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LT1576CS8-5 参数 Datasheet PDF下载

LT1576CS8-5图片预览
型号: LT1576CS8-5
PDF下载: 下载PDF文件 查看货源
内容描述: 1.5A , 200kHz的降压型开关稳压器 [1.5A, 200kHz Step-Down Switching Regulator]
分类和应用: 稳压器开关式稳压器或控制器电源电路开关式控制器光电二极管
文件页数/大小: 28 页 / 293 K
品牌: Linear [ Linear ]
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LT1576/LT1576-5  
U
W U U  
APPLICATIONS INFORMATION  
including the switch, catch diode, and input capacitor is  
the only one containing nanosecond rise and fall times. If  
you follow this path on the PC layout, you will see that it is  
irreducibly short. If you move the diode or input capacitor  
away from the LT1576, get your resumé in order. The  
other paths contain only some combination of DC and  
200kHz triwave, so are much less critical.  
higher with a poor layout, potentially exceeding the abso-  
lute max switch voltage. The path around switch, catch  
diode and input capacitor must be kept as short as  
possibletoensurereliableoperation.Whenlookingatthis,  
a >100MHz oscilloscope must be used, and waveforms  
should be observed on the leads of the package. This  
switch off spike will also cause the SW node to go below  
ground. The LT1576 has special circuitry inside which  
mitigates this problem, but negative voltages over 1V  
lasting longer than 10ns should be avoided. Note that  
100MHz oscilloscopes are barely fast enough to see the  
details of the falling edge overshoot in Figure 7.  
PARASITIC RESONANCE  
Resonance or “ringing” may sometimes be seen on the  
switch node (see Figure 7). Very high frequency ringing  
following switch rise time is caused by switch/diode/input  
capacitor lead inductance and diode capacitance. Schot-  
tky diodes have very high “Q” junction capacitance that  
can ring for many cycles when excited at high frequency.  
Iftotalleadlengthfortheinputcapacitor, diodeandswitch  
path is 1 inch, the inductance will be approximately 25nH.  
At switch off, this will produce a spike across the NPN  
output device in addition to the input voltage. At higher  
currents this spike can be in the order of 10V to 20V or  
A second, much lower frequency ringing is seen during  
switch off time if load current is low enough to allow the  
inductor current to fall to zero during part of the switch off  
time (see Figure 8). Switch and diode capacitance reso-  
nate with the inductor to form damped ringing at 1MHz to  
10 MHz. This ringing is not harmful to the regulator and it  
hasnotbeenshowntocontributesignificantlytoEMI. Any  
attempt to damp it with a resistive snubber will degrade  
efficiency.  
INPUT BYPASSING AND VOLTAGE RANGE  
Input Bypass Capacitor  
RISE AND FALL  
WAVEFORMS ARE  
SUPERIMPOSED  
(PULSE WIDTH IS  
NOT 350ns)  
5V/DIV  
Step-down converters draw current from the input supply  
in pulses. The average height of these pulses is equal to  
load current, and the duty cycle is equal to VOUT/VIN. Rise  
and fall time of the current is very fast. A local bypass  
capacitor across the input supply is necessary to ensure  
proper operation of the regulator and minimize the ripple  
current fed back into the input supply. The capacitor also  
forces switching current to flow in a tight local loop,  
minimizing EMI.  
50ns/DIV  
1374 F07  
Figure 7. Switch Node Response  
5V/DIV  
Do not cheat on the ripple current rating of the Input  
bypass capacitor, but also don’t get hung up on the value  
in microfarads. The input capacitor is intended to absorb  
all the switching current ripple, which can have an RMS  
value as high as one half of load current. Ripple current  
ratings on the capacitor must be observed to ensure  
reliable operation. In many cases it is necessary to parallel  
two capacitors to obtain the required ripple rating. Both  
capacitors must be of the same value and manufacturer to  
SWITCH NODE  
VOLTAGE  
50mA/DIV  
INDUCTOR  
CURRENT  
1µs/DIV  
1374 F08  
Figure 8. Discontinuous Mode Ringing  
17  
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