[djahng]

Generating a PWM signal based on a particular LED's characteristics is similar to designing the gain of an amplifier circuit around a particular BJT's beta. It's just not done. That's also the whole reason behind controlling LED brightness through PWM. By PWMing the drive current, you can ensure that the LED is being driven far into its conducting region, and therefore not relying on controlling dimness by varying current around the LED's "knee" (which literally is different for just about every LED in existence, even same part numbers). This way you also avoid other stuff like wavelength shifts that tend to occur near the knee.