| ??? 12/17/07 18:36 Modified: 12/17/07 18:41 Read: times |
#148398 - Answers Responding to: ???'s previous message |
Mehdi said:
what is 1N6263 and SMBJ20 role in your citcuit?
why the res 10k is not connected to gate of BS170? why 120k instead of 22k and 100k? is it for opamp protection? SMBJ20 is protection against overvoltage, undervoltage and ESD on ISO K-line. 1N6263 protects the LM393 against negative input voltages, which can result from undershots on ISO K-line. 10k resistor at output of 74HC02 shall pull-down the gate potential of BS170 when the supply voltage of 74HC02 falls down to less than 2V and the PMOS and NMOS output transistors become high ohmic. Then, an improper floating potential could turn-on the BS170. The 10k resistor forces the gate potential to 0V then. The sum of 22k and 100k is about 120k, which is the demanded impedance of ISO K-line to 0V. The voltage divider is needed to prevent the input voltage of LM393 from leaving the input common mode voltage range, which is 12V - 1.5V = 10.5V here. Kai |
