Before starting the build there is a little bit of math required to select the LED, LED current limit resistor, and power supply. LED selection:
This circuit can drive either a single led or a string of leds as long as they fall under the following conditions:
LED current limit resistor:
- Max forward current of the string is 1A (1000mA) or less
- Max forward voltage of the string is 24.5V or less
The CAT4101 datasheet includes the following table for setting LED current.
| LED current in mA|| current limit resistor value in Ohms|
| 100mA|| 4990 Ohms|
| 200mA|| 2490 Ohms|
| 300mA|| 1690 Ohms|
| 400mA|| 1270 Ohms|
| 500mA|| 1050 Ohms|
| 600mA|| 866 Ohms|
| 700mA|| 768 Ohms|
| 800mA|| 680 Ohms|
| 900mA|| 604 Ohms|
| 1000mA|| 549 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)
12V is less than 24.5V so ok there.
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.
In my junk pile I happened to have a 16V @ 4.5A power supply from an old laptop. This meets all the requirements calculated above for the LED I picked.