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LT1109CS8-12 参数 Datasheet PDF下载

LT1109CS8-12图片预览
型号: LT1109CS8-12
PDF下载: 下载PDF文件 查看货源
内容描述: 微低成本DC / DC转换器可调节和固定5V , 12V [Micropower Low Cost DC/DC Converter Adjustable and Fixed 5V, 12V]
分类和应用: 转换器开关光电二极管
文件页数/大小: 8 页 / 214 K
品牌: Linear [ Linear ]
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LT1109  
U
U
U
LT1109 S8 A D 8  
OPERATIO  
reduced by using the 8-pin version since the quiescent  
current flows from a lower voltage source. The SHUT-  
DOWNpindisablestheoscillatorwhentakentoalogic0.”  
If left floating or tied high, the converter operates nor-  
mally. With SHUTDOWN low, quiescent current remains  
at 320µA.  
The 8-pin versions of the LT1109 have separate pins for  
VIN and SENSE or FB and also have a SHUTDOWN pin.  
Separating the device VIN pin from the SENSE pin allows  
the device to be powered from the (lower) input voltage  
rather than the (higher) output voltage. Although quies-  
cent current remains constant, quiescent power will be  
O U  
W
U
PPLICATI  
A
S I FOR ATIO  
Inductor Selection  
where VD is the diode drop (0.5V for a 1N5818 Schottky).  
Energy required by the inductor per cycle must be equal or  
greater than  
A DC/DC converter operates by storing energy as mag-  
netic flux in an inductor core, and then switching this  
energy into the load. To operate as an efficient energy  
transfer element, the inductor must fulfill three require-  
ments. First, the inductance must be low enough for the  
inductor to store adequate energy under the worst case  
condition of minimum input voltage and switch-ON time.  
The inductance must also be high enough so that maxi-  
mum current ratings of the LT1109 and inductor are not  
exceeded at the other worst case condition of maximum  
input voltage and ON time. Additionally, the inductor core  
must be able to store the required flux; i.e., it must not  
saturate. At power levels generally encountered with  
LT1109 designs, small ferrite surface-mount inductors  
will function well. Lastly, the inductor must have suffi-  
ciently low DC resistance so that excessive power is not  
lost as heat in the windings. Look for DCR values in the  
inductorsspecificationtables;valuesunder0.5willgive  
best efficiency. An additional consideration is Electro-  
Magnetic Interference (EMI). Toroid and pot core type  
inductors are recommended in applications where EMI  
must be kept to a minimum; for example, where there are  
sensitiveanalogcircuitryor transducersnearby.Rodcore  
types are a less expensive choice where EMI is not a  
problem.  
P
L
02  
( )  
F
OSC  
in order for the converter to regulate the output.  
When the switch is closed, current in the inductor builds  
according to  
–R't  
L
V
R'  
IN  
I t =  
1– e  
03  
( )  
( )  
L
where R' is the sum of the switch equivalent resistance  
(0.8typicalat25°C)andtheinductorDCresistance. When  
the drop across the switch is small compared to VIN, the  
simple lossless equation  
V
L
IN  
I t =  
t
04  
( )  
( )  
L
can be used. These equations assume that at t = 0,  
inductor current is zero. This situation is called “discon-  
tinuous mode operation” in switching regulator parlance.  
Setting “t” to the switch-ON time from the LT1109 speci-  
fication table (typically 4.2µs) will yield IPEAK for a specific  
“L” and VIN. Once IPEAK is known, energy in the inductor  
at the end of the switch-ON time can be calculated as  
Specifying a proper inductor for an application requires  
first establishing minimum and maximum input voltage,  
outputvoltage,andoutputcurrent.Ina step-upconverter,  
theinductiveeventsaddtotheinputvoltagetoproducethe  
output voltage. Power required from the inductor is deter-  
mined by  
1
2
2
E = LI  
05  
( )  
L
PEAK  
EL mustbegreaterthanPL/FOSC fortheconvertertodeliver  
the required power. For best efficiency IPEAK should be  
PL = (VOUT + VD – VIN) (IOUT  
)
(01)  
5