3508 查看數據表（PDF） - Linear Technology
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3508 Datasheet PDF : 24 Pages
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LT3508
APPLICATIONS INFORMATION
VOUT
100mV/DIV
(AC)
IL
500mA/DIV
2μs/DIV
3508 F02
Figure 2. Operation Below VIN(PS). VIN = 10V, VOUT = 1.8V and
fSW = 1.5MHz
VOUT
100mV/DIV
(AC)
IL
500mA/DIV
2μs/DIV
3508 F03
Figure 3. Operation Above VIN(PS). VIN = 18V, VOUT = 1.8V
and fSW = 1.5MHz. Output Ripple and Peak Inductor Current
Increase
Table 1. Programming the Switching Frequency
SWITCHING FREQUENCY (MHz)
2.5
RT (kΩ)
7.50
2.2
9.76
2
11.5
1.8
14
1.6
16.9
1.4
20.5
1.2
26.1
1
33.2
0.9
38.3
0.8
44.2
0.7
52.3
0.6
61.9
0.5
76.8
0.45
88.7
0.4
100
0.35
115
0.3
140
0.25
169
Setting the Switching Frequency
The switching frequency is programmed either by driving
the RT/SYNC pin with a logic level SYNC signal or by tying
a resistor from the RT/SYNC pin to ground. A graph for
selecting the value of RT for a given operating frequency
is shown in the Typical Application section. Suggested
programming resistors for various switching frequencies
are shown in Table 1.
Choosing a high switching frequency will allow the smallest
overall solution size. However, at high input voltages the
efficiency can drop significantly with increasing switching
frequency. The choice of switching frequency will also
impact the input voltage range, inductor and capacitor
selection, and compensation. See the related sections
for details.
10
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
( ) L =
VOUT + VF
• 1.2MHz
f
where VF is the voltage drop of the catch diode (~0.4V) and L
is in μH. The inductor’s RMS current rating must be greater
than the maximum load current and its saturation current
should be at least 30% higher. For highest efficiency, the
series resistance (DCR) should be less than 0.1Ω. Table 2
lists several vendors and types that are suitable.
Table 2. Inductor Vendors
VENDOR
URL
Coilcraft
Murata
TDK
www.coilcraft
www.murata.com
www.component.tdk.com
Toko
www.toko.com
Sumida
www.sumida.com
PART SERIES
MSS7341
LQH55D
SLF7045
SLF10145
DC62CB
D63CB
D75C
D75F
CR54
CDRH74
CDRH6D38
CR75
TYPE
Shielded
Open
Shielded
Shielded
Shielded
Shielded
Shielded
Open
Open
Shielded
Shielded
Open
3508fb
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