Main design goals for this project were:
- Small size
- 10 dimming levels + off
- minimum 10W power handling
- Fan control (on/off + PWM)
Here is a video of me taking the prototype card though it's paces:
Step 1: Some math
This circuit can drive either a single led or a string of leds as long as they fall under the following conditions:
- Max forward current of the string is 1A (1000mA) or less
- Max forward voltage of the string is 24.5V or less
LED current limit resistor:
The CAT4101 datasheet includes the following table for setting LED current.
|LED current in mA||current limit resistor value in Ohms|
Power supply selection:
There are 3 conditions the power supply must meet to be used with this driver.
- Unloaded voltage (floating / LEDs off) less than 25V
- 0.5V greater than max LED (or string) voltage
- Less than 6V over the LED (or string) voltage when LEDs are on)
- Output current of the power supply is greater than the forward current of the LED (or string) + 100mA
In my case I am using a LED with a on voltage of 10V to 12V and a forward current of 1A (1000mA) (this is from the LED data sheet)
1A (1000mA) looked up in the current limit resistor table results in a 549 Ohm resistor
12v (LEDVmax) + 0.5v = 12.5v is the minimum output voltage that the power supply can put out and be usable.
10V (LEDVmin) + 6V = 16V is the maximum output voltage that the power supply can put out and be usable.
1A + 100mA = 1.1A is the minimum output current that the power supply can put out and be usable.