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參數(shù)資料
| 型號： | LM20242MH |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 36V, 2A PowerWise㈢ Adjustable Frequency Synchronous Buck Regulator |
| 中文描述： | 4.65 A SWITCHING REGULATOR, 825 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封裝： | TSSOP-20 |
| 文件頁數(shù)： | 12/20頁 |
| 文件大?。?/td> | 384K |
| 代理商： | LM20242MH |
Where,
Δ
V
(V) is the amount of peak to peak voltage ripple
at the power supply output, R
(
) is the series resistance
of the output capacitor, f
(Hz) is the switching frequency,
and C
(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
(F) is the minimum required output capacitance,
L (H) is the value of the inductor, V
(V) is the output
voltage drop ignoring loop bandwidth considerations,
Δ
I
OUT-
(A) is the load step change, R
(
) is the output
capacitor ESR, V
(V) is the input voltage, and V
(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
Good quality input capacitors are necessary to limit the ripple
voltage at the VIN pin while supplying most of the switch cur-
rent during the on-time. In general it is recommended to use
a ceramic capacitor for the input as they provide both a low
impedance and small footprint. One important note is to use
a good dielectric for the ceramic capacitor such as X5R or
X7R. These provide better over temperature performance
and also minimize the DC voltage derating that occurs on Y5V
capacitors. The input capacitors should be placed as close as
possible to the VIN and GND pins on both sides of the device.
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.
SETTING THE OUTPUT VOLTAGE (R
FB1
, R
FB2
)
The resistors R
and R
are selected to set the output
voltage for the device.
Table 1
provides suggestions for
R
FB1
and R
FB2
for common output voltages.
TABLE 1. Suggested Values for R
FB1
and R
FB2
R
FB1
(k
short
open
4.99
10
8.87
10.2
12.7
10.2
21.5
10.2
31.6
10.2
)
R
FB2
(k
)
V
OUT
0.8
1.2
1.5
1.8
2.5
3.3
If different output voltages are required, R
should be se-
lected to be between 4.99 k
to 49.9 k
and R
FB1
can be
calculated using the equation below.
ADJUSTING THE OPERATING FREQUENCY (R
RT
)
The operating frequency of the LM20242 can be adjusted by
connecting a resistor from the RT pin to ground. The equation
shown below can be used to calculate the value of R
RT
for a
given operating frequency.
Where, f
is the switching frequency in kHz, and R
is the
frequency adjust resistor in k
. Please refer to the curve Os-
cillator Frequency versus R
RT
in the typical performance char-
acteristics section. If the R
RT
resistor is omitted the device will
not operate.
LOOP COMPENSATION (R
C1
, C
C1
)
The purpose of loop compensation is to meet static and dy-
namic performance requirements while maintaining adequate
stability. Optimal loop compensation depends on the output
capacitor, inductor, load and the device itself.
The overall loop transfer function is the product of the power
stage and the feedback network transfer functions. For sta-
bility purposes, the objective is to have a loop gain slope that
is -20db/decade from a very low frequency to beyond the
crossover frequency.
Figure 4
shows the transfer functions
for power stage, feedback/compensation network, and the
resulting closed loop system for the LM20242.
www.national.com
12
L
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相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| LM20242MH/NOPB | 功能描述:直流/直流開關轉換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關頻率: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 安裝風格:SMD/SMT |
| LM20242MHE | 制造商:Texas Instruments 功能描述:Conv DC-DC Single Step Down 4.5V to 36V 20-Pin TSSOP EP T/R |
| LM20242MHE/NOPB | 功能描述:直流/直流開關轉換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關頻率: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 安裝風格:SMD/SMT |
| LM20242MHX | 制造商:Texas Instruments 功能描述:Conv DC-DC Single Step Down 4.5V to 36V 20-Pin TSSOP EP T/R |
| LM20242MHX/NOPB | 功能描述:直流/直流開關轉換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關頻率: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 安裝風格:SMD/SMT |
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