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LT1373CS8 参数 Datasheet PDF下载

LT1373CS8图片预览
型号: LT1373CS8
PDF下载: 下载PDF文件 查看货源
内容描述: 250kHz的低电源电流高效率1.5A开关稳压器 [250kHz Low Supply Current High Efficiency 1.5A Switching Regulator]
分类和应用: 稳压器开关光电二极管
文件页数/大小: 12 页 / 184 K
品牌: Linear [ Linear ]
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LT1373  
W U U  
U
APPLICATIO S I FOR ATIO  
inductor gets too hot, wire insulation will melt and cause  
turn-to-turn shorts). Keep in mind that all good things  
like high efficiency, low profile and high temperature  
operation will increase cost, sometimes dramatically.  
component height, output voltage ripple, EMI, fault cur-  
rent in the inductor, saturation, and of course, cost. The  
following procedure is suggested as a way of handling  
thesesomewhatcomplicatedandconflictingrequirements.  
1. Assume that the average inductor current (for a boost  
converter) is equal to load current times VOUT/VIN and  
decide whether or not the inductor must withstand  
continuous overload conditions. If average inductor  
current at maximum load current is 0.5A, for instance,  
a 0.5A inductor may not survive a continuous 1.5A  
overload condition. Also, be aware that boost convert-  
ers are not short-circuit protected, and that under  
outputshortconditions, inductorcurrentislimitedonly  
by the available current of the input supply.  
5. After making an initial choice, consider the secondary  
things like output voltage ripple, second sourcing, etc.  
Use the experts in the Linear Technology application  
department if you feel uncertain about the final choice.  
They have experience with a wide range of inductor  
types and can tell you about the latest developments in  
low profile, surface mounting, etc.  
Output Capacitor  
The output capacitor is normally chosen by its effective  
series resistance (ESR), because this is what determines  
output ripple voltage. At 500kHz, any polarized capacitor  
is essentially resistive. To get low ESR takes volume, so  
physically smaller capacitors have high ESR. The ESR  
range for typical LT1373 applications is 0.05to 0.5. A  
typical output capacitor is an AVX type TPS, 22µF at 25V,  
with a guaranteed ESR less than 0.2. This is a “D” size  
surface mount solid tantalum capacitor. TPS capacitors  
are specially constructed and tested for low ESR, so they  
give the lowest ESR for a given volume. To further reduce  
ESR, multiple output capacitors can be used in parallel.  
The value in microfarads is not particularly critical and  
values from 22µF to greater than 500µF work well, but you  
cannot cheat mother nature on ESR. If you find a tiny 22µF  
solid tantalum capacitor, it will have high ESR and output  
ripple voltage will be terrible. Table 1 shows some typical  
solid tantalum surface mount capacitors.  
2. Calculate peak inductor current at full load current to  
ensure that the inductor will not saturate. Peak current  
can be significantly higher than output current, espe-  
cially with smaller inductors and lighter loads, so don’t  
omit this step. Powered iron cores are forgiving be-  
cause they saturate softly, whereas ferrite cores satu-  
rate abruptly. Other core materials fall in between  
somewhere. The following formula assumes continu-  
ous mode operation, but it errors only slightly on the  
high side for discontinuous mode, so it can be used for  
all conditions.  
V
V
V (V  
2(f)(L)(V  
– V )  
OUT  
IN OUT  
IN  
I
= I  
+
PEAK  
OUT  
)
IN  
OUT  
V = minimum input voltage  
IN  
f = 250kHz switching frequency  
3. Decide if the design can tolerate an “open” core geom-  
etry like a rod or barrel, which have high magnetic field  
radiation, or whether it needs a closed core like a toroid  
to prevent EMI problems. One would not want an open  
core next to a magnetic storage media for instance!  
This is a tough decision because the rods or barrels are  
temptingly cheap and small, and there are no helpful  
guidelines to calculate when the magnetic field radia-  
tion will be a problem.  
Table 1. Surface Mount Solid Tantalum Capacitor  
ESR and Ripple Current  
E CASE SIZE  
ESR (MAX )  
RIPPLE CURRENT (A)  
AVX TPS, Sprague 593D  
AVX TAJ  
0.1 to 0.3  
0.7 to 0.9  
0.7 to 1.1  
0.4  
D CASE SIZE  
AVX TPS, Sprague 593D  
AVX TAJ  
0.1 to 0.3  
0.9 to 2.0  
0.7 to 1.1  
0.36 to 0.24  
C CASE SIZE  
AVX TPS  
AVX TAJ  
0.2 (Typ)  
1.8 to 3.0  
0.5 (Typ)  
4. Start shopping for an inductor which meets the require-  
ments of core shape, peak current (to avoid saturation),  
averagecurrent(tolimitheating),andfaultcurrent,(ifthe  
0.22 to 0.17  
B CASE SIZE  
AVX TAJ  
2.5 to 10  
0.16 to 0.08  
8