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简 介：测试了LM3940 3.3V稳压芯片的实际最大输入电压。尽管数据手册标明其最大输入电压为5.5V，但实测发现该芯片在输入电压高达32V时仍能稳定输出3.35V，甚至在36V下仍正常工作。当输入超过32V后，输出骤降至0V，表明内部具备约30V的过压保护机制。实验验证了该芯片实际耐压远超标称值，原因可能与设计冗余或保护电路有关，建议谨慎使用于高压环境。

关键词：LM3940

AD\Test\2026\October\TestLM3940MAXIn.SchDoc

01【LM3940最大输入电压】

一、测试电路

昨天测量了AS11173.3伏稳压芯片最大输入电压。 说选择芯片在测量时呢， 它的确可以忍受高达24伏的输入电压， 但在实际使用过程中发现，在有些时候， 12伏便可以将该芯片击穿。 现在选择另外一个芯片LM3940 300三伏的稳压电源， 它的数据手册说它的输入电压最大只能是5.5伏， 现在我们也测量一下这个芯芯片它的最大输出电压是多少？ 设计一个测试电路非常简单， 在滤波电容，采用了MLcc电容， 在焊接的时候， 焊接一个1/8瓦的47K欧姆电阻作为负载。 铺设单面PCB适合一分钟制板， 一分钟之后得到测试电路板， 电路板制作的非常完美。 下面我们测试输入电压从0至12伏变化过程中， 它的输出电压变化，看输入高压对它有什么影响。

二、测量结果

使用dh1766可编程直流电源提供从0至12伏的输入电压， 可以看到在输入电压超过3.3伏之后， 输出便趋向稳定， 但是在6伏之前，它的输出随着输入电压增加略微上升。 等数电压大于6伏之后， 输出便稳定在3.35伏。 注意此时我们可以看到它的输入电压实际上已经超过5.5伏。 这与他的数据手册所写的最大电压不超过5.5伏。 相比，已经远远超出了它最大输入可承受电压。 查看输入工作电流随着输入电压增加的情况， 在输入电压在3.3伏之前变化比较剧烈和无规律， 之后便随着输入电压上升， 工作电流也逐步上升， 当输入电压超过6.5伏之后， 工作电流便稳定在10.5毫安左右。 由此我们也验证了手边这款LM3940， 它的输电压的确可以达到12伏。

接下来我们将输入电压从0伏升高到32伏测量在这个过程中它的输出变化， 可以看到，在整个的输入电压变化过程中， 它的输出始终保持三正三副，比较稳定。 对应的电流也是类似， 只是当最后电压达到32伏的时候， 电流出现了一些突变。 但这次我们可以看到， 这颗LM3940对应的33伏稳压电源， 它居然能够在输入电压高达36伏的时候，输出仍然正常。

下面我们提供从0至48伏的输入电压， 这时我们可以看到它的输出电压， 在输入电压为32伏之后，突然下降， 之后便维持了输出电压一直很低。 对应的输入电流也是， 在当输入电压超过32伏之后也突然下降， 之后随着输入电压升高， 它的工作电流也逐步上升。 由此我们可以看到这款LM39403.3伏稳压电源， 它的最大输入电压不能超过32伏。

※总结 ※

本文测试了一款3.3伏稳压芯片， 型号为LM3940它的数据手册标明它的最大输出电压不超过5.5伏， 但实际在测量过程中发现， 当输入电压超过32伏时， 它的输出变为0伏左右。 好像内部有保护机制， 当输入电压重新回复到5伏左右， 它的输出又恢复正常， 我们可以看到它内部有过压保护模块大约是30伏， 这也印证了刚才测量的结果。 不过最终我们有疑惑， 为什么这款3.3伏稳压芯片， 它实际可以输入高达32伏的电压， 但数据手册却标明，它最大输出电压不要超过5点五伏，这究竟为什么呢？

#!/usr/local/bin/python# -*- coding: gbk -*-#============================================================# TEST1.PY -- by Dr. ZhuoQing 2026-10-12## Note:#============================================================fromheadmimport*fromtsmodule.tsvisaimport*dm3068open()vin=linspace(0,48,100)vout=[]idim=[]forvinvin:dh1766volt1(v)time.sleep(1.5)vv=dm3068vdc()vout.append(vv)i=dh1766curr1()idim.append(i)printff(v,vv,i)tspsave("LM3940",vin=vin,vout=vout,idim=idim)dh1766volt1(5)plt.plot(vin,vout,lw=3)plt.xlabel("Vin(V)",color="steelblue",fontsize=24)plt.ylabel("Vout(V)",color="steelblue",fontsize=24)plt.grid(True,which='both',linestyle='--',alpha=0.7)plt.xticks(fontsize=16)plt.yticks(fontsize=16)plt.tight_layout()plt.show()#------------------------------------------------------------# END OF FILE : TEST1.PY#============================================================

