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参数资料
| 型号： | LM2637MWC |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, UUC |
| 封装： | WAFER |
| 文件页数： | 6/20页 |
| 文件大小： | 404K |
| 代理商： | LM2637MWC |
Applications Information (Continued)
case of desktop PC applications, those ratings seem some-
what conservative. A rule-of-thumb is increase the 105C rat-
ing by 70% for desktop PC applications. The input RMS
ripple current value can be determined by the following
equation:
(14)
and the power loss in each input capacitor is:
(15)
In the case of 333 MHz Pentium II power supply, the maxi-
mum output current is around 14A. Under the worst case
when duty cycle is 50%, the maximum input capacitor RMS
ripple current is half of output current, i.e., 7A. Therefore
three Sanyo 16MV820GX capacitors are necessary under
room temperature (they are rated 1.45A at 105C). The
maximum ESR of those capacitors is 44 m
. So the maxi-
mum power loss in each of them is less than (7A)
2 x44
m
/32 = 0.24W. Note that the power loss in each capacitor
is inversely proportional to the square of the total number of
capacitors, which means the power loss in each capacitor
quickly drops when the number of capacitors increases.
Linear Section — For applications where there is a load tran-
sient requirement such as that the GTL+ supply, low ESR ca-
pacitors should be considered. Make sure that the total ESR
multiplied by the maximum load current is smaller than half
the output voltage regulation window. The output voltage
regulation window should exclude the tolerance of LM2637.
For example, for a 3.3V to 1.5V, 2A design, the initial regula-
tion window is ±9%. Assume the tolerance of the LM2637
plus margin is ±2%, then the effective window left is ±7% or
±105 mV. Therefore the ESR should be less than 105 mV ÷
2A = 52 m
. A Sanyo 6MV1200DX is sufficient. For applica-
tions where the load is static and for control bandwidth and
stability issue, refer to the guidelines in the
control loop com-
pensation section.
Inductor Selection
Output Inductor. The size of the output inductor is deter-
mined by a number of parameters. Basically the larger the
inductor, the smaller the output ripple voltage, but the slower
the converter’s response speed during a load transient. On
the other hand, a smaller inductor requires higher switching
frequency to maintain the same level of output ripple, and
probably results in a lossier converter, but has less inertia re-
sponding to load transient. In the case of MPU core power
supply, fast recovery of the load voltage from transient win-
dow back to the steady state window is important. That limits
the highest inductance value that can be used. The lowest
inductance value is limited by the highest switching fre-
quency that can be practically employed. As the switching
frequency increases, the switching loss in the FETs tends to
increase, resulting in lower overall efficiency and larger heat
sinks. A good switching frequency is probably a frequency
under which the FET conduction loss is much higher than
the switching loss because the cost of the FET is directly re-
lated to its r
DS_ON. The inductor size can be determined by
the following equation:
(16)
where V
o_rip is the peak-peak output ripple voltage, f is the
switching frequency. For commonly used low r
DS_ON FET’s,
a reasonable switching frequency is 300 kHz. Assume a
peak-peak output ripple voltage is 18 mV, the total output ca-
pacitor ESR is 9 m
, the input voltage is 5V, and output volt-
age is 2.8V, then the inductance value according to the
above equation will be 2 H. The highest slew rate of the in-
ductor current when the load changes from no load to full
load can be determined as follows:
(17)
where D
MAX is the maximum allowed duty cycle, which is
around 0.95 for LM2637. For a load transient from 0A to 14A,
the highest current slew rate of the inductor, according to the
above equation, is 0.97 A/s, and therefore the shortest pos-
sible total recovery time is 14A/(0.97 A/s) = 14.5 s. Notice
that output voltage starts to recover whenever the inductor
starts to supply current.
The highest slew rate of the inductor current when the load
changes from full load to no load can be determined from the
same equation but use D
MIN instead of DMAX.
Since the D
MIN of LM2637 is at 0%, the slew rate is therefore
1.4 A/s. So the approximate total recovery time will be
14A/(1.4 A/s) = 10 s.
