參數(shù)資料
型號： LP3923TLX-VC
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 電源管理
英文描述： 1-CHANNEL POWER SUPPLY SUPPORT CKT, PBGA30
封裝： 3 X 2.50 MM, MICRO SMD-30
文件頁數(shù)： 15/32頁
文件大?。?/td> 1718K
代理商： LP3923TLX-VC
Buck Converter Application
Information
BUCK OUTPUT VOLTAGE SELECTION
Buck output voltage can be programmed via the selection of
the external feedback resistor network forming the output
feedback between the output voltage side of the inductor and
the FB pin and the FB pin and GND.
30058618
Buck Converter Components
V
OUT will be adjusted to make the voltage at FB equal to 0.5V.
The resistor from FB to ground (R
FB2) should be around 200
k
to keep the current drawn through the resistor network to
a minimum but large enough that it is not susceptible to noise.
If R2 is 200 k
and with V
FB at 0.5V, the current through the
resistor feedback network will be 2.5 A.
The formula for output voltage selection is
V
OUT
- output voltage (V)
V
FB
- feedback voltage (0.5V)
R
FB1
- feedback resistor from V
OUT to FB
R
FB2
- feedback resistor from FB to GND
The recommended value for C1 is 2.2 pF, and for C2 is
15 pF.
Component Configurations for Various Output Voltage Values
V
OUT [V]
R
FB1 [k]
R
FB2 [k]
C1 [pF]
C2 [pF]
L [H]
C
OUT [F]
1.4
360
200
2.2
15
2.2
10
1.6
390
178
2.2
15
2.2
10
1.8
390
150
2.2
15
2.2
10
2.0
453
150
2.2
15
2.2
10
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 given at 25°C so
ratings at the application maximum ambient temperature
should be requested from the manufacturer.
There are two methods to choose the inductor saturation cur-
rent rating.
Method 1
The total current is the sum of the load and the inductor ripple
current. This can be written as:
I
LOAD
= load current
I
RIPPLE
= average to peak inductor current
V
IN
= input voltage
L
= inductor inductance
f
= switching frequency
Method 2
A more conservative approach is to choose an inductor that
can handle the maximum current limit of 1500 mA. Given a
peak-to-peak current ripple (I
PP) the inductor needs to be at
least:
A 2.2 H inductor with a saturation current rating of at least
1500 mA is recommended for most applications. The
inductor’s resistance should be less than 0.3
for good effi-
ciency. The below table suggests inductors and suppliers.
For low-cost applications, an unshielded bobbin inductor is
suggested. For noise critical applications, a toroidal or shield-
ed-bobbin inductor should be used. A good practice is to lay
out the board with overlapping footprints of both types for de-
sign flexibility. This allows substitution of a low-noise toroidal
inductor, in the event that noise from low-cost bobbin models
is unacceptable.
www.national.com
22
LP3923
相關(guān)PDF資料
PDF描述
LP3923TL-VI 1-CHANNEL POWER SUPPLY SUPPORT CKT, PBGA30
LP3946ILD 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO14
LP3972SQXA413 1-CHANNEL POWER SUPPLY SUPPORT CKT, QCC40
LP3972SQX-5810 1-CHANNEL POWER SUPPLY SUPPORT CKT, QCC40
LP3972SQXA514 1-CHANNEL POWER SUPPLY SUPPORT CKT, QCC40
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