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AAT3151B_08 参数 Datasheet PDF下载

AAT3151B_08图片预览
型号: AAT3151B_08
PDF下载: 下载PDF文件 查看货源
内容描述: 高效率1X / 1.5X电荷泵为白光LED应用 [High Efficiency 1X/1.5X Charge Pump For White LED Applications]
分类和应用:
文件页数/大小: 13 页 / 291 K
品牌: ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
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PRODUCT DATASHEET  
AAT3151B  
TM  
ChargePump  
High Efficiency 1X/1.5X Charge Pump For White LED Applications  
Charge Pump Section Efficiency  
Capacitor Characteristics  
The AAT3151B contains a fractional charge pump that  
will boost the input supply voltage when VIN is less than  
the voltage required on the constant current sink inputs.  
The efficiency (η) can be simply defined as a linear volt-  
age regulator with an effective output voltage that is  
equal to one and one half or two times the input voltage.  
Efficiency (η) for an ideal 1.5X charge pump can typi-  
cally be expressed as the output power divided by the  
input power.  
Ceramic composition capacitors are highly recommended  
over all other types of capacitors for use with the  
AAT3151B. Ceramic capacitors offer many advantages  
over their tantalum and aluminum electrolytic counter-  
parts. A ceramic capacitor has very low ESR, is lowest  
cost, has a smaller PCB footprint, and is non-polarized.  
Low ESR ceramic capacitors help maximize charge pump  
transient response. Since ceramic capacitors are non-  
polarized, they are not prone to incorrect connection  
damage.  
POUT  
PIN  
η =  
Equivalent Series Resistance  
ESR is an important characteristic to consider when  
selecting a capacitor. ESR is a resistance internal to a  
capacitor that is caused by the leads, internal connec-  
tions, size or area, material composition, and ambient  
temperature. Capacitor ESR is typically measured in mil-  
liohms for ceramic capacitors and can range to more  
than several ohms for tantalum or aluminum electrolytic  
capacitors.  
In addition, with an ideal 1.5X charge pump, the output  
current may be expressed as 2/3 of the input current.  
The expression to define the ideal efficiency (η) can be  
rewritten as:  
POUT  
PIN  
VOUT · IOUT  
VOUT  
η =  
=
=
VIN · 1.5IOUT  
1.5VIN  
Ceramic Capacitor Materials  
-or-  
Ceramic capacitors less than 0.1μF are typically made  
from NPO or C0G materials. NPO and C0G materials gen-  
erally have tight tolerance and are very stable over tem-  
perature. Larger capacitor values are usually composed  
of X7R, X5R, Z5U, or Y5V dielectric materials. Large  
ceramic capacitors (i.e., greater than 2.2μF) are often  
available in low-cost Y5V and Z5U dielectrics, but capac-  
itors greater than 1μF are not typically required for  
AAT3151B applications.  
VOUT  
η(%) = 100  
1.5V  
IN  
Capacitor Selection  
Careful selection of the four external capacitors CIN, C1,  
C2, and COUT is important because they will affect turn-on  
time, output ripple, and transient performance. Optimum  
performance will be obtained when low equivalent series  
resistance (ESR) ceramic capacitors are used. In gener-  
al, low ESR may be defined as less than 100mΩ. A value  
of 1μF for all four capacitors is a good starting point  
when choosing capacitors. If the constant current sinks  
are only programmed for light current levels, then the  
capacitor size may be decreased.  
Capacitor area is another contributor to ESR. Capacitors  
that are physically large will have a lower ESR when  
compared to an equivalent material smaller capacitor.  
These larger devices can improve circuit transient  
response when compared to an equal value capacitor in  
a smaller package size.  
w w w . a n a l o g i c t e c h . c o m  
12  
3151B.2008.02.1.1