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LP38691SD-ADJ 参数 Datasheet PDF下载

LP38691SD-ADJ图片预览
型号: LP38691SD-ADJ
PDF下载: 下载PDF文件 查看货源
内容描述: 500毫安低压降CMOS线性稳压器,可调输出/稳定器用陶瓷输出电容器 [500mA Low Dropout CMOS Linear Regulators with Adjustable Output / Stable with Ceramic Output Capacitors]
分类和应用: 稳压器电容器
文件页数/大小: 13 页 / 749 K
品牌: NSC [ National Semiconductor ]
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Z5U and Y5V dielectric ceramics have capacitance that  
drops severely with applied voltage. A typical Z5U or Y5V  
capacitor can lose 60% of its rated capacitance with half of  
the rated voltage applied to it. The Z5U and Y5V also exhibit  
a severe temperature effect, losing more than 50% of nomi-  
nal capacitance at high and low limits of the temperature  
range.  
Application Hints  
EXTERNAL CAPACITORS  
Like any low-dropout regulator, external capacitors are re-  
quired to assure stability. These capacitors must be correctly  
selected for proper performance.  
INPUT CAPACITOR: An input capacitor of at least 1µF is  
required (ceramic recommended). The capacitor must be  
located not more than one centimeter from the input pin and  
returned to a clean analog ground.  
X7R and X5R dielectric ceramic capacitors are strongly rec-  
ommended if ceramics are used, as they typically maintain a  
capacitance range within 20% of nominal over full operat-  
ing ratings of temperature and voltage. Of course, they are  
typically larger and more costly than Z5U/Y5U types for a  
given voltage and capacitance.  
OUTPUT CAPACITOR: An output capacitor is required for  
loop stability. It must be located less than 1 centimeter from  
the device and connected directly to the output and ground  
pins using traces which have no other currents flowing  
through them.  
TANTALUM: Solid Tantalum capacitors have good tempera-  
ture stability: a high quality Tantalum will typically show a  
capacitance value that varies less than 10-15% across the  
full temperature range of -40˚C to 125˚C. ESR will vary only  
about 2X going from the high to low temperature limits.  
The minimum amount of output capacitance that can be  
used for stable operation is 1µF. Ceramic capacitors are  
recommended (the LP38691/3-ADJ was designed for use  
with ultra low ESR capacitors). The LP38691/3-ADJ is stable  
with any output capacitor ESR between zero and 100 Ohms.  
The increasing ESR at lower temperatures can cause oscil-  
lations when marginal quality capacitors are used (if the ESR  
of the capacitor is near the upper limit of the stability range at  
room temperature).  
SETTING THE OUTPUT VOLTAGE: The output voltage is  
set using the external resistors R1 and R2 (see Typical  
Application Circuit). The output voltage will be given by the  
equation:  
PCB LAYOUT  
Good PC layout practices must be used or instability can be  
induced because of ground loops and voltage drops. The  
input and output capacitors must be directly connected to the  
input, output, and ground pins of the regulator using traces  
which do not have other currents flowing in them (Kelvin  
connect).  
VOUT = VADJ X (1 + R1/R2)  
Because the part has a minimum load current requirement of  
100 µA, it is recommended that R2 always be 12k Ohms or  
less to provide adequate loading. Even if a minimum load is  
always provided by other means, it is not recommended that  
very high value resistors be used for R1 and R2 because it  
can make the ADJ node susceptible to noise pickup. A  
maximum Ohmic value of 100k is recommended for R2 to  
prevent this from occurring.  
The best way to do this is to lay out CIN and COUT near the  
device with short traces to the VIN, VOUT, and ground pins.  
The regulator ground pin should be connected to the exter-  
nal circuit ground so that the regulator and its capacitors  
have a "single point ground".  
ENABLE PIN (LP38693-ADJ only): The LP38693-ADJ has  
an enable pin which turns the regulator output on and off.  
Pulling the enable pin down to a logic low will turn the part  
off. The voltage the pin has to be pulled up to in order to  
assure the part is on depends on input voltage (refer to  
Electrical Characteristics section). This pin should be tied to  
VIN if the enable function is not used.  
It should be noted that stability problems have been seen in  
applications where "vias" to an internal ground plane were  
used at the ground points of the IC and the input and output  
capacitors. This was caused by varying ground potentials at  
these nodes resulting from current flowing through the  
ground plane. Using a single point ground technique for the  
regulator and it’s capacitors fixed the problem. Since high  
current flows through the traces going into VIN and coming  
from VOUT, Kelvin connect the capacitor leads to these pins  
so there is no voltage drop in series with the input and output  
capacitors.  
FOLDBACK CURRENT LIMITING: Foldback current limit-  
ing is built into the LP38691/3-ADJ which reduces the  
amount of output current the part can deliver as the output  
voltage is reduced. The amount of load current is dependent  
on the differential voltage between VIN and VOUT. Typically,  
when this differential voltage exceeds 5V, the load current  
will limit at about 350 mA. When the VIN -VOUT differential is  
reduced below 4V, load current is limited to about 850 mA.  
RFI/EMI SUSCEPTIBILITY  
RFI (radio frequency interference) and EMI (electromagnetic  
interference) can degrade any integrated circuit’s perfor-  
mance because of the small dimensions of the geometries  
inside the device. In applications where circuit sources are  
present which generate signals with significant high fre-  
SELECTING A CAPACITOR  
It is important to note that capacitance tolerance and varia-  
tion with temperature must be taken into consideration when  
selecting a capacitor so that the minimum required amount  
of capacitance is provided over the full operating tempera-  
ture range.  
>
quency energy content ( 1 MHz), care must be taken to  
ensure that this does not affect the IC regulator.  
If RFI/EMI noise is present on the input side of the regulator  
(such as applications where the input source comes from the  
output of a switching regulator), good ceramic bypass ca-  
pacitors must be used at the input pin of the IC.  
Capacitor Characteristics  
CERAMIC: For values of capacitance in the 10 to 100 µF  
range, ceramics are usually larger and more costly than  
tantalums but give superior AC performance for bypassing  
high frequency noise because of very low ESR (typically less  
than 10 m). However, some dielectric types do not have  
good capacitance characteristics as a function of voltage  
and temperature.  
If a load is connected to the IC output which switches at high  
speed (such as a clock), the high-frequency current pulses  
required by the load must be supplied by the capacitors on  
the IC output. Since the bandwidth of the regulator loop is  
less than 100 kHz, the control circuitry cannot respond to  
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