feedback signal via the DIFFOUT pin (if
DIFFAMP is used) through the external resistive divider
and is compared to the internal reference voltage. When
the load current increases, it causes a slight decrease in
the V FB pin voltage relative to the 0.6V reference, which
in turn causes the I TH voltage to increase until each
inductor’s average current matches half of the new load
current (assuming the two current sensing resistors are
equal). In Burst Mode operation, after each 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, I REV , or the beginning
of the next cycle.
The main control loop is shut down by pulling the RUN pin
low. Releasing RUN allows an internal 1μA current source
to pull up the RUN pin. When the RUN pin reaches 1.22V,
the main control loop is enabled and the IC is powered
up. When the RUN pin is low, all functions are kept in a
controlled state.
INTV CC /EXTV CC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV CC pin.
When the EXTV CC pin is left open or tied to a voltage less
than 4.7V, an internal 5V linear regulator supplies INTV CC
power from V IN . If EXTV CC is taken above 4.7V, the 5V
regulator is turned off and an internal switch is turned on
connecting EXTV CC . Using the EXTV CC pin allows the INTV CC
power to be derived from a high efficiency external source
such as a switching regulator output. Each top MOSFET
driver is biased from the floating bootstrap capacitor, C B ,
which normally recharges during each off cycle through
an external diode when the top MOSFET turns off. If the
input voltage, V IN , decreases to a voltage close to V OUT ,
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 one-twelfth
of the clock period plus 100ns every third cycle to allow
C B to recharge. However, it is recommended that a load
be present or the IC operates at low frequency during the
dropout transition to ensure C B is recharged.
Shutdown and Start-Up (RUN and TK/SS Pins)
The LTC3856 can be shut down using the RUN pin. Pulling
the RUN pin below 1.22V shuts down the main control loop
for the controller and most internal circuits, including the
INTV CC regulator. Releasing the RUN pin allows an internal
1μA current to pull up the pin and enable the controller.
Alternatively, the RUN pin may be externally pulled up or
driven directly by logic. Be careful not to exceed the ab-
solute maximum rating of 6V on this pin. The start-up of
the controller’s output voltage, V OUT , is controlled by the
voltage on the TK/SS pin. When the voltage on the TK/SS
pin is less than the 0.6V internal reference, the LTC3856
regulates the V FB voltage to the TK/SS pin voltage instead
of the 0.6V 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 SGND. An internal 1.25μA
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 0.6V (and beyond), the output voltage, V OUT ,
rises smoothly from zero to its final value. Alternatively,
the TK/SS pin can be used to cause the start-up of V OUT
to track that of 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 INTV CC drops below its
undervoltage lockout threshold of 3.2V, the TK/SS pin is
pulled low by an internal MOSFET. When in undervoltage
lockout, all phases of the controller are disabled and the
external MOSFETs are held off.
3856f
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| LTC3856IFE#PBF | 功能描述:IC REG CTRLR BUCK PWM CM 38TFSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:PolyPhase® 标准包装:4,500 系列:PowerWise® PWM 型:控制器 输出数:1 频率 - 最大:1MHz 占空比:95% 电源电压:2.8 V ~ 5.5 V 降压:是 升压:无 回扫:无 反相:无 倍增器:无 除法器:无 Cuk:无 隔离:无 工作温度:-40°C ~ 125°C 封装/外壳:6-WDFN 裸露焊盘 包装:带卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名称:LM1771SSDX |
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