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参数资料
| 型号： | LM2734XMKX/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 2.5 A SWITCHING REGULATOR, 1900 kHz SWITCHING FREQ-MAX, PDSO6 |
| 封装： | TSOT-23, 6 PIN |
| 文件页数： | 2/24页 |
| 文件大小： | 466K |
| 代理商： | LM2734XMKX/NOPB |
capacitor section for more details on calculating output volt-
age ripple.
Now that the ripple current or ripple ratio is determined, the
inductance is calculated by:
where f
s is the switching frequency and IO is the output cur-
rent. When selecting an inductor, make sure that it is capable
of supporting the peak output current without saturating. In-
ductor saturation will result in a sudden reduction in induc-
tance and prevent the regulator from operating correctly.
Because of the speed of the internal current limit, the peak
current of the inductor need only be specified for the required
maximum output current. For example, if the designed maxi-
mum output current is 0.5A and the peak current is 0.7A, then
the inductor should be specified with a saturation current limit
of >0.7A. There is no need to specify the saturation or peak
current of the inductor at the 1.7A typical switch current limit.
The difference in inductor size is a factor of 5. Because of the
operating frequency of the LM2734, ferrite based inductors
are preferred to minimize core losses. This presents little re-
striction since the variety of ferrite based inductors is huge.
Lastly, inductors with lower series resistance (DCR) will pro-
vide better operating efficiency. For recommended inductors
see Example Circuits.
INPUT CAPACITOR
An input capacitor is necessary to ensure that V
IN does not
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, voltage,
RMS current rating, and ESL (Equivalent Series Inductance).
The recommended input capacitance is 10F, although 4.7F
works well for input voltages below 6V. The input voltage rat-
ing is specifically stated by the capacitor manufacturer. Make
sure to check any recommended deratings and also verify if
there is any significant change in capacitance at the operating
input voltage and the operating temperature. The input ca-
pacitor maximum RMS input current rating (I
RMS-IN) must be
greater than:
It can be shown from the above equation that maximum RMS
capacitor current occurs when D = 0.5. Always calculate the
RMS at the point where the duty cycle, D, is closest to 0.5.
The ESL of an input capacitor is usually determined by the
effective cross sectional area of the current path. A large
leaded capacitor will have high ESL and a 0805 ceramic chip
capacitor will have very low ESL. At the operating frequencies
of the LM2734, certain capacitors may have an ESL so large
that the resulting impedance (2
πfL) will be higher than that
required to provide stable operation. As a result, surface
mount
capacitors
are
strongly
recommended.
Sanyo
POSCAP, Tantalum or Niobium, Panasonic SP or Cornell
Dubilier ESR, and multilayer ceramic capacitors (MLCC) are
all good choices for both input and output capacitors and have
very low ESL. For MLCCs it is recommended to use X7R or
X5R dielectrics. Consult capacitor manufacturer datasheet to
see how rated capacitance varies over operating conditions.
OUTPUT CAPACITOR
The output capacitor is selected based upon the desired out-
put ripple and transient response. The initial current of a load
transient is provided mainly by the output capacitor. The out-
put ripple of the converter is:
When using MLCCs, the ESR is typically so low that the ca-
pacitive ripple may dominate. When this occurs, the output
ripple will be approximately sinusoidal and 90° phase shifted
from the switching action. Given the availability and quality of
MLCCs and the expected output voltage of designs using the
LM2734, there is really no need to review any other capacitor
technologies. Another benefit of ceramic capacitors is their
ability to bypass high frequency noise. A certain amount of
switching edge noise will couple through parasitic capaci-
tances in the inductor to the output. A ceramic capacitor will
bypass this noise while a tantalum will not. Since the output
capacitor is one of the two external components that control
the stability of the regulator control loop, most applications will
require a minimum at 10 F of output capacitance. Capaci-
tance can be increased significantly with little detriment to the
regulator stability. Like the input capacitor, recommended
multilayer ceramic capacitors are X7R or X5R. Again, verify
actual capacitance at the desired operating voltage and tem-
perature.
Check the RMS current rating of the capacitor. The RMS cur-
rent rating of the capacitor chosen must also meet the follow-
ing condition:
CATCH DIODE
The catch diode (D1) conducts during the switch off-time. A
Schottky diode is recommended for its fast switching times
and low forward voltage drop. The catch diode should be
chosen so that its current rating is greater than:
I
D1 = IO x (1-D)
The reverse breakdown rating of the diode must be at least
the maximum input voltage plus appropriate margin. To im-
prove efficiency choose a Schottky diode with a low forward
voltage drop.
BOOST DIODE
A standard diode such as the 1N4148 type is recommended.
For V
BOOST circuits derived from voltages less than 3.3V, a
small-signal Schottky diode is recommended for greater effi-
ciency. A good choice is the BAT54 small signal diode.
BOOST CAPACITOR
A ceramic 0.01F capacitor with a voltage rating of at least
6.3V is sufficient. The X7R and X5R MLCCs provide the best
performance.
OUTPUT VOLTAGE
The output voltage is set using the following equation where
R2 is connected between the FB pin and GND, and R1 is
connected between V
O and the FB pin. A good value for R2
is 10k
.
www.national.com
10
LM2734
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM2734XQMK | 制造商:Texas Instruments 功能描述:DC DC REG BUCK 1A 1.6MHZ 6TSOT 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT, Primary Input Voltage:20V, No. of Outputs:1, 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT, Primary Input Voltage:20V, No. of Outputs:1, Output Voltage:18V, Output Current:1A, Voltage Regulator Case Style:SOT-23, No. of Pins:6, Operating Temperature Min:-40C, Operating Temperature , RoHS Compliant: Yes |
| LM2734XQMK/NOPB | 功能描述:直流/直流开关调节器 RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
| LM2734XQMKE | 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT |
| LM2734XQMKE/NOPB | 功能描述:直流/直流开关调节器 RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
| LM2734XQMKX/NOPB | 功能描述:直流/直流开关调节器 RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
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