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LM2676S-5.0 参数 Datasheet PDF下载

LM2676S-5.0图片预览
型号: LM2676S-5.0
PDF下载: 下载PDF文件 查看货源
内容描述: SIMPLE SWITCHER高效率3A降压型稳压器 [SIMPLE SWITCHER High Efficiency 3A Step-Down Voltage Regulator]
分类和应用: 稳压器
文件页数/大小: 25 页 / 451 K
品牌: NSC [ National Semiconductor ]
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greater than this ripple current. The voltage rating of the  
output capacitor should be greater than 1.3 times the maxi-  
mum output voltage of the power supply. If operation of the  
system at elevated temperatures is required, the capacitor  
voltage rating may be de-rated to less than the nominal room  
temperature rating. Careful inspection of the manufacturer’s  
specification for de-rating of working voltage with tempera-  
ture is important.  
Application Hints (Continued)  
maximum ripple current of 30% of the maximum load cur-  
rent. If the ripple current exceeds this 30% limit the next  
larger value is selected.  
The inductors offered have been specifically manufactured  
to provide proper operation under all operating conditions of  
input and output voltage and load current. Several part types  
are offered for a given amount of inductance. Both surface  
mount and through-hole devices are available. The inductors  
from each of the three manufacturers have unique charac-  
teristics.  
INPUT CAPACITOR  
Fast changing currents in high current switching regulators  
place a significant dynamic load on the unregulated power  
source. An input capacitor helps to provide additional current  
to the power supply as well as smooth out input voltage  
variations.  
Renco: ferrite stick core inductors; benefits are typically  
lowest cost and can withstand ripple and transient peak  
currents above the rated value. These inductors have an  
external magnetic field, which may generate EMI.  
Like the output capacitor, the key specifications for the input  
capacitor are RMS current rating and working voltage. The  
RMS current flowing through the input capacitor is equal to  
one-half of the maximum dc load current so the capacitor  
should be rated to handle this. Paralleling multiple capacitors  
proportionally increases the current rating of the total capaci-  
tance. The voltage rating should also be selected to be 1.3  
times the maximum input voltage. Depending on the unregu-  
lated input power source, under light load conditions the  
maximum input voltage could be significantly higher than  
normal operation and should be considered when selecting  
an input capacitor.  
Pulse Engineering: powdered iron toroid core inductors;  
these also can withstand higher than rated currents and,  
being toroid inductors, will have low EMI.  
Coilcraft: ferrite drum core inductors; these are the smallest  
physical size inductors and are available only as surface  
mount components. These inductors also generate EMI but  
less than stick inductors.  
OUTPUT CAPACITOR  
The output capacitor acts to smooth the dc output voltage  
and also provides energy storage. Selection of an output  
capacitor, with an associated equivalent series resistance  
(ESR), impacts both the amount of output ripple voltage and  
stability of the control loop.  
The input capacitor should be placed very close to the input  
pin of the LM2676. Due to relative high current operation  
with fast transient changes, the series inductance of input  
connecting wires or PCB traces can create ringing signals at  
the input terminal which could possibly propagate to the  
output or other parts of the circuitry. It may be necessary in  
some designs to add a small valued (0.1µF to 0.47µF)  
ceramic type capacitor in parallel with the input capacitor to  
prevent or minimize any ringing.  
The output ripple voltage of the power supply is the product  
of the capacitor ESR and the inductor ripple current. The  
capacitor types recommended in the tables were selected  
for having low ESR ratings.  
In addition, both surface mount tantalum capacitors and  
through-hole aluminum electrolytic capacitors are offered as  
solutions.  
CATCH DIODE  
Impacting frequency stability of the overall control loop, the  
output capacitance, in conjunction with the inductor, creates  
a double pole inside the feedback loop. In addition the  
capacitance and the ESR value create a zero. These fre-  
quency response effects together with the internal frequency  
compensation circuitry of the LM2676 modify the gain and  
phase shift of the closed loop system.  
When the power switch in the LM2676 turns OFF, the current  
through the inductor continues to flow. The path for this  
current is through the diode connected between the switch  
output and ground. This forward biased diode clamps the  
switch output to a voltage less than ground. This negative  
voltage must be greater than −1V so a low voltage drop  
(particularly at high current levels) Schottky diode is recom-  
mended. Total efficiency of the entire power supply is signifi-  
cantly impacted by the power lost in the output catch diode.  
The average current through the catch diode is dependent  
on the switch duty cycle (D) and is equal to the load current  
times (1-D). Use of a diode rated for much higher current  
than is required by the actual application helps to minimize  
the voltage drop and power loss in the diode.  
As a general rule for stable switching regulator circuits it is  
desired to have the unity gain bandwidth of the circuit to be  
limited to no more than one-sixth of the controller switching  
frequency. With the fixed 260KHz switching frequency of the  
LM2676, the output capacitor is selected to provide a unity  
gain bandwidth of 40KHz maximum. Each recommended  
capacitor value has been chosen to achieve this result.  
During the switch ON time the diode will be reversed biased  
by the input voltage. The reverse voltage rating of the diode  
should be at least 1.3 times greater than the maximum input  
voltage.  
In some cases multiple capacitors are required either to  
reduce the ESR of the output capacitor, to minimize output  
ripple (a ripple voltage of 1% of Vout or less is the assumed  
performance condition), or to increase the output capaci-  
tance to reduce the closed loop unity gain bandwidth (to less  
than 40KHz). When parallel combinations of capacitors are  
required it has been assumed that each capacitor is the  
exact same part type.  
BOOST CAPACITOR  
The boost capacitor creates a voltage used to overdrive the  
gate of the internal power MOSFET. This improves efficiency  
by minimizing the on resistance of the switch and associated  
power loss. For all applications it is recommended to use a  
0.01µF/50V ceramic capacitor.  
The RMS current and working voltage (WV) ratings of the  
output capacitor are also important considerations. In a typi-  
cal step-down switching regulator, the inductor ripple current  
(set to be no more than 30% of the maximum load current by  
the inductor selection) is the current that flows through the  
output capacitor. The capacitor RMS current rating must be  
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