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LTC1624IS8 参数 Datasheet PDF下载

LTC1624IS8图片预览
型号: LTC1624IS8
PDF下载: 下载PDF文件 查看货源
内容描述: 高英法fi效率的SO-8 N沟道开关稳压器控制器 [High Efficiency SO-8 N-Channel Switching Regulator Controller]
分类和应用: 稳压器开关式稳压器或控制器电源电路开关式控制器
文件页数/大小: 28 页 / 493 K
品牌: Linear [ Linear ]
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LTC1624  
U
W U U  
APPLICATIONS INFORMATION  
The LTC1624 can be used in a wide variety of switching  
regulator applications, the most common being the step-  
down converter. Other switching regulator architectures  
includestep-up,SEPICandpositive-to-negativeconverters.  
Accepting larger values of IL allows the use of low  
inductances, but results in higher output voltage ripple  
and greater core losses. A reasonable starting point for  
setting ripple current is IL = 0.4(IMAX). Remember, the  
maximum IL occurs at the maximum input voltage.  
ThebasicLTC1624step-downapplicationcircuitisshown  
in Figure 1 on the first page. External component selection  
is driven by the load requirement and begins with the  
selection of RSENSE. Once RSENSE is known, the inductor  
can be chosen. Next, the power MOSFET and D1 are  
selected. Finally, CIN and COUT are selected. The circuit  
shown in Figure 1 can be configured for operation up to an  
input voltage of 28V (limited by the external MOSFETs).  
The inductor value also has an effect on low current  
operation. Lower inductor values (higher IL) will cause  
Burst Mode operation to begin at higher load currents,  
which can cause a dip in efficiency in the upper range of  
low current operation. In Burst Mode operation lower  
inductance values will cause the burst frequency to  
decrease. In general, inductor values from 5µH to 68µH  
are typical depending on the maximum input voltage and  
output current. See also Modifying Burst Mode Operation  
section.  
Step-Down Converter: RSENSE Selection for  
Output Current  
RSENSE is chosen based on the required output current.  
The LTC1624 current comparator has a maximum thresh-  
old of 160mV/RSENSE. The current comparator threshold  
sets the peak of the inductor current, yielding a maximum  
average output current IMAX equal to the peak value less  
half the peak-to-peak ripple current, IL.  
Step-Down Converter: Inductor Core Selection  
Once the value for L is known, the type of inductor must be  
selected. High efficiency converters generally cannot  
affordthecorelossfoundinlowcostpowderedironcores,  
forcing the use of more expensive ferrite, molypermalloy  
orKoolMµ® cores. Actualcorelossisindependentofcore  
size for a fixed inductor value, but it is very dependent on  
inductanceselected. Asinductanceincreases,corelosses  
go down. Unfortunately, increased inductance requires  
more turns of wire and, therefore, copper losses will  
increase.  
Allowing a margin for variations in the LTC1624 and  
external component values yields:  
100mV  
R
=
SENSE  
I
MAX  
The LTC1624 works well with values of RSENSE from  
0.005to 0.5.  
Ferrite designs have very low core loss and are preferred  
at high switching frequencies, so design goals can con-  
centrate on copper loss and preventing saturation. Ferrite  
core material saturates “hard,” which means that induc-  
tance collapses abruptly when the peak design current is  
exceeded. This results in an abrupt increase in inductor  
ripple current and consequent output voltage ripple. Do  
not allow the core to saturate!  
Step-Down Converter: Inductor Value Calculation  
With the operating frequency fixed at 200kHz smaller  
inductor values are favored. Operating at higher frequen-  
cies generally results in lower efficiency because of  
MOSFET gate charge losses. In addition to this basic  
trade-off, the effect of inductor value on ripple current and  
low current operation must also be considered.  
Molypermalloy (from Magnetics, Inc.) is a very good, low  
losscorematerialfortoroids,butitismoreexpensivethan  
ferrite. A reasonable compromise from the same manu-  
facturer is Kool Mµ. Toroids are very space efficient,  
especially when you can use several layers of wire.  
Because they generally lack a bobbin, mounting is more  
difficult. However, designs for surface mount that do not  
increase the height significantly are available.  
Theinductorvaluehasadirecteffectonripplecurrent.The  
inductor ripple current IL decreases with higher induc-  
tance and increases with higher VIN or VOUT  
:
V V  
V
+ V  
IN  
OUT OUT D  
I =  
L
V + V  
f L  
( )( )  
IN  
D
where VD is the output Schottky diode forward drop.  
Kool Mu is a registered trademark of Magnetics, Inc.  
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