UC3843 Clamping Error Amplifier Voltage
Current sensing is used to limit the output current to the voltage imposed by the error amplifier. If you force this to some predefined value you will still have output current given by the output voltage. It doesn't work as a current generator because if you'll change the load you'll get a different current, set by the voltage at the COMP pin. Therefore you will have a voltage generator, with a current-mode control, but without voltage conrtol (not taken into account in the feedback loop).
If you want a current generator, then you'll need a second current sensor (resistor, what have you) on the load side, where the DC current is, and which will give you the voltage to feed to the IC as the current to regulate. This will require an external amplifier since the reference voltage for the internal error amplifier is a fixed 2.5 V. So you'll need to amplify the sensed current to this value. This means that your loop will have to account for this amplification factor, which may be in the order of tens (e.g. a 0.25 Ω resistor for a 0.5 A will mean a 0.125 V, which will need 20x amplification). So, unless you're making this a high-current generator, it might not be a good choice. There are constant output current controllers. The fact that this is a current-mode controller, it doesn't mean that it refers to the output DC current, rather to its controlling of the pulsed current, to serve the output voltage.