#!/usr/local/bin/python# -*- coding: gbk -*-#============================================================# TEST2.PY -- by Dr. ZhuoQing 2026-10-12## Note:#============================================================fromheadmimport*vin,vout,idim=tspload('LM3940','vin','vout','idim')plt.plot(vin,idim,lw=3)plt.xlabel("Vin(V)",color="steelblue",fontsize=24)plt.ylabel("I(A)",color="steelblue",fontsize=24)plt.grid(True,which='both',linestyle='--',alpha=0.7)plt.xticks(fontsize=16)plt.yticks(fontsize=16)plt.tight_layout()plt.show()#------------------------------------------------------------# END OF FILE : TEST2.PY#============================================================

vin=[0.0000,0.4848,0.9697,1.4545,1.9394,2.4242,2.9091,3.3939,3.8788,4.3636,4.8485,5.3333,5.8182,6.3030,6.7879,7.2727,7.7576,8.2424,8.7273,9.2121,9.6970,10.1818,10.6667,11.1515,11.6364,12.1212,12.6061,13.0909,13.5758,14.0606,14.5455,15.0303,15.5152,16.0000,16.4848,16.9697,17.4545,17.9394,18.4242,18.9091,19.3939,19.8788,20.3636,20.8485,21.3333,21.8182,22.3030,22.7879,23.2727,23.7576,24.2424,24.7273,25.2121,25.6970,26.1818,26.6667,27.1515,27.6364,28.1212,28.6061,29.0909,29.5758,30.0606,30.5455,31.0303,31.5152,32.0000,32.4848,32.9697,33.4545,33.9394,34.4242,34.9091,35.3939,35.8788,36.3636,36.8485,37.3333,37.8182,38.3030,38.7879,39.2727,39.7576,40.2424,40.7273,41.2121,41.6970,42.1818,42.6667,43.1515,43.6364,44.1212,44.6061,45.0909,45.5758,46.0606,46.5455,47.0303,47.5152,48.0000]vout=[0.1022,0.0218,0.0082,0.0802,1.9291,2.4092,2.8934,3.2603,3.2747,3.2873,3.2988,3.3096,3.3213,3.3393,3.3439,3.3445,3.3451,3.3456,3.3462,3.3468,3.3474,3.3480,3.3486,3.3493,3.3500,3.3506,3.3513,3.3521,3.3528,3.3536,3.3544,3.3552,3.3559,3.3568,3.3577,3.3585,3.3594,3.3603,3.3612,3.3622,3.3630,3.3640,3.3650,3.3660,3.3670,3.3680,3.3691,3.3702,3.3713,3.3724,3.3735,3.3746,3.3759,3.3770,3.3782,3.3794,3.3806,3.3819,3.3832,3.3844,3.3857,3.3870,3.3883,3.3896,3.3910,3.3922,3.4116,0.3989,0.0599,0.0149,0.0066,0.0043,0.0033,0.0027,0.0023,0.0020,0.0018,0.0017,0.0015,0.0014,0.0013,0.0013,0.0012,0.0011,0.0011,0.0010,0.0010,0.0009,0.0009,0.0009,0.0008,0.0008,0.0008,0.0008,0.0008,0.0007,0.0007,0.0007,0.0007,0.0007]idim=[0.0000,0.0000,0.0000,0.0000,0.0046,0.0088,0.0093,0.0084,0.0087,0.0091,0.0094,0.0097,0.0100,0.0103,0.0104,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0105,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0106,0.0071,0.0019,0.0019,0.0019,0.0020,0.0020,0.0020,0.0021,0.0021,0.0021,0.0022,0.0022,0.0022,0.0023,0.0023,0.0023,0.0024,0.0024,0.0024,0.0025,0.0025,0.0025,0.0026,0.0026,0.0026,0.0027,0.0027,0.0027,0.0028,0.0028,0.0028,0.0029,0.0029,0.0029]