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參數(shù)資料
| 型號： | MAX649MJA |
| 廠商： | MAXIM INTEGRATED PRODUCTS INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 5V/3.3V/3V or Adjustable, High-Efficiency, |
| 中文描述： | SWITCHING CONTROLLER, 300 kHz SWITCHING FREQ-MAX, CDIP8 |
| 封裝： | 0.300 INCH, CERDIP-8 |
| 文件頁數(shù)： | 10/16頁 |
| 文件大?。?/td> | 266K |
| 代理商： | MAX649MJA |
__________________Design Proc edure
S etting the Output V oltage
The MAX649/MAX651/MAX652 are preset for 5V, 3.3V,
and 3V output voltages, respectively. Tie FB to GND
for fixed-output operation. They may also be adjusted
from 1.5V (the reference voltage) to the input voltage,
using external resistors R2 and R3 configured as
shown in Figure 4. For adjustable-output operation,
150k
is recommended for resistor R3. 150k
is a
good value—high enough to avoid wasting energy, yet
low enough to avoid RC delays caused by parasitic
capacitance at FB. R2 is given by:
V
V
REF
R2 = R3 x
[
——— -1
]
where V
REF
= 1.5V.
When using external resistors, it does no harm to con-
nect OUT and the output together, or to leave OUT
unconnected.
Current-S ense Resistor S elec tion
The current-sense resistor limits the peak switch cur-
rent to 210mV/R
SENSE
, where R
SENSE
is the value of
the current-sense resistor, and 210mV is the current-
limit trip level (see Electrical Characteristics).
To maximize efficiency and reduce the size and cost
of external components, minimize the peak current.
However, since the available output current is a func-
tion of the peak current, the peak current must not
be too low.
To choose the proper current-sense resistor for a par-
ticular output voltage, determine the minimum input
voltage and the maximum load current. Next, referring
to Figures 5a, 5b, or 5c, using the minimum input volt-
age, find the curve with the largest sense resistor that
provides sufficient output current. It is not necessary
to perform worst-case calculations. These curves take
into account the worst-case values for sense resistor
(±5%), inductor (22μH ±10%), diode drop (0.6V), and
the IC’s current-sense trip level; an external MOSFET
on-resistance of 0.13
is assumed for V
GS
= -4.5V.
M
5V /3.3V /3V or Adjustable, High-Effic ienc y,
Low I
Q
, S tep-Down DC-DC Controllers
10
______________________________________________________________________________________
3.0
0
2.5
M
2.0
16
M
INPUT VOLTAGE (V)
14
13
15
0.5
1.0
1.5
12
MAX651
V
OUT
= 3.3V
10
9
11
8
6
5
7
4
3
R
S
= 0.06
R
S
= 0.07
R
S
= 0.08
R
S
= 0.10
R
S
= 0.12
R
S
= 0.14
Figure 5b. MAX651 Current-Sense Resistor Graph
3.0
0
2.5
M
2.0
16
M
INPUT VOLTAGE (V)
14
13
15
0.5
1.0
1.5
12
MAX652
V
OUT
= 3.0V
10
9
11
8
6
5
7
4
3
R
S
= 0.06
R
S
= 0.07
R
S
= 0.08
R
S
= 0.10
R
S
= 0.12
R
S
= 0.14
Figure 5c. MAX652 Current-Sense Resistor Graph
3.0
0
2.0
2.5
M
9
16
M
INPUT VOLTAGE (V)
7
12
10 11
8
13
0.5
1.0
1.5
6
15
14
4
5
3
MAX649
V
OUT
= 5V
R
S
= 0.06
R
S
= 0.07
R
S
= 0.08
R
S
= 0.10
R
S
= 0.12
R
S
= 0.14
Figure 5a. MAX649 Current-Sense Resistor Graph
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