3/20/08 SP6699 Evaluation Board Manual © 2008 Exar Corporation
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_______________________________________________ USING THE EVALUATION BOARD
1) Powering Up the SP6699 Circuit
The SP6699 Evaluation Board can be powered from a 2.5V to 16V power supply. Connect the power
supply with short leads directly to the “VIN” and “GND” posts.
2) Setting the output current
The typical SP6699EB configuration is for 5 WLEDs mounted in series and with the output current set for
20mA. Other LEDs could be externally connected but need to be connected anode to the VOUT pin and
cathode to the Vfb pin. As many as 5 series connected LEDs may be used, depending on the input
voltage range available. The total output current of the SP6699 is controlled by the VFB pin voltage
setpoint of 200mV. Use a resistor at R2 to set the output current by the formula:
Iout = Vfb/R2
Standard SP6699EB Evaluation Board Example:
Iout = Vfb/R2 = 200mV/10ohm = 20mA
3) Dimming
The SP6699 can be pulse width modulated using a signal applied to the CTRL post. The CTRL signal
will turn on the SP6699 when CTRL = H and turn-off the SP6699 when CTRL = L. The CTRL signal
needs to be greater than 1.8V minimum to turn-on the SP6699 and less than 50mV to fully turn-off the
SP6699. It is recommended to use a signal with CTRL = 1.8V or more for ON and 0V for OFF.
Recommended modulation frequencies are from 100Hz to 200Hz.
4) Efficiency and Output Current
The SP6699 evaluation board efficiency is shown in figure 2 for 5 WLEDs at 20mA and 25mA. The
efficiency is as high as 82% for 4.2V input which is very good for a high voltage boost regulator. The
output current curves are shown in figure 3 and show that the output current remains very stable for a
range of input voltage from 3V to 5V input.
