For a stable system, the gain must be less than one by the time a 360^o phase shift occurs. The error amplifier inverts the signal (180^o phase shift); added to the 180^o phase shift of the L-C filter results in a 360^o phase shift. If there is gain at the L-C corner frequency, a voltage mode converter will oscillate. For voltage and current mode controllers, compensation is required to get the adequate phase and gain margins required for stability. Designers should keep in mind that the desired phase margin is 45 - 60°. Compensation can be fixed internally (minimizes external components but limits choice of output filters).