參數(shù)資料
型號(hào)： LM2832XSDX
廠(chǎng)商： NATIONAL SEMICONDUCTOR CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： High Frequency 2.0A Load - Step-Down DC-DC Regulator
中文描述： 3.25 A SWITCHING REGULATOR, 1950 kHz SWITCHING FREQ-MAX, DSO6
封裝： LLP-6
文件頁(yè)數(shù)： 12/27頁(yè)
文件大?。?/td> 895K
代理商： LM2832XSDX
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (V
O
) to input voltage (V
IN
):
The catch diode (D1) forward voltage drop and the voltage
drop across the internal PMOS must be included to calculate
a more accurate duty cycle. Calculate D by using the follow-
ing formula:
V
SW
can be approximated by:
V
SW
= I
OUT
x R
DSON
The diode forward drop (V
) can range from 0.3V to 0.7V
depending on the quality of the diode. The lower the V
, the
higher the operating efficiency of the converter. The inductor
value determines the output ripple current. Lower inductor
values decrease the size of the inductor, but increase the
output ripple current. An increase in the inductor value will
decrease the output ripple current.
One must ensure that the minimum current limit (2.4A) is not
exceeded, so the peak current in the inductor must be
calculated. The peak current (I
LPK
) in the inductor is calcu-
lated by:
I
LPK
= I
OUT
+
i
L
In general,
i
L
= 0.1 x (I
OUT
)
→
0.2 x (I
OUT
)
If
i
L
= 20% of 2A, the peak current in the inductor will be
2.4A. The minimum guaranteed current limit over all operat-
ing conditions is 2.4A. One can either reduce
i
L
, or make
the engineering judgment that zero margin will be safe
enough. The typical current limit is 3.25A.
The LM2832 operates at frequencies allowing the use of
ceramic output capacitors without compromising transient
response. Ceramic capacitors allow higher inductor ripple
without significantly increasing output ripple. See the output
capacitor section for more details on calculating output volt-
age ripple. Now that the ripple current is determined, the
inductance is calculated by:
Where
When selecting an inductor, make sure that it is capable of
supporting the peak output current without saturating. Induc-
tor saturation will result in a sudden reduction in inductance
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 maxi-
mum output current. For example, if the designed maximum
output current is 1.0Aand the peak current is 1.25A, then the
inductor should be specified with a saturation current limit of
>
1.25A. There is no need to specify the saturation or peak
current of the inductor at the 3.25A typical switch current
limit. The difference in inductor size is a factor of 5. Because
of the operating frequency of the LM2832, ferrite based
inductors are preferred to minimize core losses. This pre-
sents little restriction since the variety of ferrite-based induc-
tors is huge. Lastly, inductors with lower series resistance
(R
) will provide better operating efficiency. For recom-
mended inductors see Example Circuits.
INPUT CAPACITOR
An input capacitor is necessary to ensure that V
does not
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, volt-
age, RMS current rating, and ESL (Equivalent Series Induc-
tance). The recommended input capacitance is 22 μF.The
input voltage rating is specifically stated by the capacitor
manufacturer. Make sure to check any recommended derat-
ings and also verify if there is any significant change in
capacitance at the operating input voltage and the operating
temperature. The input capacitor maximum RMS input cur-
rent rating (I
RMS-IN
) must be greater than:
Neglecting inductor ripple simplifies the above equation to:
It can be shown from the above equation that maximum
RMS capacitor current occurs when D = 0.5. Always calcu-
late 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 frequen-
cies of the LM2832, leaded capacitors may have an ESL so
large that the resulting impedance (2
π
fL) will be higher than
that required to provide stable operation. As a result, surface
mount capacitors are strongly recommended.
20197505
FIGURE 3. Inductor Current
L
www.national.com
12
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