For DC-to-DC (DC/DC) powered LED lighting applications, discussions can be based on different power ranges.
1) 1~3W DC/DC LED step-down application
Typical 1~3W DC/DC LED step-down lighting applications include MR11/MR16, automotive lighting, and solar power. These applications feature input voltages from 5 to 28 Vdc, support for 350mA and 700mA constant current output, frequency up to 500kHz to 2MHz, energy efficiency of no less than 90%, and an operating temperature range of -40°C to 125°C. In this type of application, ON Semiconductor's CAT4201 step-down LED driver can be used. The device's patented switch-control architecture drives up to seven LEDs in series (at 24V input) with up to 94% efficiency and provides protection features such as LED open-circuit protection, current limiting and thermal protection.
Figure 1: CAT4201 1~3W DC/DC LED scheme.
2) 1~30W DC/DC LED step-down application
Typical 1~30W DC/DC LED step-down applications include MR16 spotlights, secondary side DC/DC LED drivers in street lighting. In this type of application, the input voltage range is 7 to 120 Vdc, the output voltage range is 6 to 110 Vdc, and the 350mA, 700mA or 1A constant current output is supported, and the energy efficiency is not less than 90%. This type of application can use ON Semiconductor's NCL30100. Buck LED driver, this device's external switching MOSFET, provides flexible input voltage and output current design with energy efficiency above 95%. See Figure 2 for the application circuit diagram.
Figure 2: NHL30010 based 1-30W LED step-down application and NCP3066 based 3-20W LED boost application.