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參數(shù)資料
| 型號： | LM2704MF-ADJ |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Micropower Step-up DC/DC Converter with 550mA Peak Current Limit |
| 中文描述： | 0.62 A SWITCHING REGULATOR, PDSO5 |
| 封裝： | SOT-23, 5 PIN |
| 文件頁數(shù)： | 8/11頁 |
| 文件大?。?/td> | 312K |
| 代理商： | LM2704MF-ADJ |
Operation
(Continued)
The LM2704 features a constant off-time control scheme.
Operation can be best understood by referring to
Figure 2
and
Figure 3
. Transistors Q1 and Q2 and resistors R3 and
R4 of
Figure 2
form a bandgap reference used to control the
output voltage. When the voltage at the FB pin is less than
1.237V, the Enable Comp in
Figure 2
enables the device and
the NMOS switch is turned on pulling the SW pin to ground.
When the NMOS switch is on, current begins to flow through
inductor L while the load current is supplied by the output
capacitor C
. Once the current in the inductor reaches the
peak current limit, the CL Comp trips and the 400ns One
Shot turns off the NMOS switch. The SW voltage will then
rise to the output voltage plus a diode drop and the inductor
current will begin to decrease as shown in
Figure 3
. During
this time the energy stored in the inductor is transferred to
C
and the load. After the 400ns off-time the NMOS
switch is turned on and energy is stored in the inductor
again. This energy transfer from the inductor to the output
causes a stepping effect in the output ripple as shown in
Figure 3
.
This cycle is continued until the voltage at FB reaches
1.237V. When FB reaches this voltage, the enable compara-
tor then disables the device turning off the NMOS switch and
reducing the Iq of the device to 40uA. The load current is
then supplied solely by C
OUT
indicated by the gradually
decreasing slope at the output as shown in
Figure 3
. When
the FB pin drops slightly below 1.237V, the enable compara-
tor enables the device and begins the cycle described pre-
viously. The SHDN pin can be used to turn off the LM2704
and reduce the I
q
to 0.01μA. In shutdown mode the output
voltage will be a diode drop lower than the input voltage.
Application Information
INDUCTOR SELECTION
The appropriate inductor for a given application is calculated
using the following equation:
where V
is the schottky diode voltage, I
is the switch
current limit found in the
Typical Performance Characteris-
tics
section, and T
is the switch off time. When using this
equation be sure to use the minimum input voltage for the
application, such as for battery powered applications. For
the LM2704 constant-off time control scheme, the NMOS
power switch is turned off when the current limit is reached.
There is approximately a 200ns delay from the time the
current limit is reached in the NMOS power switch and when
the internal logic actually turns off the switch. During this
200ns delay, the peak inductor current will increase. This
increase in inductor current demands a larger saturation
current rating for the inductor. This saturation current can be
approximated by the following equation:
Choosing inductors with low ESR decrease power losses
and increase efficiency.
Care should be taken when choosing an inductor. For appli-
cations that require an input voltage that approaches the
output voltage, such as when converting a Li-Ion battery
voltage to 5V, the 400ns off time may not be enough time to
discharge the energy in the inductor and transfer the energy
to the output capacitor and load. This can cause a ramping
effect in the inductor current waveform and an increased
ripple on the output voltage. Using a smaller inductor will
cause the I
PK
to increase and will increase the output voltage
ripple further. This can be solved by adding a 4.7pF capaci-
tor across the R
F1
feedback resistor (
Figure 2
) and slightly
increasing the output capacitor. A smaller inductor can then
be used to ensure proper discharge in the 400ns off time.
DIODE SELECTION
To maintain high efficiency, the average current rating of the
schottky diode should be larger than the peak inductor cur-
rent, I
. Schottky diodes with a low forward drop and fast
switching speeds are ideal for increasing efficiency in por-
table applications. Choose a reverse breakdown of the
schottky diode larger than the output voltage.
CAPACITOR SELECTION
Choose low ESR capacitors for the output to minimize output
voltage ripple. Multilayer ceramic capacitors are the best
choice. For most applications, a 1μF ceramic capacitor is
sufficient. For some applications a reduction in output volt-
age ripple can be achieved by increasing the output capaci-
tor.
Local bypassing for the input is needed on the LM2704.
Multilayer ceramic capacitors are a good choice for this as
well. A 4.7μF capacitor is sufficient for most applications. For
additional bypassing, a 100nF ceramic capacitor can be
used to shunt high frequency ripple on the input.
LAYOUT CONSIDERATIONS
The input bypass capacitor C
IN
, as shown in
Figure 1
, must
be placed close to the IC. This will reduce copper trace
resistance which effects input voltage ripple of the IC. For
additional input voltage filtering, a 100nF bypass capacitor
can be placed in parallel with C
IN
to shunt any high fre-
quency noise to ground. The output capacitor, C
, should
also be placed close to the IC. Any copper trace connections
for the Cout capacitor can increase the series resistance,
which directly effects output voltage ripple. The feedback
network, resistors R1 and R2, should be kept close to the FB
pin to minimize copper trace connections that can inject
noise into the system. The ground connection for the feed-
back resistor network should connect directly to an analog
ground plane. The analog ground plane should tie directly to
the GND pin. If no analog ground plane is available, the
ground connection for the feedback network should tie di-
rectly to the GND pin. Trace connections made to the induc-
tor and schottky diode should be minimized to reduce power
dissipation and increase overall efficiency.
L
www.national.com
8
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|---|---|
| LM2704MF-ADJ/HALF | 制造商:Texas Instruments 功能描述:IC STEP UP CONV 550MA POWERWISE |
| LM2704MF-ADJ/NOPB | 功能描述:DC/DC 開關控制器 MICROPWR STEP-UP DC/DC CONVERTER RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 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 安裝風格: 封裝 / 箱體:CPAK |
| LM2704MFX-ADJ | 功能描述:IC CONV DC/DC 550MA ADJ SOT23-5 RoHS:否 類別:集成電路 (IC) >> PMIC - LED 驅動器 系列:- 標準包裝:1 系列:- 恒定電流:- 恒定電壓:- 拓撲:PWM，切換式電容器（充電泵） 輸出數(shù):1 內部驅動器:是 類型 - 主要:背光 類型 - 次要:白色 LED 頻率:642kHz 電源電壓:2.7 V ~ 5.5 V 輸出電壓:5V 安裝類型:表面貼裝 封裝/外殼:10-VFDFN 裸露焊盤 供應商設備封裝:10-VSON 包裝:剪切帶 (CT) 工作溫度:-30°C ~ 85°C 產品目錄頁面:1371 (CN2011-ZH PDF) 其它名稱:BD1603NUV-E2CT |
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| LM2705 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Micropower Step-up DC/DC Converter with 150mA Peak Current Limit |
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