Several driver options are at the fingertips of designers with the FAN56XX family of LED drivers from Fairchild Semiconductor. For instance, driver-only parallel connection drivers with no boost circuit can either drive matched LEDs (LEDs binned for brightness and forward voltage, or VF) or unmatched LEDs (those binned only for brightness). These ICs, which drive up to two to four LEDs in parallel, feature resistor-programmed current and on/off enable. Those drivers targeting unmatched LEDs contain the previously described VF compensation circuit.
The driver family also includes drivers with dc-dc boost circuits and parallel connection of four LEDs. These devices improve conversion efficiency by measuring the VF of the LEDs and adjusting the driver's boost voltage to a value 150 mV above the highest measured VF. That differs from the typical LED driver, with which the boost voltage is fixed.
By adjusting the boost voltage, the headroom between boost voltage and VF is minimized, which in turn minimizes the power dissipated in the driver. As a result, Fairchild's driver ICs can attain up to 97.5% efficiency and require no external current-matching resistors. In addition, the company offers drivers with dc-dc boost converters for serial and parallel connection of LEDs. These ICs drive multiple channels of serial-connected LEDs.
Fairchild's driver ICs feature three mechanisms of brightness control. The first method, analog control via a dc voltage, is fairly common among LED drivers. However, the other methods of control are less common. The drivers feature a two-line digital input that permits four levels of brightness control. In addition, users may apply a pulse-width modulation (PWM) signal on the drivers' input, as well as control LED brightness by varying the PWM duty cycle.
Fairchild Semiconductor
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