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LM1084IS-ADJ 参数 Datasheet PDF下载

LM1084IS-ADJ图片预览
型号: LM1084IS-ADJ
PDF下载: 下载PDF文件 查看货源
内容描述: 5A低压差正稳压器 [5A Low Dropout Positive Regulators]
分类和应用: 稳压器
文件页数/大小: 13 页 / 748 K
品牌: NSC [ National Semiconductor ]
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adjustment terminal. The adjust pin can take a transient  
signal of 25V with respect to the output voltage without  
damaging the device.  
Application Note (Continued)  
Figure 3 shows a typical application using a fixed output  
regulator. Rt1 and Rt2 are the line resistances. VLOAD is less  
than the VOUT by the sum of the voltage drops along the line  
resistances. In this case, the load regulation seen at the  
RLOAD would be degraded from the data sheet specification.  
To improve this, the load should be tied directly to the output  
terminal on the positive side and directly tied to the ground  
terminal on the negative side.  
When an output capacitor is connected to a regulator and  
the input is shorted, the output capacitor will discharge into  
the output of the regulator. The discharge current depends  
on the value of the capacitor, the output voltage of the  
regulator, and rate of decrease of VIN. In the LM1084 regu-  
lator, the internal diode between the output and input pins  
can withstand microsecond surge currents of 10A to 20A.  
With an extremely large output capacitor (1000 µf), and  
with input instantaneously shorted to ground, the regulator  
could be damaged. In this case, an external diode is recom-  
mended between the output and input pins to protect the  
regulator, shown in Figure 5.  
10094618  
FIGURE 3. Typical Application using Fixed Output  
Regulator  
When the adjustable regulator is used (Figure 4), the best  
performance is obtained with the positive side of the resistor  
R1 tied directly to the output terminal of the regulator rather  
than near the load. This eliminates line drops from appearing  
effectively in series with the reference and degrading regu-  
lation. For example, a 5V regulator with 0.05resistance  
between the regulator and load will have a load regulation  
due to line resistance of 0.05x IL. If R1 (=125) is con-  
nected near the load the effective line resistance will be  
0.05(1 + R2/R1) or in this case, it is 4 times worse. In  
addition, the ground side of the resistor R2 can be returned  
near the ground of the load to provide remote ground sens-  
ing and improve load regulation.  
10094615  
FIGURE 5. Regulator with Protection Diode  
OVERLOAD RECOVERY  
Overload recovery refers to regulator’s ability to recover from  
a short circuited output. A key factor in the recovery process  
is the current limiting used to protect the output from drawing  
too much power. The current limiting circuit reduces the  
output current as the input to output differential increases.  
Refer to short circuit curve in the curve section.  
During normal start-up, the input to output differential is  
small since the output follows the input. But, if the output is  
shorted, then the recovery involves a large input to output  
differential. Sometimes during this condition the current lim-  
iting circuit is slow in recovering. If the limited current is too  
low to develop a voltage at the output, the voltage will  
stabilize at a lower level. Under these conditions it may be  
necessary to recycle the power of the regulator in order to  
get the smaller differential voltage and thus adequate start  
up conditions. Refer to curve section for the short circuit  
current vs. input differential voltage.  
10094619  
THERMAL CONSIDERATIONS  
FIGURE 4. Best Load Regulation using Adjustable  
Output Regulator  
ICs heats up when in operation, and power consumption is  
one factor in how hot it gets. The other factor is how well the  
heat is dissipated. Heat dissipation is predictable by knowing  
the thermal resistance between the IC and ambient (θJA).  
Thermal resistance has units of temperature per power (C/  
W). The higher the thermal resistance, the hotter the IC.  
PROTECTION DIODES  
Under normal operation, the LM1084 regulator does not  
need any protection diode. With the adjustable device, the  
internal resistance between the adjustment and output ter-  
minals limits the current. No diode is needed to divert the  
current around the regulator even with a capacitor on the  
The LM1084 specifies the thermal resistance for each pack-  
age as junction to case (θJC). In order to get the total  
resistance to ambient (θJA), two other thermal resistance  
www.national.com  
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