A similar version of this article appeared in the Decmeber 15, 2006 issue of EDN
USB ports can be a handy source of 5V power. Not only can a USB port power a microcontroller and other essential circuitry, it often has enough current headroom left over to charge an energy-storage element such as a small battery or super-capacitor. The typical approach is to estimate the maximum current drawn by the essential circuitry, and then place an appropriate current-limiting device in the path of the energy-storage device (Figure 1
). Though simple, this method doesn't utilize all of the available USB current. It therefore takes longer to charge the energy-storage device.
Figure 1. In this typical method for drawing power from a USB port, the storage-element current is limited to a fixed value that is less than optimal.
The circuit of Figure 2
exploits all of the available USB power by dynamically adjusting the amount of current delivered to the energy-storage device, thereby siphoning a relatively constant (and maximum) current from the USB port. U1 (MAX4173
), U2 (MAX6129
), and the load-switch configuration (P1, N1, R2, and C4) form a control loop that limits the current flowing through P1. The circuit maximizes current flowing to the energy-storage element (Figure 3
) by ensuring that the sum of battery and essential-circuitry currents never exceeds the maximum allowed for a USB high-power device (500mA).
Figure 2. This circuit continuously monitors the total current drawn from the USB port, and dynamically adjusts the storage-element current as required to avoid exceeding the port's maximum output capability.
Figure 3. These waveforms from the Figure 2 circuit show that the sum of the essential-circuitry current (middle trace) and storage-element current (bottom trace) is always less than the 500mA maximum specified for the USB port (top trace).
To reconfigure the circuit for USB low-power operation (100mA max), you can replace U1 with a MAX4173HEUT
(a device with 100V/V gain) and R1 with a 250mΩ resistor.