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LM51031D 参数 Datasheet PDF下载

LM51031D图片预览
型号: LM51031D
PDF下载: 下载PDF文件 查看货源
内容描述: 快PFET降压控制器 [Fast PFET Buck Controller]
分类和应用: 控制器
文件页数/大小: 9 页 / 269 K
品牌: HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
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Fast PFET Buck Controller  
LM51031  
2) Switching Frequency and On and Off Time  
Calculations  
impedance aluminum are less expensive.  
of suppliers and are the best choice for surface mount  
Given that fSW = 200 kHz and DMAX = 0.80  
Solid tantalum chip capacitors are available from a numb  
of suppliers and are the best choice for surface mount  
applications.  
The output capacitor limits the output ripple voltage. The  
LM51031 needs a maximum of 20 mV of output ripple for  
the feedback comparator to change state. If we assume  
that all the inductor ripple current flows through the output  
capacitor and that it is an ideal capacitor (i.e. zero ESR),  
the minimum capacitance needed to limit the output ripple  
to 50 mV peak−to−peak is given by:  
3) Oscillator Capacitor Selection  
The switching frequency is set by COSC, whose  
value is given by:  
The minimum ESR needed to limit the output voltage  
ripple to 50 mV peak−to−peak is:  
4) Inductor Selection  
The output capacitor should be chosen so that its ESR is  
less than 83 mΩ.  
The inductor value is chosen for continuous mode  
operation down to 0.3 Amps.  
The ripple current ∆I = 2 × IOUTmin = 2 × 0.3 A  
= 0.6 A  
During the minimum off time, the ripple current is 0.4 A  
and the output voltage ripple will be:  
This is the minimum value of inductor to keep the  
ripple current < 0.6 A during normal operation.  
A smaller inductor will result in larger ripple  
6) VFB Divider  
current. Ripple current at a minimum off time is:  
The input bias current to the comparator is 4.0 uA. The  
resistor divider current should be considerably higher than  
this to ensure that there is sufficient bias current. If we  
choose the divider current to be at least 250 times the bia  
current this permits a divider current of 1.0 mA and  
simplifies the calculations.  
The core must not saturate with the maximum  
expected current, here given by:  
5) Output Capacitor  
Let R2 = 1.0 K  
The output capacitor and the inductor form a low  
pass filter. The output capacitor should have a low  
ESL and ESR. Low impedance aluminum electrolytic,  
tantalum or organic semiconductor capacitors are  
a good choice for an output capacitor. Low  
Rearranging the divider equation gives:  
HTC  
7 −  
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