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參數(shù)資料
| 型號： | LP3972SQ-5810 |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 電源管理 |
| 英文描述： | 1-CHANNEL POWER SUPPLY SUPPORT CKT, QCC40 |
| 封裝： | 5 X 5 MM, LLP-40 |
| 文件頁數(shù)： | 51/60頁 |
| 文件大?。?/td> | 1171K |
| 代理商： | LP3972SQ-5810 |
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BUCK CONSIDERATIONS
Inductor Selection
There are two main considerations when choosing an induc-
tor; the inductor should not saturate, and the inductor current
ripple is small enough to achieve the desired output voltage
ripple. Different saturation current rating specs are followed
by different manufacturers so attention must be given to de-
tails. Saturation current ratings are typically specified at 25°C
so ratings at max ambient temperature of application should
be requested from manufacturer.
There are two methods to choose the inductor saturation cur-
rent rating.
Method 1
The saturation current is greater than the sum of the maxi-
mum load current and the worst case average to peak induc-
tor current. This can be written as
I
RIPPLE: Average to peak inductor current
I
OUTMAX: Maximum load current (1500 mA)
V
IN: Maximum input voltage in application
L: Min inductor value including worst case tolerances (30%
drop can be considered for method 1)
f: Minimum switching frequency (1.6 MHz)
V
OUT: Output voltage
Method 2
A more conservative and recommended approach is to
choose an inductor that has saturation current rating greater
than the max current limit of 3A.
A 2.2
μH inductor with a saturation current rating of at least
3A is recommended for most applications. The inductor’s re-
sistance should be less than 0.3
for a good efficiency. Table
1 lists suggested inductors and suppliers. For low-cost appli-
cations, an unshielded bobbin inductor could be considered.
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 flex-
ibility. This allows substitution of a low-noise shielded induc-
tor, in the event that noise from low-cost bobbin models is
unacceptable.
Input Capacitor Selection
A ceramic input capacitor of 10
μF, 6.3V is sufficient for most
applications. Place the input capacitor as close as possible to
the V
IN pin of the device. A larger value may be used for im-
proved input voltage filtering. Use X7R or X5R types, do not
use Y5V. DC bias characteristics of ceramic capacitors must
be considered when selecting case sizes like 0805 and 0603.
The input filter capacitor supplies current to the PFET switch
of the converter in the first half of each cycle and reduces
voltage ripple imposed on the input power source. A ceramic
capacitor’s low ESR provides the best noise filtering of the
input voltage spikes due to this rapidly changing current. Se-
lect a capacitor with sufficient ripple current rating. The input
current ripple can be calculated as:
The worst case is when V
IN = 2 * VOUT
TABLE 1. Suggested Inductors and Their Suppliers
Model
Vendor
Dimensions LxWxH (mm)
D.C.R. (Typ)
FDSE0312-2R2M
Toko
3.0 x 3.0 x 1.2
160 m
DO1608C-222
Coilcraft
6.6 x 4.5 x 1.8
80 m
Output Capacitor Selection
Use a 10
μF, 6.3V ceramic capacitor. Use X7R or X5R types,
do not use Y5V. DC bias characteristics of ceramic capacitors
must be considered when selecting case sizes like 0805 and
0603. DC bias characteristics vary from manufacturer to man-
ufacturer and dc bias curves should be requested from them
as part of the capacitor selection process. The output filter
capacitor smooths out current flow from the inductor to the
load, helps maintain a steady output voltage during transient
load changes and reduces output voltage ripple. These ca-
pacitors must be selected with sufficient capacitance and
sufficiently low ESR to perform these functions.
The output voltage ripple is caused by the charging and dis-
charging of the output capacitor and also due to its ESR and
can be calculated as:
Voltage peak-to-peak ripple due to ESR can be expressed as
follows
V
PP-ESR = (2 * IRIPPLE) * RESR
Because these two components are out of phase the rms val-
ue can be used to get an approximate value of peak-to-peak
ripple.
Voltage peak-to-peak ripple, root mean squared can be ex-
pressed as follows
Note that the output voltage ripple is dependent on the induc-
tor current ripple and the equivalent series resistance of the
output capacitor (R
ESR).
The R
ESR is frequency dependent (as well as temperature
dependent); make sure the value used for calculations is at
the switching frequency of the part.
55
www.national.com
LP3972
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相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| LP3972SQ-5810/NOPB | 功能描述:監(jiān)控電路 Pwr Mgmt Unit RoHS:否 制造商:STMicroelectronics 監(jiān)測電壓數(shù): 監(jiān)測電壓: 欠電壓閾值: 過電壓閾值: 輸出類型:Active Low, Open Drain 人工復位:Resettable 監(jiān)視器:No Watchdog 電池備用開關:No Backup 上電復位延遲（典型值）:10 s 電源電壓-最大:5.5 V 最大工作溫度:+ 85 C 安裝風格:SMD/SMT 封裝 / 箱體:UDFN-6 封裝:Reel |
| LP3972SQ-A413 | 功能描述:PMIC 解決方案 RoHS:否 制造商:Texas Instruments 安裝風格:SMD/SMT 封裝 / 箱體:QFN-24 封裝:Reel |
| LP3972SQ-A413/NOPB | 功能描述:PMIC 解決方案 RoHS:否 制造商:Texas Instruments 安裝風格:SMD/SMT 封裝 / 箱體:QFN-24 封裝:Reel |
| LP3972SQ-A514 | 功能描述:PMIC 解決方案 RoHS:否 制造商:Texas Instruments 安裝風格:SMD/SMT 封裝 / 箱體:QFN-24 封裝:Reel |
| LP3972SQ-A514/NOPB | 功能描述:PMIC 解決方案 RoHS:否 制造商:Texas Instruments 安裝風格:SMD/SMT 封裝 / 箱體:QFN-24 封裝:Reel |
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