Dynamic Siphon Steals Current from USB Port

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Abstract

This circuit exploits all the power available from a USB port by dynamically adjusting the amount of current delivered to the load, thereby siphoning a relatively constant (and maximum) current from the USB port. Included are a current-sense amplifier (MAX4173), voltage reference (MAX6129), and precision op amp (MAX4238).

A similar version of this article appeared in the December 15, 2006 issue of EDN magazine.

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.

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 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).

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.