Cirrus Logic AN375 - Manual (Page 6)

AN375
6 AN375REV4
3.1 Operating Parameters
To initiate the design procedure, a set of operating parameters is required. Operating parameters required for
the analytical process are outlined in the table below. Parameters critical to the overall design, but not
specifically addressed in this document, include EMI compliance, efficiency, form factor, layout, and operating
temperature.
3.2 Overview of Design Steps
The AC line current is the sum of the flyback current flowing in power FET Q4 and the current flowing in dimmer
compatibility FET Q2, the main element of the dimmer compatibility circuit. When defining the flyback current
components, two modes of operation must be considered: Dimmer Mode and No-dimmer Mode.
Dimmer Mode
When a dimmer is detected, the flyback current appears constant on the AC line side of the EMI filter. The
flyback operates until the charge taken from the line in each half line-cycle equals the desired LED current. For
correct dimmer operation, it is desirable to quickly reach the dimmer TRIAC holding current, and for this reason
a constant current is preferable to a slow rising sine wave. Current through FET Q2 is added at critical phases
to keep the TRIAC in conduction and to discharge the EMI filter capacitor to reveal when the dimmer opens.
The need to set the constant current value based on the dimmer requirement sets a minimum input power that
may be greater than the power required by the load. This is particularly true when a low-wattage LED is
dimmed significantly. This excess power is dissipated by FET Q2 and resistor R15.
No-dimmer Mode
For optimal power factor and lowest THD when no dimmer is present, the flyback operates in a PFC mode that
regulates the AC line current to be nearly sinusoidal.
Parameters Symbol
Output Power P
OUT
AC Line Input Voltage V
IN
Output Voltage V
OUT
Load Current I
OUT
Maximum Switching Frequency * F
sw(max)
* Increasing switching frequency F
sw
may reduce the size of the magnetics but increase switching losses in the
FET and transformer.