Dynamic siphon steals current from USB port

A USB port offers a handy source of 5V power for auxiliary devices. A USB port not only supplies power to a microcontroller and other essential circuitry, but also provides enough extra current head room to charge a small battery or supercapacitor energy-storage element. One typical approach to exploiting a USB port's leftover-current capability begins with an estimation of the essential circuitry's maximum current drain. You then place an appropriate current-limiting device in the path of the energy-storage device (Figure 1). Although easy to implement, this method doesn't use all of the current available from the USB port, and the energy-storage device slowly charges or recharges.

The circuit in Figure 2 uses all available USB power by dynamically adjusting the amount of current delivered to the energy-storage device and thereby siphoning a relatively constant and maximum current from the USB port. IC₁, a Maxim MAX4173FEUT; IC₂, a Maxim MAX6123AEUK25; and the load-switch circuit comprising Q₁, Q₂, R₂, and C₄ form a control loop that limits the current flowing through Q₁. 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 of 500 mA for a high-power USB device. To reconfigure the circuit for low-power USB operation of 100 mA maximum, you can replace IC₁ with a MAX4173HEUT, a device with 100V/V gain, and R₁ with a 0.25Ω resistor.