Dynamic siphon steals current from USB port

Maxim MAX4173 MAX4238 MAX6129

A USB port offers a handy source of 5 V 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.

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 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 MAX4173; IC2, a Maxim MAX6129; 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 100 V/V gain, and R1 with a 0.25 Ω resistor.

This circuit continuously monitors the total current drawn from the USB port and dynamically adjusts the storage-element current 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 to avoid exceeding the port’s maximum output capability.
 
These waveforms taken from Figure 2 show that the sum of the essential circuitry current (middle trace) and storage-element current (bottom trace) never exceeds the 500 mA maximum that the USB port (top trace) specifies.
Figure 3. These waveforms taken from Figure 2 show that the sum of the essential circuitry
current (middle trace) and storage-element current (bottom trace) never exceeds
the 500 mA maximum that the USB port (top trace) specifies.

Materials on the topic

  1. Datasheet Maxim MAX4173
  2. Datasheet Maxim MAX4238
  3. Datasheet Maxim MAX6129
  4. Datasheet Fairchild FDN359AN
  5. Datasheet Fairchild FDS6679

EDN

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