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LM2676S-5.0-NOPB 参数 Datasheet PDF下载

LM2676S-5.0-NOPB图片预览
型号: LM2676S-5.0-NOPB
PDF下载: 下载PDF文件 查看货源
内容描述: LM2676 SIMPLE SWITCHER®高效率3A降压型稳压器 [LM2676 SIMPLE SWITCHER® High Efficiency 3A Step-Down Voltage Regulator]
分类和应用: 稳压器
文件页数/大小: 35 页 / 1445 K
品牌: TI [ TEXAS INSTRUMENTS ]
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LM2676  
SNVS031J APRIL 2000REVISED APRIL 2013  
www.ti.com  
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.  
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 capacitance. The voltage rating should also be selected to be 1.3 times the maximum  
input voltage. Depending on the unregulated 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.  
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.  
CATCH DIODE  
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 recommended. Total efficiency of the entire  
power supply is significantly 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.  
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.  
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.  
ADDITIONAL APPLICATION INFORMATION  
When the output voltage is greater than approximately 6V, and the duty cycle at minimum input voltage is greater  
than approximately 50%, the designer should exercise caution in selection of the output filter components. When  
an application designed to these specific operating conditions is subjected to a current limit fault condition, it may  
be possible to observe a large hysteresis in the current limit. This can affect the output voltage of the device until  
the load current is reduced sufficiently to allow the current limit protection circuit to reset itself.  
Under current limiting conditions, the LM267x is designed to respond in the following manner:  
1. At the moment when the inductor current reaches the current limit threshold, the ON-pulse is immediately  
terminated. This happens for any application condition.  
2. However, the current limit block is also designed to momentarily reduce the duty cycle to below 50% to avoid  
subharmonic oscillations, which could cause the inductor to saturate.  
3. Thereafter, once the inductor current falls below the current limit threshold, there is a small relaxation time  
during which the duty cycle progressively rises back above 50% to the value required to achieve regulation.  
If the output capacitance is sufficiently ‘large’, it may be possible that as the output tries to recover, the output  
capacitor charging current is large enough to repeatedly re-trigger the current limit circuit before the output has  
fully settled. This condition is exacerbated with higher output voltage settings because the energy requirement of  
the output capacitor varies as the square of the output voltage (½CV2), thus requiring an increased charging  
current.  
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