"Current controller 112 controls active and average values of LED current i.sub.LED by controlling the conductivity of n-channel field effect transistor (FET) Q1. Current controller 112 generates a gate control signal C.sub.G0 to charge and discharge a gate of FET Q1. The control signal C.sub.G0 has two relevant frequencies, an active frequency and a duty cycle modulated frequency. During an active period of LED current i.sub.LED, the control signal C.sub.G0 has an active frequency in the range of, for example, 20 kHz to 500 kHz. As described subsequently in more detail, the duty cycle modulated frequency is less than the active frequency. The active period of LED current i.sub.LED is the period of time when the average value of LED current i.sub.LED equals current value i.sub.FULL. The time period for this average is, for example, one or a few (such as 3-5) periods of the active frequency.
"When the control signal C.sub.G0 is a logical 'one', FET Q1 conducts, i.e. is 'ON', and when the control signal C.sub.G0 is a logical 'zero', FET Q1 is nonconductive, i.e. is 'OFF'. When the FET Q1 is 'ON', diode D1 is reversed bias and, LED current i.sub.LED flows through the LEDs 102 and charges inductor L.sub.1. When FET Q1 is 'OFF', the voltage across inductor L.sub.1 changes polarity, and diode D.sub.1 creates a current path for the LED current i.sub.LED. The inductor L.sub.1 is chosen so as to store enough energy to maintain an approximately constant active value of LED current i.sub.LED when MOSFET Q1 is 'OFF'. Capacitor C1 helps 'smooth' LED current i.sub.LED. As subsequently explained in more, detail, the active value of the LED current i.sub.LED is the average LED current i.sub.LED when the current control system 112 is active, i.e. during the active period of LED current i.sub.LED. The LED current i.sub.LED includes a ripple 201 due to, for example, the charging and discharging of inductor L1. The frequency of the ripple 201 is the active frequency. It is desirable, for LED efficiency, to keep the LED current relatively constant, to reduce heating effects.
"FIG. 2 depicts a graphical representation 200 of the LED current i.sub.LED for various dimming levels indicated by the phase modulated signal V.sub..PHI.. Referring to FIGS. 1 and 2, when the phase modulated signal V.sub..PHI.indicates a full dimming level, i.e. full brightness for LEDs; 102, current controller 112 controls the LED current i.sub.LED so that the active value of LED current i.sub.LED is continuous and constant over time and equals i.sub.FULL, as indicated by LED current i.sub.LED waveform 202. 'i.sub.FULL' represents the active value of LED current i.sub.LED that causes the LEDs 102 to illuminate at full brightness.
"The current controller 112 uses feedback information from feedback signal LEDi.sub.sense to sense the active value of LED current i.sub.LED. The feedback signal LEDi.sub.sense represents a voltage V.sub.fb across sense resistor R.sub.SENSE. The voltage V.sub.fb represents LED current i.sub.LED when FET Q1 is ON. Thus, from the feedback signal LEDi.sub.sense, the current controller 112 obtains the value of LED current i.sub.LED and can adjust the duty cycle of control signal C.sub.G0.sub.--.sub.FULL to maintain the active value of LED current i.sub.LED at the full active value i.sub.FULL during the active period of LED current i.sub.LED. As subsequently explained in more detail, the control signal C.sub.G0.sub.--.sub.FULL is also duty cycle modulated at the duty cycle modulation frequency in response to dimming levels indicated by phase modulated signal V.sub..PHI.to generate control signal C.sub.G0.
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