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參數(shù)資料
| 型號： | LM27402SQX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 1350 kHz SWITCHING FREQ-MAX, QCC16 |
| 封裝： | 4 X 4 MM, 0.80 MM HEIGHT, 0.65 MM PITCH, LLP-16 |
| 文件頁數(shù)： | 6/32頁 |
| 文件大?。?/td> | 653K |
| 代理商： | LM27402SQX |
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Design Guide
The Design Guide assists the designer with the steps neces-
sary to select the external components to build a fully func-
tional power supply. As with any DC-DC converter numerous
tradeoffs are possible to optimize the design for efficiency,
size, or performance. These tradeoffs will be taken into ac-
count and highlighted throughout the discussion. To facilitate
component selection, the circuit shown in Figure 4 below may
be used as a reference. Unless otherwise indicated, all for-
mulae assume units of Amps (A) for current, Farads (F) for
capacitance, Henries (H) for inductance and Volts (V) for volt-
age.
30092601
FIGURE 4. Typical Application Circuit
The above schematic shows R
F and CF acting as an RC filter
to the input of the LM27402. The filter is used to attenuate
voltage ripple that may exist on the input rail particularly dur-
ing high output currents. The recommended values of R
F and
C
F are 2.2 and 1 F, respectively. There is a practical limit
to the size of R
F as it can cause a large voltage drop if large
operating bias currents are present. The VIN pin of the
LM27402 should not exceed 150 mV difference from the input
voltage rail (V
IN).
The first equation to calculate for any buck converter is duty
ratio:
Due to the resistive powertrain losses, the duty ratio will in-
crease based on the overall efficiency,
η. Calculation of η can
be found in the POWER/EFFICIENCY CALCULATIONS sec-
tion of this datasheet.
INDUCTOR SELECTION (L)
The inductor value is determined based on the operating fre-
quency, load current, ripple current, and duty ratio. The se-
lected inductor should have a saturation current rating greater
than the peak current limit of the LM27402. To optimize the
performance, the inductance is typically selected such that
the ripple current,
ΔI
L, is between 20% and 40% of the rated
output current. Figure 5 illustrates the switch voltage and in-
ductor ripple current waveforms. Once the nominal input volt-
age, output voltage, operating frequency, and desired ripple
current are known, the minimum inductance value can be
calculated by:
30092670
FIGURE 5. Switch Voltage and Inductor Current
Waveforms
The peak inductor current at maximum load, I
OUT + ΔIL/2,
should be kept adequately below the peak current limit set-
point of the device.
OUTPUT CAPACITOR SELECTION (C
OUT)
The output capacitor, C
OUT, filters the inductor ripple current
and provides a source of charge for transient load events. A
wide range of output capacitors may be used with the
LM27402 that provide excellent performance. The best per-
formance is typically obtained using ceramic, tantalum, or
electrolytic type chemistries. Typically, ceramic capacitors
provide extremely low ESR to reduce the output ripple voltage
and noise spikes, while tantalum and electrolytic capacitors
provide a large bulk capacitance in a small size for transient
loading events. When selecting the output capacitance value,
the two performance characteristics to consider are the output
voltage ripple and transient response. The output voltage rip-
ple can be approximated by:
where
ΔV
OUT (V) is the amount of peak to peak voltage ripple
at the power supply output, R
ESR () is the series resistance
of the output capacitor, f
SW (Hz) is the switching frequency,
and C
OUT (F) is the output capacitance used in the design.
The amount of output ripple that can be tolerated is applica-
tion specific; however a general recommendation is to keep
the output ripple less than 1% of the rated output voltage. Note
that ceramic capacitors are sometimes preferred because
they have very low ESR; however, depending on package
and voltage rating of the capacitor, the value of capacitance
can drop significantly with applied voltage and operating tem-
perature.
The output capacitor will affect the output voltage droop dur-
ing a load transient. The peak output voltage deviation is
dependent on many factors such as output capacitance, out-
put capacitor ESR, inductor size, control loop bandwidth,
powertrain parasitics, etc. Given sufficient control loop band-
www.national.com
14
LM27402
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM27402SQX/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 |
| LM27403 | 制造商:TI 制造商全稱:Texas Instruments 功能描述:Synchronous Buck Controller with Temperature-Compensated, Inductor-DCR-Based Overcurrent Protection and Programmable Thermal Shutdown |
| LM27403EVM | 制造商:Texas Instruments 功能描述:LM27403EVM - Boxed Product (Development Kits) |
| LM27403EVM-POL600 | 制造商:Texas Instruments 功能描述:BOARD EVAL LM27403 |
| LM27403SQ | 制造商:TI 制造商全稱:Texas Instruments 功能描述:Synchronous Buck Controller with Temperature-Compensated, Inductor-DCR-Based Overcurrent Protection and Programmable Thermal Shutdown |
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