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AAT2552_08 参数 Datasheet PDF下载

AAT2552_08图片预览
型号: AAT2552_08
PDF下载: 下载PDF文件 查看货源
内容描述: 用于便携式应用的总电源解决方案 [Total Power Solution for Portable Applications]
分类和应用: 便携式
文件页数/大小: 31 页 / 824 K
品牌: ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
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PRODUCT DATASHEET  
AAT2552178  
SystemPowerTM  
Total Power Solution for Portable Applications  
The maximum input capacitor RMS current is:  
the converter performance, a high ESR tantalum or alu-  
minum electrolytic capacitor should be placed in parallel  
with the low ESR, ESL bypass ceramic capacitor. This  
dampens the high Q network and stabilizes the system.  
The linear regulator and the step-down convertor share  
the same input capacitor on the evaluation board.  
VO  
VIN  
VO ⎞  
VIN ⎠  
1
2
· 1 -  
=
D · (1 - D) = 0.52 =  
The input capacitor RMS ripple current varies with the  
input and output voltage and will always be less than or  
equal to half of the total DC load current.  
Linear Regulator Output Capacitor (C5)  
For proper load voltage regulation and operational stabil-  
ity, a capacitor is required between OUT and GND. The  
COUT capacitor connection to the LDO regulator ground  
pin should be made as directly as practically possible for  
maximum device performance. Since the regulator has  
been designed to function with very low ESR capacitors,  
ceramic capacitors in the 1.0μF to 10μF range are rec-  
ommended for best performance. Applications utilizing  
the exceptionally low output noise and optimum power  
supply ripple rejection should use 2.2μF or greater for  
VO  
VIN  
VO ⎞  
VIN ⎠  
IRMS = IO ·  
· 1 -  
for VIN = 2 · VO  
IO  
IRMS(MAX)  
=
2
COUT. In low output current applications, where output  
load is less than 10mA, the minimum value for COUT can  
be as low as 0.47μF.  
The term appears in both the input voltage ripple and  
input capacitor RMS current equations and is a maxi-  
mum when VO is twice VIN. This is why the input voltage  
ripple and the input capacitor RMS current ripple are a  
maximum at 50% duty cycle.  
Battery Charger Output Capacitor (C2)  
The battery charger of the AAT2552 only requires a 1μF  
ceramic capacitor on the BAT pin to maintain circuit stabil-  
ity. This value should be increased to 10μF or more if the  
battery connection is made any distance from the charger  
output. If the AAT2552 is to be used in applications where  
the battery can be removed from the charger, such as  
with desktop charging cradles, an output capacitor great-  
er than 10μF may be required to prevent the device from  
cycling on and off when no battery is present.  
The input capacitor provides a low impedance loop for the  
edges of pulsed current drawn by the step-down con-  
verter. Low ESR/ESL X7R and X5R ceramic capacitors are  
ideal for this function. To minimize stray inductance, the  
capacitor should be placed as closely as possible to the IC.  
This keeps the high frequency content of the input current  
localized, minimizing EMI and input voltage ripple.  
The proper placement of the input capacitor (C6) can be  
seen in the evaluation board layout in Figure 7.  
Step-Down Converter Output Capacitor (C3)  
The output capacitor limits the output ripple and pro-  
vides holdup during large load transitions. A 4.7μF to  
10μF X5R or X7R ceramic capacitor typically provides  
sufficient bulk capacitance to stabilize the output during  
large load transitions and has the ESR and ESL charac-  
teristics necessary for low output ripple. For enhanced  
transient response and low temperature operation appli-  
cations, a 10μF (X5R, X7R) ceramic capacitor is recom-  
mended to stabilize extreme pulsed load conditions.  
A laboratory test set-up typically consists of two long  
wires running from the bench power supply to the evalu-  
ation board input voltage pins. The inductance of these  
wires, along with the low-ESR ceramic input capacitor,  
can create a high Q network that may affect converter  
performance. This problem often becomes apparent in  
the form of excessive ringing in the output voltage dur-  
ing load transients. Errors in the loop phase and gain  
measurements can also result.  
The output voltage droop due to a load transient is dom-  
inated by the capacitance of the ceramic output capacitor.  
During a step increase in load current, the ceramic output  
capacitor alone supplies the load current until the loop  
responds. Within two or three switching cycles, the loop  
responds and the inductor current increases to match the  
load current demand. The relationship of the output volt-  
Since the inductance of a short PCB trace feeding the  
input voltage is significantly lower than the power leads  
from the bench power supply, most applications do not  
exhibit this problem.  
In applications where the input power source lead induc-  
tance cannot be reduced to a level that does not affect  
w w w . a n a l o g i c t e c h . c o m  
2552.2008.02.1.2  
21  
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