Preliminary WS2801
3-Channel Constant Current
LED Driver With Programmable
PWM Output
Fig8, Constant Voltage Driving with External Power Switches.
This driving mode is mainly used in high input voltage, high LED driving current (ILED>300mA). Its operating principle is using the
external power NPN switches to driving the LED lamps. The LED driving current can be adjusted by the external current limiting
resistor RL, RL can be derived by the following equation:
RL
= VDD
− VLED
I LED
− VCE
------------------------------------------------------------- (2)
The external NPN transistor operates in saturation region, VCE is the saturation voltage, and VCE=0.5~0.8V, the base resistor
RB=2~5K.
Constant Current Driving Mode with External Switches
This driving mode makes WS2801 can drive the cascading LED lamps with extreme low cost, at this mode, POL=’HIGH’ or
floating, see Fig10.
WS-Preliminay-V0.3
Fig9, Constant current drive with external switches
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