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AAT2688IFK-AI-T1 参数 Datasheet PDF下载

AAT2688IFK-AI-T1图片预览
型号: AAT2688IFK-AI-T1
PDF下载: 下载PDF文件 查看货源
内容描述: 对于12V适配器系统4.5A PMIC解决方案有2个输出高性能降压型转换器 [4.5A PMIC Solution for 12V Adapter Systems with 2-Output High Performance Step-Down Converters]
分类和应用: 转换器集成电源管理电路
文件页数/大小: 20 页 / 3687 K
品牌: ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
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PRODUCT DATASHEET  
AAT2688  
TM  
SystemPower  
4.5A PMIC Solution for 12V Adapter Systems with 2-Output High Performance Step-Down Converters  
Channel 1 Regulator  
Output Capacitor Selection  
Applications Information  
Output 1 is a high voltage DC/DC step-down converter  
providing an output voltage from 0.8V to 5.5V. The inte-  
grated high-side N-channel MOSFET device provides up  
to 4.5A output current. Input voltage range is 6.0V to  
24.0V. The step-down converter utilizes constant fre-  
quency (PWM-mode) voltage mode control to achieve  
high operating efficiency while maintaining extremely  
low output noise across the operating range. High  
490kHz (nominal) switching frequency allows small  
external filtering components, achieving minimum cost  
and solution size. External compensation and an option-  
al feed forward capacitor allows the designer to optimize  
the transient response while achieving stability across  
the operating range.  
Three 22μF ceramic output capacitors are required to  
filter the inductor current ripple and supply the load  
transient current for IOUT = 4.5A. The 1206 package with  
10V minimum voltage rating is recommended for the  
output capacitors to maintain a minimum capacitance  
drop with DC bias.  
Channel 1 Output Inductor Selection  
The step-down converter utilizes constant frequency  
(PWM-mode) voltage mode control. A 4.7H inductor  
value is selected to maintain the desired output current  
ripple and minimize the converter’s response time to  
load transients. The peak switch current should not  
exceed the inductor saturation current or the MOSFETs.  
Output 2 is a low voltage low dropout (LDO) linear regu-  
lator providing 1.8V with up to 600mA output current.  
The input voltage range is 2.7V to 5.5V. The LDO pro-  
vides very low noise output which can be derived direct-  
ly from Output 1.  
Channel 1 MOSFET Selection  
The step-down (buck) converter utilizes synchronous  
rectification (Q1) for constant frequency (PWM mode)  
voltage mode control. The synchronous rectifier is select-  
ed based on the desired RDS(ON) value and QG (total gate  
charge), these two critical parameters are weighed  
against each other. To get a low RDS(ON) value, the MOSFET  
must be of a very large size and a larger MOSFET will  
have a large QG. Conversely to get a low QG, the MOSFET  
must be small and thus have a large RDS(ON) value. In  
addition to the trade off between RDS(ON) and QG, the  
maximum voltage rating for the external synchronous  
MOSFET must exceed the maximum application input  
voltage value (VDS[MAX] > VIN[MAX]).  
Output Voltage—Channel 11  
The output voltage is set using an external resistor  
divider as shown in Table 1. Minimum output voltage is  
0.8V and maximum output voltage is 5.5V. Typical max-  
imum duty cycle is 85%. Example: with R4 = 1.96K,  
(VOUT - 0.585) · R4  
R3 =  
0.585  
1
VOUT (V)  
R3 (kΩ)  
The QG affects the turn-on/turn-off time of the synchro-  
nous MOSFET, the longer the turn-on/turn-off time the  
more likely the step-down converter will have “shoot  
through” current issues. “Shoot through” current occurs  
when the AAT2688 internal top-side MOSFET and the  
external synchronous MOSFET are conducting current at  
the same time. This will result in a low impedance path  
to ground from the input voltage through the two  
MOSFETs, and the current may exceed the maximum  
current rating of the AAT2688 and external synchronous  
MOSFET. Exceeding the maximum current ratings will  
lead to the destructive derating of the AAT2688 and  
external synchronous MOSFET.  
0.8  
1.0  
1.2  
1.5  
1.8  
2.0  
2.5  
3.0  
3.3  
5.0  
0.715  
1.37  
2.05  
3.09  
4.02  
4.75  
6.49  
8.06  
9.09  
14.7  
Table 1: External Resistor Values (Standard 1%  
Resistors are Substituted for Calculated Values).  
1. The R3 and R4 feedback resistors are separate from the compensation network. When changing either R3 and/or R4, the compensation network will have to be altered. Contact  
the Applications Engineering department for compensation network recommendations for specific output voltages.  
w w w . a n a l o g i c t e c h . c o m  
2688.2008.06.1.0  
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