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參數(shù)資料
| 型號： | LP3971 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | POWER MANAGEMENT UNIT FOR ADVANCED APPLICATION PROCESSORS |
| 中文描述： | 電源管理單元提供高級應(yīng)用處理器 |
| 文件頁數(shù)： | 40/42頁 |
| 文件大?。?/td> | 1350K |
| 代理商： | LP3971 |
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Application Hints
(Continued)
have to be larger in capacitance (which means bigger and
more costly) than a ceramic capacitor with the same ESR
value. It should also be noted that the ESR of a typical
tantalum will increase about 2:1 as the temperature goes
from 25C down to 40C, so some guard band must be
allowed.
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
details. Saturation current ratings are typically specified at
25C so ratings at max ambient temperature of application
should be requested from manufacturer.
There are two methods to choose the inductor saturation
current rating.
Method 1:
The saturation current is greater than the sum of the maxi-
mum load current and the worst case average to peak
inductor 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 2.1A.
A 2.2 μH inductor with a saturation current rating of at least
1150 mA is recommended for most applications. The induc-
tor’s resistance should be less than 0.3
for good efficiency.
Table 1
lists suggested inductors and suppliers. For low-cost
applications, an unshielded bobbin inductor could be consid-
ered. 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 shielded
inductor, 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
improved 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. Select a capacitor with sufficient ripple current rat-
ing. The input current ripple can be calculated as:
The worst case is when V
IN
= 2 * V
OUT
TABLE 1. Suggested Suppliers
Vendor
Toko
Toko
Coilcraft
Coilcraft
Coilcraft
Dimensions LxWxH (mm)
2.8 x 3.0 x 1.2
3.0 x 3.0 x 1.2
3.76 x 4.2 x 1.8
4.45 x 6.6 x 2.92
3.3 x 3.3 x 1.4
D.C.R (Max)
70 m
160 m
70 m
70 m
200 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 capaci-
tors must be considered when selecting case sizes like 0805
and 0603. DC bias characteristics vary from manufacturer to
manufacturer and dc bias curves should be requested from
them as part of the capacitor selection process. The output
filter capacitor smoothes 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 capacitors must be selected with sufficient capaci-
tance and sufficiently low ESR to perform these functions.
The output voltage ripple is caused by the charging and
discharging of the output capacitor and also due to its ESR
and can be calculated as:
Voltage peak-to-peak ripple due to capacitance can be ex-
pressed as follows
Voltage peak-to-peak ripple due to ESR can be expressed
as follows
V
PP-ESR
= (2 * I
RIPPLE
) * R
ESR
L
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