The circuit presented here serves as a step-up converter designed to power 20 LEDs, intended for use as a DIY ceiling night light in a child’s bedroom. Traditional night lights of this kind typically consist of a string of Christmas tree lights with 20 bulbs, each consuming 1 W, totaling 20 W. In an effort to conserve power and prolong operational lifespan, this circuit provides an updated approach by utilizing LEDs. Power can be sourced from an unregulated 12 V mains adapter, provided it can deliver a minimum of approximately 330 mA. The circuit employs a cost-effective current-mode controller, specifically the UCC3800N, which has been reconfigured into voltage mode to create a step-up converter with straightforward compensation.
Versatility through External Component Changes
Modifying the circuit for various applications is effortless by adjusting the external components. To convert a current-mode controller into a voltage-mode controller. It’s crucial to couple a sawtooth ramp rising from 0 V to 0.9 V to the CS (current sense) pin. This pin serves as an input to the internal PWM comparator. The necessary ramp is generated on the RC pin of the IC and is adjusted to the appropriate voltage range by the voltage divider created by R3 and R2.
Setting Switching Frequency and Output Voltage
The switching frequency, set at approximately 525 kHz, is determined by the RC network comprising R4 and C6. The comparator compares the ramp with a scaled-down version of the output voltage from the potential divider (R6 and R7). Trimmer P1 facilitates output voltage adjustment, allowing precise control of the current flowing through the LEDs, tailored to the specific devices employed.
Operating Range and Adjustment Process
The UCC3800N initializes with an input voltage of 7.2 V and ceases operation if the input falls below 6.9 V. The circuit accommodates output voltages between 20 V and 60 V, suitable for various scenarios, considering most white LEDs have specified forward voltages ranging from 3 V to 4.5 V. For the depicted setup with two parallel chains of ten LEDs in series, a voltage between 30 V and 45 V is needed.
Optimizing Power Components and Calibration
D1, T1, and L1 in the power section are overdimensioned due to the circuit’s original application requiring higher power. To fine-tune the circuit, start by setting the potentiometer to maximum resistance. Insert a multimeter in series with the LEDs and set it to a 200 mA DC current range. Gradually turn P1 until a constant current of 40 mA flows. This calibration ensures the step-up converter operates correctly and is ready for deployment.