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AAT2120_08 参数 Datasheet PDF下载

AAT2120_08图片预览
型号: AAT2120_08
PDF下载: 下载PDF文件 查看货源
内容描述: 500毫安低噪声降压转换器 [500mA Low Noise Step-Down Converter]
分类和应用: 转换器
文件页数/大小: 19 页 / 679 K
品牌: ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
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PRODUCT DATASHEET  
AAT2120  
SwitchRegTM  
500mA Low Noise Step-Down Converter  
transient response and low temperature operation appli-  
cation, a 10μF (X5R, X7R) ceramic capacitor is recom-  
mended to stabilize extreme pulsed load conditions.  
With enhanced transient response for extreme pulsed  
load application, an external feed-forward capacitor, (C3  
in Figure 1), can be added.  
The output voltage droop due to a load transient is domi-  
nated 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-  
age droop during the three switching cycles to the output  
capacitance can be estimated by:  
R2 = 59kΩ  
R1 (kΩ)  
R2 = 221kΩ  
R1 (kΩ)  
VOUT (V)  
0.8  
0.9  
1.0  
1.1  
1.2  
1.3  
1.4  
1.5  
1.8  
1.85  
2.0  
2.5  
3.3  
19.6  
29.4  
39.2  
49.9  
59.0  
68.1  
78.7  
88.7  
118  
124  
137  
187  
267  
75  
113  
150  
187  
221  
261  
301  
332  
442  
464  
523  
715  
1000  
3 · ΔILOAD  
=
COUT  
V
DROOP · FS  
Once the average inductor current increases to the DC  
load level, the output voltage recovers. The above equa-  
tion establishes a limit on the minimum value for the  
output capacitor with respect to load transients.  
Table 2: Adjustable Resistor Values for  
Step-Down Converter.  
The internal voltage loop compensation also limits the  
minimum output capacitor value to 4.7μF. This is due to  
its effect on the loop crossover frequency (bandwidth),  
phase margin, and gain margin. Increased output capac-  
itance will reduce the crossover frequency with greater  
phase margin.  
Thermal Calculations  
There are three types of losses associated with the  
AAT2120 step-down converter: switching losses, conduc-  
tion losses, and quiescent current losses. Conduction  
losses are associated with the RDS(ON) characteristics of the  
power output switching devices. Switching losses are  
dominated by the gate charge of the power output switch-  
ing devices. At full load, assuming continuous conduction  
mode (CCM), a simplified form of the losses is given by:  
The maximum output capacitor RMS ripple current is  
given by:  
1
V
OUT · (VIN(MAX) - VOUT  
)
IRMS(MAX)  
=
·
L · FS · VIN(MAX)  
2 · 3  
IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO])  
Dissipation due to the RMS current in the ceramic output  
capacitor ESR is typically minimal, resulting in less than  
a few degrees rise in hot-spot temperature.  
PTOTAL  
=
VIN  
+ (tsw · FS · IO + IQ) · VIN  
Adjustable Output Resistor Selection  
IQ is the step-down converter quiescent current. The  
term tsw is used to estimate the full load step-down con-  
verter switching losses.  
Resistors R1 and R2 of Figure 1 program the output to  
regulate at a voltage higher than 0.6V. To limit the bias  
current required for the external feedback resistor string  
while maintaining good noise immunity, the suggested  
value for R2 is 59kΩ. Decreased resistor values are nec-  
essary to maintain noise immunity on the FB pin, result-  
ing in increased quiescent current. Table 2 summarizes  
the resistor values for various output voltages.  
For the condition where the step-down converter is in  
dropout at 100% duty cycle, the total device dissipation  
reduces to:  
PTOTAL = IO2 · RDSON(H) + IQ · VIN  
Since RDS(ON), quiescent current, and switching losses all  
vary with input voltage, the total losses should be inves-  
tigated over the complete input voltage range.  
V
V
3.3V  
0.6V  
R1 =  
OUT -1 · R2 =  
- 1 · 59kΩ = 267kΩ  
REF  
w w w . a n a l o g i c t e c h . c o m  
12  
2120.2008.02.1.2