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參數(shù)資料
| 型號(hào)： | LM3743 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | N-Channel FET Synchronous Buck Controller for Low Output Voltages |
| 中文描述： | N溝道FET的同步降壓控制器，用于低輸出電壓 |
| 文件頁數(shù)： | 16/23頁 |
| 文件大小： | 1145K |
| 代理商： | LM3743 |
Application Information
(Continued)
be followed to create many other designs with varying input
voltages, output voltages, load currents, and switching fre-
quency.
Switching Frequency
Selection of the operating switching frequency is based on
trade-offs between size, cost, efficiency, and response time.
For example, a lower frequency will require larger more
expensive inductors and capacitors. While a higher switch-
ing frequency will generally reduce the size of these compo-
nents, but will have a reduction in efficiency. Fast switching
converters allow for higher loop gain bandwidths, which in
turn have the ability to respond quickly to load and line
transients. For the example application we have chosen a
300 kHz switching frequency because it will reduce the
switching power losses and in turn allow for higher conduc-
tion losses considering the same power loss criteria, thus it
is possible to sustain a higher load current.
Output Inductor
The output inductor is responsible for smoothing the square
wave created by the switching action and for controlling the
output current ripple (
I
) also called the AC component
of the inductor current. The DC current into the load is equal
to the average current flowing in the inductor. The induc-
tance is chosen by selecting between trade-offs in efficiency,
size, and response time. The recommended percentage of
AC component to DC current is 30% to 40%, this will provide
the best trade-off between energy requirements and size,
(readAN-1197 for theoretical analysis).Another criteria is the
ability to respond to large load transient responses; the
smaller the output inductor, the more quickly the converter
can respond. The equation for output inductor selection is:
Here we have plugged in the values for input voltage, output
voltage, switching frequency, and 30% of the maximum load
current. This yields an inductance of 1.34 μH. The output
inductor must be rated to handle the peak current (also equal
to the peak switch current), which is (I
OUT
+ (0.5 x
I
OUT
)) =
11.5A, for a 10A design and a AC current of 3A.
The Coiltronics DR125–1R5 is 1.5 μH, is rated to 13.8ARMS
current, and has a direct current resistance (DCR) of 3 m
.
After selecting the Coiltronics DR125–1R5 for the output
inductor, actual inductor current ripple must be re-calculated
with the selected inductance value. This information is
needed to determine the RMS current through the input and
output capacitors. Re-arranging the equation used to select
inductance yields the following:
V
is assumed to be 10% above the steady state input
voltage, or 5.5V at V
= 5.0V. The re-calculated current
ripple will then be 2.69A. This gives a peak inductor/switch
current will be 11.35A.
Output Capacitor
The output capacitor in a switching regulator is selected on
the basis of capacitance, equivalent series resistance (ESR),
size, and cost. In this example the output current is 10A and
the expected type of capacitor is an aluminum electrolytic, as
with the input capacitors. An important specification in
switching converters is the output voltage ripple
V
. At
300 kHz the impedance of most capacitors is very small
compared to ESR, hence ESR becomes the main selection
criteria. In this design the load requires a 2% ripple , which
results in a
V
OUT
of 36 mV
P-P
. Thus the maximum ESR is
then:
ESR
is 13 m
. Aluminum electrolytic (Al-E), tantalum
(Ta), solid aluminum, organic, and niobium (Nb) capacitors
are all popular in switching converters. In general, by the
time enough capacitors have been paralleled to obtain the
desired ESR, the bulk capacitance is more than enough to
supply the load current during a transient from no-load to full
load. The number and type of capacitors used depends
mainly on their size and cost. One exception to this is
multi-layer ceramic capacitors. MLCCs have very low ESR,
but also low capacitance in comparison with other types.
This makes them attractive for lower power designs. For
higher power or for fast load transients the number of ML-
CCs needed often increases the size and cost to unaccept-
able levels. Because the load could transition quickly from 0
to 10A, more bulk capacitance is needed than the MLCCs
can provide. One compromise is a solid electrolytic POS-
CAP from Sanyo or SP-caps from Panasonic. POSCAP and
SPcaps often have large capacitances needed to supply
currents for load transients, and low ESRs. The 6TPD470M
by Sanyo has 470 μF, and a maximum ESR of 10 m
. Solid
electrolytics have stable ESR relative to temperature, and
capacitance change is relatively immune to bias voltage.
Tantalums (Ta), niobium (Nb), and Al-E are good solutions
for ambient operating temperatures above 0C, however
their ESR tends to increase quickly below 0C ambient op-
erating temperature, so these capacitor types are not rec-
ommended for this area of operation.
Input Capacitor
The input capacitors in a buck converter are subjected to
high RMS current stress. Input capacitors are selected for
their ability to withstand the heat generated by the RMS
current and the ESR as specified by the manufacturer. Input
RMS ripple current is approximately:
L
www.national.com
16
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM3743-1000EVAL | 功能描述:電源管理IC開發(fā)工具 LM3743 EVAL BOARD RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評(píng)估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
| LM3743-300EVAL | 功能描述:電源管理IC開發(fā)工具 LM3743 EVAL BOARD RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評(píng)估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
| LM3743MM-1000 | 功能描述:電壓模式 PWM 控制器 RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓?fù)浣Y(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
| LM3743MM-1000/NOPB | 功能描述:電壓模式 PWM 控制器 RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓?fù)浣Y(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
| LM3743MM-300 | 功能描述:電壓模式 PWM 控制器 RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓?fù)浣Y(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
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