500 mA is about battery charging current, so same design would works. Download AVR451 app. note from atmel.com, there is drawings 3-3 which shows two channel PWM - current converter based on OPA + MOSFET. Be aware, that PWM may be not right approach, as there is low response time and low resolution 8-bit (default). External DAC may be better solution, interfaced with same OPA+MOSFET driver.