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ISO15 and DCP020503 tolerance

Jamaal_Charles
Genius 3245 points
Other Parts Discussed in Thread: ISO15, TPS54232, DCP020503, DCR010503

We’re implementing an RS485 interface using the ISO15 (http://www.ti.com/lit/ds/symlink/iso15.pdf) with a circuit similar to that described in this app note (http://www.ti.com/lit/an/slla299/slla299.pdf).

 

The ISO15 requires the isolated VCC2 to be between 3.15V and 3.6V

 

We’re looking at the TI DCP020503 as the isolated power supply for this design (rather than the recom in the app note, which is no longer manufactured and is EOL).

 

However, the TI DCP020503 has the following output voltage range: 3.13 - .3.46

 

So we have the potential of being 3.13 Vout with a Vin(min) of 3.15. 

 

Not a big margin of error issue, but something we need to be careful with.

Our main power supply that drives the circuit is the TPS54232, which will have a pretty tight (<2%?) output voltage, so can we safely assume that we’ll be inside the ISO15 requirements thru the DCP020503?

Thanks

  • 0
    TI__Mastermind 28265 points

    Eric,

    these 20mV cut it tight. It's not so much the line regulation that should concern you but the load regulation. The 3.13V are given for a perfect 5V input. when you look at the max. load regulation of 30%, this is what concerns me most. Should your RS485 link experince large common-mode voltages (of up to +/- 7V per EIA485) the driver current and with it the load current of the DCP02 will change drastically. If you then have a dvice sample whose output voltage is already close to minimum, the load regulation of the device might cause Vout to drop below the 3.13V.

    Unless you need the 2W of the DCP02 for other output circuitry , I recommend using the DCR010503. This device has a regulated 3.3V output and a maximum variance in regulation over line and load of 2.5%.

    Hope that helps.

    Regards, Thomas