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参数资料
| 型号： | LTC3568IDD#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 4 A SWITCHING REGULATOR, 4000 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封装： | 3 X 3 MM, LEAD FREE, PLASTIC, MO-229WEED-2, DFN-10 |
| 文件页数： | 16/18页 |
| 文件大小： | 300K |
| 代理商： | LTC3568IDD#PBF |
LTC3568
3568fa
operaTion
The LTC3568 uses a constant frequency, current mode
architecture. The operating frequency is determined by
the value of the RT resistor or can be synchronized to an
external oscillator. To suit a variety of applications, the
selectable Mode pin, allows the user to trade-off noise
for efficiency.
The output voltage is set by an external divider returned
to the VFB pin. An error amplfier compares the divided
output voltage with a reference voltage of 0.8V and adjusts
the peak inductor current accordingly. Overvoltage and
undervoltage comparators will pull the PGOOD output
low if the output voltage is not within ±7.5%.
MainControlLoop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle
when the VFB voltage is below the the reference voltage.
The current into the inductor and the load increases until
the current limit is reached. The switch turns off and
energy stored in the inductor flows through the bottom
switch (N-channel MOSFET) into the load until the next
clock cycle.
The peak inductor current is controlled by the voltage on
the ITH pin, which is the output of the error amplifier.This
amplifier compares the VFB pin to the 0.8V reference.
Whentheloadcurrentincreases,theVFBvoltagedecreases
slightly below the reference. This decrease causes the er-
ror amplifier to increase the ITH voltage until the average
inductor current matches the new load current.
ThemaincontrolloopisshutdownbypullingtheSHDN/RT
pin to SVIN. A digital soft-start is enabled after shutdown,
which will slowly ramp the peak inductor current up over
1024 clock cycles or until the output reaches regulation,
whicheverisfirst.Soft-startcanbelengthenedbyramping
the voltage on the ITH pin (see Applications Information
section).
LowCurrentOperation
Three modes are available to control the operation of the
LTC3568 at low currents. All three modes automatically
switch from continuous operation to to the selected mode
when the load current is low.
To optimize efficiency, the Burst Mode operation can be
selected. When the load is relatively light, the LTC3568
automaticallyswitchesintoBurstModeoperationinwhich
the PMOS switch operates intermittently based on load
demand. By running cycles periodically, the switching
losses which are dominated by the gate charge losses
of the power MOSFETs are minimized. The main control
loop is interrupted when the output voltage reaches the
desiredregulatedvalue.Thehystereticvoltagecomparator
B trips when ITH is below 0.24V, shutting off the switch and
reducing the power. The output capacitor and the inductor
supply the power to the load until ITH/RUN exceeds 0.31V,
turning on the switch and the main control loop which
starts another cycle.
For lower output voltage ripple at low currents, pulse
skipping mode can be used. In this mode, the LTC3568
continues to switch at a constant frequency down to
very low currents, where it will eventually begin skipping
pulses.
Finally, in forced continuous mode, the inductor current
is constantly cycled which creates a fixed output voltage
ripple at all output current levels. This feature is desirable
in telecommunications since the noise is at a constant
frequency and is thus easy to filter out. Another advan-
tage of this mode is that the regulator is capable of both
sourcing current into a load and sinking some current
from the output.
DropoutOperation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100% which
is the dropout condition. In dropout, the PMOS switch is
turnedoncontinuouslywiththeoutputvoltagebeingequal
to the input voltage minus the voltage drops across the
internal P-channel MOSFET and the inductor.
LowSupplyOperation
The LTC3568 incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below about 2V.
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