Often times the power supply designer may have to use a
custom-made inductor for best performance/price ratio. Mi-
crometals offers cost effective iron powder cores that are
widely adopted by motherboard supplies and OEMs. One
important rule when designing an iron power inductor is
never saturate the core or else it will exhibit extremely poor
dynamic performance. Useful inductor design tools can also
be found on their web page, www.micrometals.com. The
user
of
LM2637
can
also
contact
National
for
a
custom-made inductor.
Alternatively the designer may use an open core inductor,
which is lower cost due to its ease of mass production. How-
ever, the open magnetic field may cause some noise prob-
lems to nearby circuitry and may cause EMI issues. How-
ever, no negative reports have been heard so far. Coilcraft
(www.coilcraft.com) offers a wide range of open core in-
ductors. Custom-made parts are also possible. Other than
low cost, the advantages of open core inductors are less
board space and superior dynamic performance.
Input Inductor. The input inductor is for limiting the input
current slew rate during a load transient and normal opera-
tion. In the case that low ESR aluminum electrolytic capaci-
tors are used for the input capacitor bank, input capacitor
voltage change due to capacitor charging/discharging is usu-
ally negligible for the first 20 s. ESR is by far the dominant
factor in determining the amount of capacitor voltage
undershoot/overshoot during a fast load transient. So the
worst case is when the load changes between no load and
full load. Under that condition the input inductor sees the
highest voltage change across the input capacitors. Assume
the input capacitor bank consists of three 16MV820GX, i.e.,
a total ESR of 15 m
. Whenever there is a sudden load
change, the change in input current has to be initially sup-
ported by the input capacitor bank instead of the input induc-
tor. So for a fast load-swing between 0A and 14A, the voltage
change seen by the input inductor is a ramp from 0V to a
V
or vice versa, whereas
V = 14A x15m = 210 mV. So this
situation is just as bad as operating under heaviest load. Use
the following equation to determine the minimum inductance
value:
www.national.com
14
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相关代理商/技术参数 |
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| LM2637MX/NOPB | 功能描述:IC MOTHERBRD PWR SUPPLY 24-SOIC RoHS:是 类别:集成电路 (IC) >> PMIC - 电源管理 - 专用 系列:- 应用说明:Ultrasound Imaging Systems Application Note 产品培训模块:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 标准包装:37 系列:- 应用:医疗用超声波成像，声纳 电流 - 电源:- 电源电压:2.37 V ~ 6 V 工作温度:0°C ~ 70°C 安装类型:表面贴装 封装/外壳:56-WFQFN 裸露焊盘 供应商设备封装:56-TQFN-EP（8x8） 包装:管件 |
| LM2638 | 制造商:未知厂家 制造商全称:未知厂家 功能描述: |
| LM2638M | 制造商:Texas Instruments 功能描述: |
| LM2638M/NOPB | 功能描述:IC MOTHERBRD PWR SUPPLY 24-SOIC RoHS:是 类别:集成电路 (IC) >> PMIC - 电源管理 - 专用 系列:- 应用说明:Ultrasound Imaging Systems Application Note 产品培训模块:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 标准包装:37 系列:- 应用:医疗用超声波成像，声纳 电流 - 电源:- 电源电压:2.37 V ~ 6 V 工作温度:0°C ~ 70°C 安装类型:表面贴装 封装/外壳:56-WFQFN 裸露焊盘 供应商设备封装:56-TQFN-EP（8x8） 包装:管件 |
| LM2638MX | 功能描述:IC MOTHERBRD PWR SUPPLY 24-SOIC RoHS:是 类别:集成电路 (IC) >> PMIC - 电源管理 - 专用 系列:- 应用说明:Ultrasound Imaging Systems Application Note 产品培训模块:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 标准包装:37 系列:- 应用:医疗用超声波成像，声纳 电流 - 电源:- 电源电压:2.37 V ~ 6 V 工作温度:0°C ~ 70°C 安装类型:表面贴装 封装/外壳:56-WFQFN 裸露焊盘 供应商设备封装:56-TQFN-EP（8x8） 包装:管件 |
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