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參數(shù)資料
| 型號(hào)： | MAX1921EUT-T |
| 廠商： | Maxim Integrated Products, Inc. |
| 元件分類： | DC/DC變換器 |
| 英文描述： | Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 |
| 中文描述： | 低電壓，400mA的降壓型DC - DC轉(zhuǎn)換器采用SOT23 |
| 文件頁數(shù)： | 7/9頁 |
| 文件大?。?/td> | 370K |
| 代理商： | MAX1921EUT-T |
Capacitor Selection
For nearly all applications, the input capacitor, C
IN
,
may be as small as 2.2μF ceramic with X5R or X7R
dielectric. The input capacitor filters peak currents and
noise at the voltage source and, therefore, must meet
the input ripple requirements and voltage rating.
Calculate the maximum RMS input current as:
The output capacitor, C
OUT
, may be either ceramic or
tantalum depending upon the chosen application cir-
cuit (see Figures 1 through 4). Table 3 lists some sug-
gested capacitor suppliers.
Ceramic Output Capacitor
For ceramic C
OUT
, use the application circuit of Figure 1
or Figure 2. Calculate the minimum capacitor value as:
Select the next standard value larger than C
OUT
(MIN).
The C
OUT
(MIN) calculation already includes a margin
for capacitor tolerance. Values much larger than
C
OUT
(MIN) always improve transient performance and
stability, but capacitor size and cost increase.
Tantalum Output Capacitor
For tantalum C
OUT
, use the application circuit of Figure
3 or Figure 4. With tantalum C
OUT
, the equivalent series
resistance (ESR) of C
OUT
must be large enough for sta-
bility. Generally, 25mV of ESR-ripple at the feedback
node is sufficient. The simplified calculation is:
Because tantalum capacitors rarely specify minimum
ESR, choose a capacitor with typical ESR that is about
twice as much as ESR
COUT
(MIN). Although ESRs
greater than this work, output ripple becomes larger.
For tantalum C
OUT
, calculate the minimum output
capacitance as:
The 1.25 multiplier is for capacitor tolerance. Select any
standard value larger than C
OUT
(MIN).
Feedback and Compensation
The MAX1921 has factory preset output voltages of
1.5V, 1.8V, 2.5V, 3V, and 3.3V, while the MAX1920 is
externally adjusted by connecting FB to a resistive volt-
age-divider. When using a ceramic output capacitor,
the feedback network must include a compensation
feed-forward capacitor, C
FF
.
C
MIN
(
L I
MAX
V
×
ESR
MIN
(
OUT
OUT
COUT
CRITICAL
)
.
(
)
)
=
×
1 25
ESR
MIN
(
V
COUT
OUT
)
.
=
×
8 0 10
2
C
MIN
(
V
OUT
CRITICAL
)
.
=
×
2 5 10
6
I
RMS
(
I
MAX
(
V
V
V
V
IN
OUT
OUT
IN
OUT
IN
)
)
(
)
=
×
M
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
_______________________________________________________________________________________
7
MAX1920
AGND
IN
SHDN
LX
PGND
FB
OFF
ON
R1
C
OUT
L
C
FF
OUTPUT
UP TO 400mA
INPUT
2V TO 5.5V
C
IN
R2
2
1
3
5
6
4
Figure 2. Typical Application Circuit (MAX1920)
MAX1921
AGND
IN
SHDN
LX
PGND
OUT
OFF
ON
C
OUT
L
OUTPUT
UP TO 400mA
INPUT
2V TO 5.5V
C
IN
2
1
3
5
6
4
Figure 3. MAX1921 Application Circuit Using Tantalum Output
Capacitor
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