USB Power Adapter Overview
A USB power adapter serves the purpose of generating the standard 5-volt DC power required for USB devices. The amperage typically varies, ranging from 0.07 A to 2.4 A. This adapter is designed to plug into an AC outlet, and a USB cable connects to the adapter, facilitating the power supply to various USB devices.
Addressing Power Shortage Issues
Problems related to power shortages often occur when multiple USB devices connected to a PC operate simultaneously. Devices such as scanners, modems, thermal printers, mice, USB hubs, and external storage devices draw power from the PC. Due to the limited power supply capacity of a PC, external power sources become necessary to accommodate these power-demanding devices. The purpose of this circuit is to augment the power supply along a USB cable line.
Utilizing a sealed 12-V 750 mA unregulated wall cube proves to be a cost-effective and safe solution. To convert 12 V to 5 V, two types of regulators, switching, and linear are available, each with its own advantages and drawbacks. The switching regulator, chosen for this circuit, is particularly suitable due to its high efficiency and compact design, especially as most digital circuits can now tolerate voltage ripple developed during switching.
Stable 5 V Output with LM2575-5 Switcher
In this setup, the LM2575-5 switcher is selected to maintain a stable 5 V output voltage. Remarkably simple, this switcher requires only three components: an inductor, a capacitor, and a high-speed or fast-recovery diode. Its operation relies on the internal power transistor, which switches on and off in response to a feedback signal. The chopped or switched voltage undergoes conversion to DC, displaying minimal ripple, thanks to components D1, L1, and C2.
Controlled Operation and Power Indication
The LM2575-5 features an ON/OFF pin that activates when pulled to the ground. T1, R2, and R1, acting as a pull-up resistor, ground the ON/OFF pin upon receiving a power signal from the PC or +5 V source. To signify a stable 5 V power condition, D2, a red LED with current-limiting resistor R3, illuminates. Additionally, C3 serves as a high-frequency decoupling capacitor. A notable point is the author’s ingenious approach, splitting a USB cable without cutting the data wires. It’s advisable to consult the USB cable pin assignment for safety considerations.