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参数资料
| 型号： | LT3845MPFE#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 13/26页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 16TSSOP |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 500kHz |
| 电源电压： | 4 V ~ 60 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： | -55°C ~ 125°C |
| 封装/外壳： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 包装： | 带卷 (TR) |
LT3845
APPLICATIONS INFORMATION
at start-up the soft-start circuit will discharge the V C 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 C SS pin
voltage has been charged to 200mV above the internal
reference of 1.231V.
During a V IN UVLO, V CC UVLO or SHDN UVLO event, the
C SS pin voltage is discharged with a 50μA current source.
In normal operation the C SS pin voltage is clamped to a
diode above the V FB pin voltage. Therefore, the value of the
C SS capacitor is relevant to how long of a fault event will
retrigger a soft-start. If any of the above UVLO conditions
occur, the C SS 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 C SS pin voltage enters
its active region and the soft-start function is enabled.
Also, since the C SS pin voltage is clamped to a diode above
the V FB pin voltage, during a short circuit the C SS pin volt-
age is pulled low because the V FB pin voltage is low. Once
the short has been removed the V FB pin voltage starts to
recover. The soft-start circuit takes control of the output
voltage slew rate once the V FB pin voltage has exceeded
the slowly ramping C SS 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
arti?cial ramp on the sensed current to increase the rising
slope as duty cycle increases. Unfortunately, this additional
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 V CC regulator pin. When the IC is
enabled via the SHDN pin, only the V CC regulator is enabled.
Switching remains disabled until the UVLO threshold is
achieved at the V CC 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 V IN 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.
R SENSE Selection
The current sense resistor, R SENSE , 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 ampli?er
3845fd
13
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LT3846 | 制造商:LINER 制造商全称:Linear Technology 功能描述:Current Mode PWM Controller |
| LT3846J | 制造商:Linear Technology 功能描述:Current Mode PWM Controller 200mA 16-Pin CDIP |
| LT3846N | 制造商:LINER 制造商全称:Linear Technology 功能描述:Current Mode PWM Controller |
| LT3847 | 制造商:LINER 制造商全称:Linear Technology 功能描述:Current Mode PWM Controller |
| LT3847J | 制造商:Linear Technology 功能描述:Current Mode PWM Controller 200mA 16-Pin CDIP |
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