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參數(shù)資料
| 型號： | LT3845EFE#TR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 600 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 4.40 MM, PLASTIC, TSSOP-16 |
| 文件頁數(shù)： | 4/24頁 |
| 文件大?。?/td> | 263K |
| 代理商： | LT3845EFE#TR |
LT3845
12
3845fb
at start-up the soft-start circuit will discharge the VC pin
voltage below the DC control voltage equivalent to zero
inductor current. This will reduce the input supply inrush
current. The soft-start circuit is disabled once the CSS pin
voltage has been charged to 200mV above the internal
reference of 1.231V.
During a VIN UVLO, VCC UVLO or SHDN UVLO event, the
CSS pin voltage is discharged with a 50μA current source.
In normal operation the CSS pin voltage is clamped to a
diode above the VFBpin voltage. Therefore, the value of the
CSS capacitor is relevant to how long of a fault event will
retrigger a soft-start. If any of the above UVLO conditions
occur, the CSS pin voltage will be discharged with a 50μA
current source. There is a diode worth of voltage headroom
to ride through the fault before the CSS pin voltage enters
its active region and the soft-start function is enabled.
Also, since the CSS pin voltage is clamped to a diode above
the VFB pin voltage, during a short circuit the CSS pin volt-
age is pulled low because the VFB pin voltage is low. Once
the short has been removed the VFB pin voltage starts to
recover. The soft-start circuit takes control of the output
voltage slew rate once the VFB pin voltage has exceeded
the slowly ramping CSS pin voltage, reducing the output
voltage overshoot during a short circuit recovery.
Adaptive Nonoverlap (NOL) Output Stage
The FET driver output stages implement adaptive nonover-
lap control. This feature maintains a constant dead time,
preventing shoot-through switch currents, independent
of the type, size or operating conditions of the external
switch elements.
Each of the two switch drivers contains a NOL control
circuit, which monitors the output gate drive signal of the
other switch driver. The NOL control circuits interrupt the
“turn on” command to their associated switch driver until
the other switch gate is fully discharged.
Antislope Compensation
Most current mode switching controllers use slope com-
pensation to prevent current mode instability. The LT3845
is no exception. A slope-compensation circuit imposes an
articial ramp on the sensed current to increase the rising
slope as duty cycle increases. Unfortunately, this additional
APPLICATIONS INFORMATION
ramp corrupts the sensed current value, reducing the
achievable current limit value by the same amount as the
added ramp represents. As such, current limit is typically
reduced as duty cycles increase. The LT3845 contains
circuitry to eliminate the current limit reduction typically
associated with slope compensation. As the slope-com-
pensation ramp is added to the sensed current, a similar
ramp is added to the current limit threshold reference.
The end result is that current limit is not compromised,
so an LT3845 converter can provide full power regardless
of required duty cycle.
Shutdown
The LT3845 SHDN pin uses a bandgap generated reference
threshold of 1.35V. This precision threshold allows use of
the SHDN pin for both logic-level controlled applications
and analog monitoring applications such as power supply
sequencing.
The LT3845 operational status is primarily controlled by
a UVLO circuit on the VCC regulator pin. When the IC is
enabled via the SHDN pin, only the VCCregulator is enabled.
Switching remains disabled until the UVLO threshold is
achieved at the VCC pin, when the remainder of the IC is
enabled and switching commences.
Because an LT3845 controlled converter is a power
transfer device, a voltage that is lower than expected on
the input supply could require currents that exceed the
sourcing capabilities of that supply, causing the system
to lock up in an undervoltage state. Input supply start-up
protection can be achieved by enabling the SHDN pin
using a resistive divider from the VIN supply to ground.
Setting the divider output to 1.35V when that supply is at
an adequate voltage prevents an LT3845 converter from
drawing large currents until the input supply is able to
provide the required power. 120mV of input hysteresis
on the SHDN pin allows for almost 10% of input supply
droop before disabling the converter.
RSENSE Selection
The current sense resistor, RSENSE, monitors the inductor
current of the supply (See Typical Application on front
page). Its value is chosen based on the maximum required
output load current. The LT3845 current sense amplier
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