Hi，when read the datasheet of AD5421，with its REGout current is 3.15mA， How can We get the 6mA current so that to drive the outside circuit，please give some advises，thanks.
The regulator can only supply up to 3.15mA. If the load at REGOUT pin is 6mA, you won't be able to set the loop current less than 6mA. You need to select your external circuitry so that its total current consumption will not exceed 3.15mA.
If this is not possible, you have to use an another supply to provide power to your external components.