This is an application example for an ON Semiconductor CAT4104 Quad Channel Constant Current LED Driver. It demonstrates the part's resistor programmable and independent current control for each channel, as well as its PWM dimming capability. By choosing the value of r_set from 768 Ohm up to 12 kOhm, the current in each channel is controlled from 175 mA down to 10 mA. That is, the channel current is 100 times the current in r_set, which is driven by an internal 1.2 V source from the CAT4104 "rset" pin. The LED strings can be driven by parallel channels to reach even higher string current levels.
For LED dimming, the CAT4104 "en_pwm" pin can be driven by a PWM signal. In this application circuit, that signal is provided by an LM555 timer module. The potentiometer setting controls the PWM duty cycle, and the element's total resistance value determines the PWM frequency. The initial potentiometer wiper ratio of 0.5 and resistance of 20 kOhm, gives a 50% PWM duty cycle at 500 Hz.
The LED string model, for which the user can select the number of LEDs in the string as well as the individual LED operating parameters, has an internal quantity that gives the "perceived" luminous output. This includes a 15 Hz Low-pass filter, to represent the light-averaging effect of the human eye. This is shown in the dark blue waveform. The user can explore the effect of changing the potentiometer settings, which in turn change the PWM characteristics, on the perceived light output. The initial LED parameters for this example represent a CREE XPCWHT-L1-R250-009E6.