MAX3222: different data rate on driver channels and ESD selection guide

Hi Charles,

1. Yes - you can have a different data rate at DIN1 than the data rate at DIN2 as long as its less than or equal to max data rate (typ. 250kbps at 1000pF load)

2. For ESD selection - we don't really have a direct guide - but we have some best practices when selecting the diode. 

2a) Generally we see them on the DOUT and RIN pins

2b) They need to be bi-directional

2c) for DOUT they should be selected to have a working voltage of  >= 6V and <= -6V with a max clamping level at <= 13.2V and >= -13.2V.

2d) For RIN - the working voltage will depend on if this device will only interact with other +/-5.5V RS-232 signal level devices or if it will need to handle up to +/-15V - if this part is only going to be used with other +/-5.5V RS-232 signal level devices the working voltage should >=6V and <= -6V but if they need the full +/-15V then the working voltage goes to >15V and <-15V. The max clamping level - regardless of RS-232 signal levels used - is <= 25V and >= -25V

2e) If there is only 1 protection diode on the line it needs to clamp below the lowest abs. max rating on the bus - so the DOUT pin limits should be followed in that case. 

2f) For the capacitance - RS-232 has a Capacitive load budget - so this device will typically max out at  250kbps with 1000pF of capacitive loading (but can be a minimum of 150kbps)  - most of that budget is reserved for capacitance due to cabling - so adding diodes will give you less budget for cabling which could shorten the max distance of the bus. Generally there is ~167pF/m on most common RS-232 cabling - so for example lets say there is one diode per line at 110pF, and there is 50pF of additional parasitic capacitance (connectors/trace/IC etc...) which would leave 840pF of the 1000pF budget we started with - assuming we are using the 167pF/m cable our max cable than our max cable length where we could typically hit a max data rate of 250kbps is ~5m. So diodes reduce max bus length - so lower capacitance is better overall.

Ultimately with all that being said the vast majority (I'd say >80% of schematics I have seen) will include protection diodes on the RS_232 bus lines - they are extremely common in application and as long as you follow the general rules above customer should be able to select proper diode that works well with their system.  

Please let me know if you have any other questions!

Best,

Parker Dodson