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LT1940EFE#PBF 参数 Datasheet PDF下载

LT1940EFE#PBF图片预览
型号: LT1940EFE#PBF
PDF下载: 下载PDF文件 查看货源
内容描述: [LT1940 - Dual Monolithic 1.4A, 1.1MHz Step-Down Switching Regulator; Package: TSSOP; Pins: 16; Temperature Range: -40°C to 85°C]
分类和应用: 开关光电二极管
文件页数/大小: 20 页 / 309 K
品牌: Linear [ Linear ]
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LT1940/LT1940L  
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W U U  
APPLICATIO S I FOR ATIO  
is likely to see high surge currents when the input source  
is applied, tantalum capacitors should be surge rated. The  
manufacturer may also recommend operation below the  
rated voltage of the capacitor. Be sure to place the 1µF  
ceramic as close as possible to the VIN and GND pins on  
the IC for optimal noise immunity.  
where IL is the peak-to-peak ripple current in the induc-  
tor. The RMS content of this ripple is very low, and the  
RMS current rating of the output capacitor is usually not  
of concern.  
Another constraint on the output capacitor is that it must  
havegreaterenergystoragethantheinductor;ifthestored  
energy in the inductor is transferred to the output, you  
would like the resulting voltage step to be small compared  
totheregulationvoltage. Fora5%overshoot, thisrequire-  
ment becomes COUT > 10L(ILIM/VOUT)^2.  
A final caution is in order regarding the use of ceramic  
capacitors at the input. A ceramic input capacitor can  
combine with stray inductance to form a resonant tank  
circuit. If power is applied quickly (for example by plug-  
ging the circuit into a live power source) this tank can ring,  
doubling the input voltage and damaging the LT1940. The  
solution is to either clamp the input voltage or dampen the  
tank circuit by adding a lossy capacitor in parallel with the  
ceramic capacitor. For details, see AN88.  
Finally, there must be enough capacitance for good tran-  
sient performance. The last equation gives a good starting  
point. Alternatively, you can start with one of the designs  
in this data sheet and experiment to get the desired  
performance. This topic is covered more thoroughly in the  
section on loop compensation.  
Output Capacitor Selection  
The high performance (low ESR), small size and robust-  
ness of ceramic capacitors make them the preferred type  
for LT1940 applications. However, all ceramic capacitors  
are not the same. As mentioned above, many of the higher  
value capacitors use poor dielectrics with high tempera-  
ture and voltage coefficients. In particular, Y5V and Z5U  
typeslosealargefractionoftheircapacitancewithapplied  
voltage and temperature extremes. Because the loop  
stability and transient response depend on the value of  
COUT, you may not be able to tolerate this loss. Use X7R  
and X5R types.  
For 5V and 3.3V outputs with greater than 1A output, a  
10µF 6.3V ceramic capacitor (X5R or X7R) at the output  
resultsinverylowoutputvoltagerippleandgoodtransient  
response. For lower voltages, 10µF is adequate but in-  
creasing COUT to 15µF or 22µF will improve transient  
performance. Other types and values can be used; the  
following discusses tradeoffs in output ripple and tran-  
sient performance.  
The output capacitor filters the inductor current to gener-  
ate an output with low voltage ripple. It also stores energy  
inordersatisfytransientloadsandtostabilizetheLT1940’s  
control loop. Because the LT1940 operates at a high  
frequency, youdon’tneedmuchoutputcapacitance. Also,  
the current mode control loop doesn’t require the pres-  
enceofoutputcapacitorseriesresistance(ESR).Forthese  
reasons, you are free to use ceramic capacitors to achieve  
very low output ripple and small circuit size.  
Youcanalsouseelectrolyticcapacitors. TheESRsofmost  
aluminum electrolytics are too large to deliver low output  
ripple. Tantalum and newer, lower ESR organic electro-  
lytic capacitors intended for power supply use are suit-  
able, and the manufacturers will specify the ESR. The  
choiceofcapacitorvaluewillbebasedontheESRrequired  
for low ripple. Because the volume of the capacitor deter-  
mines its ESR, both the size and the value will be larger  
than a ceramic capacitor that would give you similar ripple  
performance. One benefit is that the larger capacitance  
may give better transient response for large changes in  
load current. Table 2 lists several capacitor vendors.  
Estimate output ripple with the following equations:  
VRIPPLE = IL/(8f COUT) for ceramic capacitors, and  
VRIPPLE = IL ESR for electrolytic capacitors (tantalum  
and aluminum);  
1940fa  
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