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high voltage comparator

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taofeng

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Hi gurus,

Attached is the schematic of a high voltage comparator. The reference voltage is 10v, the Vin changes widely from 4V to 10V. All the transistors are high voltage transistors which means that the Vds can sustain as high as 40V.
However, there is serious reliability problem. When Vin changes, the Vcm normally follows. So the Vgs of MP1 is still less than the max allowed Vgs rating(3.6V). The issue is the Vgs of MP2, especially when Vin is going close to 4V.
Is there any special design tricks to solve this ?

thanks,

jeffrey
 

Common design technique is to insert 2 forward biased diodes between Vcm node and sources of MP1 and MP2. Gates of MP2 and MP2 must be protected e.g. by zener diodes.
But your circuitry doesn't allow such changes because your common mode level is close to battery. You have to change design solution. Use PMOS diff. pair and all that I've mentioned above.
 

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