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參數(shù)資料
| 型號： | LM21215MHX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 23.5 A SWITCHING REGULATOR, 525 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封裝： | TSSOP-20 |
| 文件頁數(shù)： | 7/24頁 |
| 文件大?。?/td> | 512K |
| 代理商： | LM21215MHX |
INDUCTOR SELECTION
The inductor (L) used in the application will influence the ripple
current and the efficiency of the system. The first selection
criteria is to define a ripple current,
ΔI
L. In a buck converter,
it is typically selected to run between 20% to 30% of the max-
imum output current. Figure 7 shows the ripple current in a
standard buck converter operating in continuous conduction
mode. Larger ripple current will result in a smaller inductance
value, which will lead to a lower series resistance in the in-
ductor, and improved efficiency. However, larger ripple cur-
rent will also cause the device to operate in discontinuous
conduction mode at a higher average output current.
30103707
FIGURE 7. Switch and Inductor Current Waveforms
Once the ripple current has been determined, the appropriate
inductor size can be calculated using the following equation:
OUTPUT CAPACITOR SELECTION
The output capacitor, C
OUT, filters the inductor ripple current
and provides a source of charge for transient load conditions.
A wide range of output capacitors may be used with the
LM21215 that provide various advantages. The best perfor-
mance is typically obtained using ceramic, SP or OSCON
type chemistries. Typical trade-offs are that the ceramic ca-
pacitor provides extremely low ESR to reduce the output
ripple voltage and noise spikes, while the SP and OSCON
capacitors provide a large bulk capacitance in a small volume
for transient loading conditions.
When selecting the value for the output capacitor, the two
performance characteristics to consider are the output volt-
age ripple and transient response. The output voltage ripple
can be approximated by using the following formula:
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.
Keep in mind ceramic capacitors are sometimes preferred
because they have very low ESR; however, depending on
package and voltage rating of the capacitor the value of the
capacitance can drop significantly with applied voltage. The
output capacitor selection will also affect the output voltage
droop during a load transient. The peak droop on the output
voltage during a load transient is dependent on many factors;
however, an approximation of the transient droop ignoring
loop bandwidth can be obtained using the following equation:
where, C
OUT (F) is the minimum required output capacitance,
L (H) is the value of the inductor, V
DROOP (V) is the output
voltage drop ignoring loop bandwidth considerations,
ΔI
OUT-
STEP (A) is the load step change, RESR () is the output
capacitor ESR, V
IN (V) is the input voltage, and VOUT (V) is
the set regulator output voltage. Both the tolerance and volt-
age coefficient of the capacitor should be examined when
designing for a specific output ripple or transient droop target.
INPUT CAPACITOR SELECTION
Quality input capacitors are necessary to limit the ripple volt-
age at the VIN pin while supplying most of the switch current
during the on-time. Additionally, they help minimize input volt-
age droop in an output current transient condition. In general,
it is recommended to use a ceramic capacitor for the input as
it provides both a low impedance and small footprint. Use of
a high grade dielectric for the ceramic capacitor, such as X5R
or X7R, will provide improved over-temperature performance
and also minimize the DC voltage derating that occurs with
Y5V capacitors. The input capacitors C
IN1 and CIN2 should be
placed as close as possible to the PVIN and PGND pins.
Non-ceramic input capacitors should be selected for RMS
current rating and minimum ripple voltage. A good approxi-
mation for the required ripple current rating is given by the
relationship:
As indicated by the RMS ripple current equation, highest re-
quirement for RMS current rating occurs at 50% duty cycle.
For this case, the RMS ripple current rating of the input ca-
pacitor should be greater than half the output current. For best
performance, low ESR ceramic capacitors should be placed
in parallel with higher capacitance capacitors to provide the
best input filtering for the device.
When operating at low input voltages (3.3V or lower), addi-
tional capacitance may be necessary to protect from trigger-
ing an under-voltage condition on an output current transient.
This will depend on the impedance between the input voltage
supply and the LM21215, as well as the magnitude and slew
rate of the output transient.
The AVIN pin requires a 1 F ceramic capacitor to AGND and
a 1
resistor to PVIN. This RC network will filter inherent
noise on PVIN from the sensitive analog circuitry connected
to AVIN.
PROGRAMMABLE CURRENT LIMIT
A resisitor from the ILIM pin to GND will set the internal current
limit on the LM21215. The current limit should be pro-
grammed such that the peak inductor current (I
L) does not
trigger the current limit in normal operation. This requires set-
15
www.national.com
LM21215
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM21215MHX/NOPB | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
| LM212B | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Optoelectronic |
| LM212D | 制造商:TI 制造商全稱:Texas Instruments 功能描述:OPERATIONAL AMPLIFIERS/BUFFERS |
| LM212H | 制造商:Motorola Inc 功能描述: |
| LM212JG | 制造商:TI 制造商全稱:Texas Instruments 功能描述:OPERATIONAL AMPLIFIERS/BUFFERS |
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