SP6648LED Datasheet, PDF (2/5 Pages) Sipex Corporation – Evaluation Board Manual

USING THE EVALUATION BOARD
1) Powering Up the SP6648LED Circuit
The SP6648LED Evaluation Board can be powered from inputs from a +1.8V to +3.2V power
supply or from 2 alkaline cells. Connect with short leads directly to the “VIN” and “GND” posts.
Plug the 1W LumiLED between the “VOUT” and “REF” posts. Unlike other boost regulators, the
part has self-protection feature. It will not be damaged when the output is open as long as there
is a low resistance from FB to GND. As shown in Figures 1 and 2, if the board is powered up
before the LumiLED is plugged in, the circuit will bring the Feedback pin to 0V and the SP6648
has a feature to set the output voltage to be 3.3V. Once the LumiLED is plugged in, the
Feedback pin will go up to 1.25V and begin to regulate. The output voltage will go from 3.3V to
3.68V (=V_F+0.61V) accordingly (as shown in Fig. 1), where V_F is the forward voltage of the
LumiLED. When the LumiLED is open, the Feedback pin voltage will go to 0V and the output
voltage will go to 3.3V which will protect the part (as shown in Fig 2).
V_OUT
V_OUT
V_REF
V_REF
Fig 1. Plug in the LumiLED
Fig. 2. Unplug the LumiLED
2) Using the J1 Jumper: Enabling the SP6648 Output and using the Shutdown Mode
The SP6648 output will be enabled if the J1 Jumper is in the top or pin 1 to 2 position. If J1 is in
the pin 2 to 3 position, the Shutdown pin is brought to GND, which puts the SP6648 in the low
quiescent Shutdown Mode.
3) Selecting the Bias Resistor
In the white LEDs application, the SP6648 is generally programmed as a current source. The
bias resistor is used to set the operating current of the white LED as equation
R_b
=
V_REF
I_F
where V_REF is around 0.61V, I_F is the operating current of the Lumiled. To set the operating
current to be about 350mA, R_b is selected as 1.8 Ohm as shown in the schematic.
4) Vout Programming
The SP6648 can be programmed as either a voltage source or a current source. To program the
SP6648 as voltage source, the SP6648 requires 2 feedback resistors R₆ & R₇ to control the
output voltage. To set Vout in the voltage mode, use the equation:
R₆
=


V_out
0.61
−
1

•
R
7