Tool/software:
I am using below circuit to generate 4 to 20ma output .I have changed input voltage from 3V to 3.3V .i would like to know weather below circuit will generate the 4 to 20ma output without any heating issue.
Secondly if there is issue then can i use directly 3.3v or 3V low quiscent current LDO to generate the 4 to 20ma .
Regards,
Vipin
Vipin,
As is, I don't think your schematic will work. There are two connections that I think the schematic is missing. Here is the schematic given in TIPD158:
First, the NPN collector should be connected to the output of the bridge, and not to the subregulated voltage. The NPN pulls most of the 4-20mA current. Second from the output of the DAC, you have R20 and R21. The right side of R21 should be connected to the + input of the opamp. With this connection, the opamp feedback sets the loop current based on the voltage output of the DAC7311. After making the connection changes, I think the topology should be ok.
I haven't analyzed the setup of this schematic, however a 3V supply should be enough to drive the base of the NPN when the loop current is at the maximum of 20mA. Note that the bulk of the current is going to the NPN. If your loop voltage is high, then the NPN may need to dissipate more than half a Watt.
Joseph Wu
