欢迎访问ic37.com |
会员登录 免费注册
发布采购

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 ]
 浏览型号LM1084IS-ADJ的Datasheet PDF文件第3页浏览型号LM1084IS-ADJ的Datasheet PDF文件第4页浏览型号LM1084IS-ADJ的Datasheet PDF文件第5页浏览型号LM1084IS-ADJ的Datasheet PDF文件第6页浏览型号LM1084IS-ADJ的Datasheet PDF文件第8页浏览型号LM1084IS-ADJ的Datasheet PDF文件第9页浏览型号LM1084IS-ADJ的Datasheet PDF文件第10页浏览型号LM1084IS-ADJ的Datasheet PDF文件第11页  
STABILITY CONSIDERATION  
Application Note  
Stability consideration primarily concern the phase response  
of the feedback loop. In order for stable operation, the loop  
must maintain negative feedback. The LM1084 requires a  
certain amount series resistance with capacitive loads. This  
series resistance introduces a zero within the loop to in-  
crease phase margin and thus increase stability. The equiva-  
lent series resistance (ESR) of solid tantalum or aluminum  
electrolytic capacitors is used to provide the appropriate zero  
(approximately 500 kHz).  
GENERAL  
Figure 1 shows a basic functional diagram for the LM1084-  
Adj (excluding protection circuitry) . The topology is basically  
that of the LM317 except for the pass transistor. Instead of a  
Darlingtion NPN with its two diode voltage drop, the LM1084  
uses a single NPN. This results in a lower dropout voltage.  
The structure of the pass transistor is also known as a quasi  
LDO. The advantage a quasi LDO over a PNP LDO is its  
inherently lower quiescent current. The LM1084 is guaran-  
teed to provide a minimum dropout voltage 1.5V over tem-  
perature, at full load.  
The Aluminum electrolytic are less expensive than tantal-  
ums, but their ESR varies exponentially at cold tempera-  
tures; therefore requiring close examination when choosing  
the desired transient response over temperature. Tantalums  
are a convenient choice because their ESR varies less than  
2:1 over temperature.  
The recommended load/decoupling capacitance is a 10uF  
tantalum or a 50uF aluminum. These values will assure  
stability for the majority of applications.  
The adjustable versions allows an additional capacitor to be  
used at the ADJ pin to increase ripple rejection. If this is done  
the output capacitor should be increased to 22uF for tantal-  
ums or to 150uF for aluminum.  
Capacitors other than tantalum or aluminum can be used at  
the adjust pin and the input pin. A 10uF capacitor is a  
reasonable value at the input. See Ripple Rejection section  
regarding the value for the adjust pin capacitor.  
10094665  
It is desirable to have large output capacitance for applica-  
tions that entail large changes in load current (microproces-  
sors for example). The higher the capacitance, the larger the  
available charge per demand. It is also desirable to provide  
low ESR to reduce the change in output voltage:  
FIGURE 1. Basic Functional Diagram for the LM1084,  
excluding Protection circuitry  
OUTPUT VOLTAGE  
V = I x ESR  
The LM1084 adjustable version develops at 1.25V reference  
voltage, (VREF), between the output and the adjust terminal.  
As shown in figure 2, this voltage is applied across resistor  
R1 to generate a constant current I1. This constant current  
then flows through R2. The resulting voltage drop across R2  
adds to the reference voltage to sets the desired output  
voltage.  
It is common practice to use several tantalum and ceramic  
capacitors in parallel to reduce this change in the output  
voltage by reducing the overall ESR.  
Output capacitance can be increased indefinitely to improve  
transient response and stability.  
RIPPLE REJECTION  
The current IADJ from the adjustment terminal introduces an  
output error . But since it is small (120uA max), it becomes  
negligible when R1 is in the 100range.  
For fixed voltage devices, R1 and R2 are integrated inside  
the devices.  
Ripple rejection is a function of the open loop gain within the  
feed-back loop (refer to Figure 1 and Figure 2). The LM1084  
exhibits 75dB of ripple rejection (typ.). When adjusted for  
voltages higher than VREF, the ripple rejection decreases as  
function of adjustment gain: (1+R1/R2) or VO/VREF. There-  
fore a 5V adjustment decreases ripple rejection by a factor of  
four (−12dB); Output ripple increases as adjustment voltage  
increases.  
However, the adjustable version allows this degradation of  
ripple rejection to be compensated. The adjust terminal can  
be bypassed to ground with a capacitor (CADJ). The imped-  
ance of the CADJ should be equal to or less than R1 at the  
desired ripple frequency. This bypass capacitor prevents  
ripple from being amplified as the output voltage is in-  
creased.  
1/(2π*fRIPPLE*CADJ) R1  
LOAD REGULATION  
The LM1084 regulates the voltage that appears between its  
output and ground pins, or between its output and adjust  
pins. In some cases, line resistances can introduce errors to  
the voltage across the load. To obtain the best load regula-  
tion, a few precautions are needed.  
10094617  
FIGURE 2. Basic Adjustable Regulator  
7
www.national.com