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參數(shù)資料
| 型號： | MAX776MJA |
| 廠商： | MAXIM INTEGRATED PRODUCTS INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Triple 3-Input Positive-NAND Gates 14-CFP -55 to 125 |
| 中文描述： | SWITCHING CONTROLLER, 300 kHz SWITCHING FREQ-MAX, CDIP8 |
| 封裝： | 0.300 INCH, CERDIP-8 |
| 文件頁數(shù)： | 12/16頁 |
| 文件大小： | 163K |
| 代理商： | MAX776MJA |
M
Choosing an Induc tor
Practical inductor values range from 10μH to 50μH.
The maximum inductor value is not particularly critical.
For highest current at high
V
OUT
to V+ ratios, the
inductor should not be so large that the peak current
never reaches the current limit. That is:
SW
(max)
]
x 12μs
L(max)
≤
__V+(min) - V
I
LIM
(max)
This is only important if
IN
t
OFF
(min)
t
ON
(max)
_V
< 1
V
OUT
6
More important is that the inductor not be so small that the
current rises much faster than the current-limit comparator
can respond. This would be wasteful and reduce effi-
ciency. Calculate the minimum inductor value as follows:
SW
(min)
]
x 0.3μs
L(min)
≥
__V+(max) - V
δ
(I) x I
LIM
(min)
Where L is in μH, 0.3μs is an ample time for the com-
parator response, I
LIM
is the current limit (see Current-
Sense Resistor section), and
δ
(I) is the allowable per-
centage of overshoot. As an example, Figure 2's circuit
uses a 3A peak current. If we allow a 15% overshoot
and 15V is the maximum input voltage, then L(min) is
16μH. The actual value of L above this limit has minimal
effect on this circuit's operation.
For highest efficiency, use a coil with low DC resistance.
Coils with 30m
or lower resistance are available. To
minimize radiated noise, use a torroid, pot-core, or shield-
ed-bobbin inductor. Inductors with a ferrite core or equiv-
alent are recommended. Make sure that the inductor’s
saturation current rating is greater than I
LIM
(max).
Diode S elec tion
The ICs’ high switching frequencies demand a high-
speed rectifier. Schottky diodes such as the 1N5817 to
1N5822 families are recommended. Choose a diode
with an average current rating approximately equal to
or greater than I
LIM
(max) and a voltage rating higher
than V
IN
(max) + V
OUT
. For high-temperature applica-
tions, Schottky diodes may be inadequate due to their
high leakage currents; instead, high-speed silicon
diodes may be used. At heavy loads and high temper-
ature, the benefits of a Schottky diode’s low forward
voltage may outweigh the disadvantages of its high
leakage current.
Current-S ense Resistor
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-
sense comparator threshold (see Current-Limit Trip
Level in the Electrical Characteristics).
To maximize efficiency and reduce the size and cost of
external components, minimize the peak current.
However, since the output current is a function of the
peak current, do not set the limit too low. Refer to
Figures 6–9 to determine the sense resistor (and, there-
fore, peak current) for the required load current.
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
Inverting DC-DC Controllers
12
______________________________________________________________________________________
M
INPUT VOLTAGE (V)
0
500
1000
1500
2000
2500
3 4
5 6
7
8
9 10 11 12 13 14 15
V
OUT
= -5V
R
SENSE
= 0.05
R
SENSE
= 0.06
R
SENSE
= 0.08
R
SENSE
= 0.09
R
SENSE
= 0.07
M
Figure 6. MAX774 Maximum Output Current vs. Input Voltage
(V
OUT
= -5V)
Figure 7. MAX775 Maximum Output Current vs. Input Voltage
(V
OUT
= -12V)
M
0
200
400
600
800
1000
9
INPUT VOLTAGE (V)
M
3
4
5
6
7
8
R
SENSE
= 0.05
R
SENSE
= 0.06
R
SENSE
= 0.07
R
SENSE
= 0.08
R
SENSE
= 0.09
V
OUT
= -12V
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