欢迎访问ic37.com |
会员登录 免费注册
发布采购

2576 参数 Datasheet PDF下载

2576图片预览
型号: 2576
PDF下载: 下载PDF文件 查看货源
内容描述: 5安培可调稳压器 [5-Amp Adjustable Regulators]
分类和应用: 稳压器
文件页数/大小: 16 页 / 399 K
品牌: NSC [ National Semiconductor ]
 浏览型号2576的Datasheet PDF文件第1页浏览型号2576的Datasheet PDF文件第2页浏览型号2576的Datasheet PDF文件第3页浏览型号2576的Datasheet PDF文件第4页浏览型号2576的Datasheet PDF文件第6页浏览型号2576的Datasheet PDF文件第7页浏览型号2576的Datasheet PDF文件第8页浏览型号2576的Datasheet PDF文件第9页  
Load Regulation  
Application Hints (Continued)  
The LM138 is capable of providing extremely good load  
regulation but a few precautions are needed to obtain maxi-  
mum performance. The current set resistor connected be-  
tween the adjustment terminal and the output terminal (usu-  
ally 240) should be tied directly to the output of the  
regulator (case) rather than near the load. This eliminates  
line drops from appearing effectively in series with the refer-  
ence and degrading regulation. For example, a 15V regula-  
tor with 0.05resistance between the regulator and load will  
have a load regulation due to line resistance of 0.05x IL. If  
the set resistor is connected near the load the effective line  
resistance will be 0.05(1 + R2/R1) or in this case, 11.5  
times worse.  
Figure 2 shows the effect of resistance between the regula-  
tor and 240set resistor.  
DS009060-6  
FIGURE 1.  
Since the 50 µA current from the adjustment terminal repre-  
sents an error term, the LM138 was designed to minimize  
IADJ and make it very constant with line and load changes.  
To do this, all quiescent operating current is returned to the  
output establishing a minimum load current requirement. If  
there is insufficient load on the output, the output will rise.  
External Capacitors  
An input bypass capacitor is recommended. A 0.1 µF disc or  
1 µF solid tantalum on the input is suitable input bypassing  
for almost all applications. The device is more sensitive to  
the absence of input bypassiing when adjustment or output  
capacitors are used but the above values will eliminate the  
possiblity of problems.  
DS009060-7  
FIGURE 2. Regulator with Line  
Resistance in Output Lead  
With the TO-3 package, it is easy to minimize the resistance  
from the case to the set resistor, by using 2 separate leads to  
the case. The ground of R2 can be returned near the ground  
of the load to provide remote ground sensing and improve  
load regulation.  
The adjustment terminal can be bypassed to ground on the  
LM138 to improve ripple rejection. This bypass capacitor  
prevents ripple from being amplified as the output voltage is  
increased. With a 10 µF bypass capacitor 75 dB ripple rejec-  
tion is obtainable at any output level. Increases over 20 µF  
do not appreciably improve the ripple rejection at frequen-  
cies above 120 Hz. If the bypass capacitor is used, it is  
sometimes necessary to include protection diodes to prevent  
the capacitor from discharging through internal low current  
paths and damaging the device.  
Protection Diodes  
When external capacitors are used with any IC regulator it is  
sometimes necessary to add protection diodes to prevent  
the capacitors from discharging through low current points  
into the regulator. Most 20 µF capacitors have low enough  
internal series resistance to deliver 20A spikes when  
shorted. Although the surge is short, there is enough energy  
to damage parts of the IC.  
In general, the best type of capacitors to use are solid tanta-  
lum. Solid tantalum capacitors have low impedance even at  
high frequencies. Depending upon capacitor construction, it  
takes about 25 µF in aluminum electrolytic to equal 1 µF  
solid tantalum at high frequencies. Ceramic capacitors are  
also good at high frequencies; but some types have a large  
decrease in capacitance at frequencies around 0.5 MHz. For  
this reason, 0.01 µF disc may seem to work better than a  
0.1 µF disc as a bypass.  
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 regu-  
lator, and the rate of decrease of VIN. In the LM138 this dis-  
charge path is through a large junction that is able to sustain  
25A surge with no problem. This is not true of other types of  
positive regulators. For output capacitors of 100 µF or less at  
output of 15V or less, there is no need to use diodes.  
Although the LM138 is stable with no output capacitors, like  
any feedback circuit, certain values of external capacitance  
can cause excessive ringing. This occurs with values be-  
tween 500 pF and 5000 pF. A 1 µF solid tantalum (or 25 µF  
aluminum electrolytic) on the output swamps this effect and  
insures stability.  
The bypass capacitor on the adjustment terminal can dis-  
charge through a low current junction. Discharge occurs  
when either the input or output is shorted. Internal to the  
LM138 is a 50resistor which limits the peak discharge cur-  
rent. No protection is needed for output voltages of 25V or  
less and 10 µF capacitance. Figure 3 shows an LM138 with  
protection diodes included for use with outputs greater than  
25V and high values of output capacitance.  
5
www.national.com