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參數(shù)資料
| 型號： | LM2852XMXA-3.3 |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 2A 500/1500kHz SIMPLE SYNCHRONOUS Buck Regulator |
| 中文描述： | 4.95 A SWITCHING REGULATOR, 1825 kHz SWITCHING FREQ-MAX, PDSO14 |
| 封裝： | TSSOP-14 |
| 文件頁數(shù)： | 8/13頁 |
| 文件大?。?/td> | 618K |
| 代理商： | LM2852XMXA-3.3 |
Block Diagram
20127012
Applications Information
The LM2852 is a DC-DC buck converter belonging to Na-
tional Semiconductor’s SIMPLE SYNCHRONOUS
family.
Integration of the PWM controller, power switches and com-
pensation network greatly reduces the component count
required to implement a switching power supply. A typical
application requires only four components: an input capaci-
tor, a soft-start capacitor, an output filter capacitor and an
output filter inductor.
INPUT CAPACITOR (C
IN
)
Fast switching of large currents in the buck converter places
a heavy demand on the voltage source supplying PVIN. The
input capacitor, C
IN
, supplies extra charge when the switcher
needs to draw a burst of current from the supply. The RMS
current rating and the voltage rating of the C
IN
capacitor are
therefore important in the selection of C
IN
. The RMS current
specification can be approximated to be the load current
times the square root of the duty cycle:
where D is the duty cycle, V
/V
. C
also provides
filtering of the supply. Trace resistance and inductance de-
grade the benefits of the input capacitor, so C
should be
placed very close to PVIN in the layout. A 22 μF or 47 μF
ceramic capacitor is typically sufficient for C
. In parallel
with the large input capacitance a smaller capacitor may be
added such as a 1μF ceramic for higher frequency filtering.
SOFT-START CAPACITOR (C
SS
)
The DAC that sets the reference voltage of the error amp
sources a current through a resistor to set the reference
voltage. The reference voltage is one half of the output
voltage of the switcher due to the 200k
divider connected
to the SNS pin. Upon start-up, the output voltage of the
switcher tracks the reference voltage with a two to one ratio
as the DAC current charges the capacitance connected to
the reference voltage node. Internal capacitance of 20pF is
permanently attached to the reference voltage node which is
also connected to the soft-start pin, SS. Adding a soft-start
capacitor externally increases the time it takes for the output
voltage to reach its final level.
The charging time required for the reference voltage can be
estimated using the RC time constant of the DAC resistor
and the capacitance connected to the SS pin. Three RC time
constant periods are needed for the reference voltage to
reach 95% of its final value. The actual start-up time will vary
with differences in the DAC resistance and higher-order
effects.
If little or no soft-start capacitance is connected, then the
start-up time may be determined by the time required for the
current limit current to charge the output filter capacitance.
The capacitor charging equation I = C
V/
t can be used to
estimate the start-up time in this case. For example, a part
with a 3V output, a 100 μF output capacitance and a 3A
current limit threshold would require a time of 100 μs:
Since it is undesirable for the power supply to start up in
current limit, a soft-start capacitor must be chosen to force
the LM2852 to start up in a more controlled fashion based on
the charging of the soft-start capacitance. In this example,
suppose a 3 ms start time is desired. Three time constants
are required for charging the soft-start capacitor to 95% of
the final reference voltage. So in this case RC=1ms. The
DAC resistor, R, is 400 k
so C can be calculated to be
2.5nF. A 2.7nF ceramic capacitor can be chosen to yield
approximately a 3ms start-up time.
L
www.national.com
8
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM2852XMXAX-0.8 | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
| LM2852XMXAX-0.8/NOPB | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
| LM2852XMXAX-1.0 | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
| LM2852XMXAX-1.0/NOPB | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
| LM2852XMXAX-1.2 | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
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