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參數(shù)資料
| 型號： | LM3670 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | RECTIFIER STANDARD SINGLE 1A 100V 100 30A-ifsm 5uA-ir 1V-vf A405 5K/REEL-13 |
| 中文描述： | 微型降壓型DC - DC轉(zhuǎn)換器的超低電壓電路 |
| 文件頁數(shù)： | 13/17頁 |
| 文件大?。?/td> | 997K |
| 代理商： | LM3670 |
Application Information
OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE
LM3670
The output voltage of the adjustable parts can be pro-
grammed through the resistor network connected from V
OUT
to V
FB
the to GND. V
OUT
will be adjusted to make V
FB
equal
to .5V. The resistor from V
FB
to GND (R2) should be at least
100K
to keep the current sunk through this network well
below the 15μA quiescent current level (PFM mode with no
switching) but large enough that it is not susceptible to noise.
If R
2
is 200K
, and given the V
FB
is .5V, then the current
through the resistor feedback network will be 2.5μA ( I
FB
=.5V/R
2
). The output voltage formula is:
For any output voltage greater than or equal to .8V a fre-
quency zero must be added at 10KHz for stability. The
formula is:
V
OUT
Output Voltage (V)
V
FB
Feedback Voltage (.5V typ)
R
1
Resistor from V
OUT
to V
FB
(
)
R
2
Resistor from V
OUT
to GND (
)
For output voltages between .7 and .8V a pole must also be
placed at 10KHz as well. The lowest output voltage possible
is .7V. At the low voltages the duty cycle is very small. In
addition, as the input voltage increases the duty cycle de-
creases even further. Since the duty cycle is so low any
change due to noise is an appreciable percentage. In other
words, it is susceptible to noise. The C
and C
act as noise
filters at this point rather than frequency poles and zeroes. If
tghe pole and zero are at the sasme frequency the formula
is:
A pole can be usesd at higher output voltages too. For
example, in the table "Adjustable LM3670 Configurations for
Various VOUT"
Table 3
there is an entry for 1.24V with both a
pole and zero at approximately 10KHz for noise rejection.
INDUCTOR SELECTION
There are two main considerations when choosing an induc-
tor; the inductor current should not saturate, and the inductor
current ripple is small enough to achieve the desired output
voltage ripple.
There are two methods to choose the inductor current rating.
Method 1:
The total current is the sum of the load and the inductor
ripple current. This can be written as
Method 2:
A more conservative approach is to choose an inductor that
can handle the current limit of 700 mA.
Given a peak-to-peak current ripple (I
PP
) the inductor needs
to be at least
I
LOAD
load current
V
IN
input voltage
L inductor
f switching frequency
I
RIPPLE
peak-to-peak
A 10 μH inductor with a saturation current rating of at least
800 mA is recommended for most applications. The induc-
tor’s resistance should be less than around 0.3
for good
efficiency.
Table 1
lists suggested inductors and suppliers.
For low-cost applications, an unshielded bobbin inductor is
suggested. For noise critical applications, a toroidal or
shielded-bobbin inductor should be used. A good practice is
to lay out the board with overlapping footprints of both types
for design flexibility. This allows substitution of a low-noise
toroidal inductor, in the event that noise from low-cost bobbin
models is unacceptable.
INPUT CAPACITOR SELECTION
A ceramic input capacitor of 4.7 μF is sufficient for most
applications. A larger value may be used for improved input
voltage filtering. The input filter capacitor supplies current to
the PFET switch of the LM3670 in the first half of each cycle
and reduces voltage ripple imposed on the input power
source. A ceramic capcitor’s low ESR provides the best
noise filtering of the input voltage spikes due to this rapidly
changing current. Select an input filter capacitor with a surge
current rating sufficient for the power-up surge from the input
power source. The power-up surge current is approximately
the capacitor’s value (μF) times the voltage rise rate (V/μs).
The input current ripple can be calculated as:
L
www.national.com
13
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM3670_06 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits |
| LM3670B-0250 WAF | 制造商:Texas Instruments 功能描述: |
| LM3670MF-1.2 | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| LM3670MF-1.2/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| LM3670MF-1.2/NOPB | 制造商:Texas Instruments 功能描述:DC/DC Converter IC |
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