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AAT3170_08 参数 Datasheet PDF下载

AAT3170_08图片预览
型号: AAT3170_08
PDF下载: 下载PDF文件 查看货源
内容描述: 高电流LED闪光灯驱动器电荷泵IC [High Current LED Flash Driver Charge Pump IC]
分类和应用: 驱动器闪光灯
文件页数/大小: 15 页 / 372 K
品牌: ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
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PRODUCT DATASHEET  
AAT3170  
ChargePumpTM  
High Current LED Flash Driver Charge Pump IC  
When enabling constant voltage mode, the LED channels  
F1 and F2 are automatically disabled so current does not  
flow through the flash LED during constant voltage  
mode. Because of this, very little programming is  
required. The user just addresses the voltage mode reg-  
ister and programs it to voltage mode.  
In 1.5X mode, the output voltage can be boosted to 3/2  
the input voltage. The 3/2 conversion ratio introduces a  
corresponding 1/2 increase in input current. For ideal  
conversion, the 1.5X mode efficiency is given by:  
VF · ILED  
VF  
η =  
=
VIN · 1.5IIN 1.5 · VIN  
LED Selection  
Similarly, when the input falls further, such that 1.5X  
mode can no longer sustain LED current, the device will  
automatically switch to 2X mode. In 2X mode, the out-  
put voltage can be boosted to twice the input voltage.  
The doubling conversion ratio introduces a correspond-  
ing doubling of the input current. For ideal conversion,  
the 2X mode efficiency is given by:  
The AAT3170 is designed to drive high-intensity white  
LEDs. It is particularly suitable for LEDs with an operat-  
ing forward-voltage in the range of 4.2V to 1.5V.  
The charge pump device can also drive other loads that  
have similar characteristics to white LEDs. For various  
load types, the AAT3170 provides a high-current, pro-  
grammable ideal constant current source.  
VF · ILED  
VIN · 2IIN  
VF  
Device Power Efficiency  
η =  
=
2 · VIN  
The AAT3170 power conversion efficiency depends on  
the charge pump mode. By definition, device efficiency  
is expressed as the output power delivered to the LED  
divided by the total input power consumed.  
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 equiva-  
lent series resistance (ESR) (<100mΩ) ceramic capaci-  
tors are used. A value of 1μF for the flying capacitors is  
a good starting point when choosing capacitors. If the  
LED current sinks are only programmed for light current  
levels, then the capacitor size may be decreased.  
POUT  
η =  
PIN  
When the input voltage is sufficiently greater than the  
LED forward voltage, the device optimizes efficiency by  
operating in 1X mode. In 1X mode, the device is working  
as a bypass switch and passing the input supply directly  
to the output. The power conversion efficiency can be  
approximated by:  
Ceramic composition capacitors are highly recommended  
over all other types of capacitors for use with the  
AAT3170. Ceramic capacitors offer many advantages  
over their tantalum and aluminum electrolytic counter-  
parts. A ceramic capacitor typically 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.  
VF · ILED VF  
η =  
VIN · IIN  
VIN  
Due to the very low 1X mode quiescent current, the  
input current nearly equals the current delivered to the  
LED. Further, the low-impedance bypass switch intro-  
duces negligible voltage drop from input to output.  
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  
milliohms for ceramic capacitors and can range to more  
than several ohms for tantalum or aluminum electrolytic  
capacitors.  
The AAT3170 further maintains optimized performance  
and efficiency by detecting when the input voltage is not  
sufficient to sustain LED current. The device automati-  
cally switches to 1.5X mode when the input voltage  
drops too low in relation to the LED forward voltage.  
w w w . a n a l o g i c t e c h . c o m  
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3170.2007.11.1.2