An LED driver circuit and an isolated DC-DC converter therefor are presented, in which a primary side state estimator circuit is provided for inferred output current sensing for closed loop control of pulse width modulated flyback or buck converters. Specifically, the estimator circuit includes a capacitance having a first terminal coupled to a comparator input of a pulse width modulation controller, and a second terminal coupled to an error input of an error amplifier, and a switching device having a first power terminal coupled to the second terminal of the capacitance, a second power terminal coupled to the circuit ground, and a control input terminal coupled to a drive output of the pulse width modulation controller.