Simple circuit prevents USB current overshoot during insertion

You can inadvertently trigger an unwanted system reset by inserting a simple pen drive or other peripheral into a USB port. One should, therefore, provide a current limiter in front of the port, for protection against shorts and overloads that may be connected to the port itself.

One characteristic of this interface is the possibility of supplying power to the various peripherals you might connect, such as a pen drive, small hard disk, modem, MP3 reader, or memory card. A server board fed by a 12-V main power rail, for example, includes a buck converter that steps the rail down to 3.3 V for powering digital logic. It also includes a step-up circuit that boosts the 3.3-V rail to the standard voltage for a USB port (5 V, with 500 mA current capability).

A problem can occur when you insert peripherals: the sudden load on the port's 5-V pin usually is not purely resistive, but has a capacitive component that can cause a rapid and high-amplitude rise in current, to levels well above the 500 mA limit for a USB port.

A similar problem occurs when you connect a too-large hard disk to the port. The capacitive load is not an issue; rather, the disk's normal operating current may exceed 500 mA, causing a system reset or causing the power supply to go into a protective mode. To limit inrush current and restrict the maximum output current to 500 mA, we usually add a protection device between the 5-V supply and the port's VOUT pin.

Because the protection device cannot limit current immediately after a load is connected (without causing a false current limitation and oscillation) it must be blinded for a short interval after insertion, to let the system stabilize. The power supply, however, must withstand the inrush current during this blind period. Step-down converters can handle the inrush because IOUT is sensed by ILIMIT resistance, and the current loop acts to keep IOUT within the value set by that resistance. Step-up converters are a different story.

The inductor in a step-up converter provides a continuous path between Vin and the load. When the power supply cannot provide enough current to the load, it starts to charge the step-up converter's voltage source. In some cases, this action can cause the supervisor to assert a system reset, and thereby crash the system.

A simple modification to the 5-V boost converter circuit solves these problems (Figure 1 ): insert a low-value resistor (∼0.5 Ω) between the boost-converter output and the resistive divider used for feedback. Because the 5-V converter (U1, MAX1760 DC/DC converter) now compensates for voltage drops across this resistor, the voltage applied to the IOUT limiter (U2, Maxim MAX8586 current-limited switch) is independent of the 5-V load current.

Figure 1: While generating 5 V for the USB port, this circuit also limits peak inrush currents at the moment of insertion (at the USB connector), and limits operating current to 500 mA as required by the port.

(Click on image to enlarge)

This circuit change prevents the high and narrow current peaks that otherwise occur during the insertion of low-capacitance loads such as a pen drive (1 μF). Such current peaks can cause a system reset by overloading the boost converter first, and then the 3.3-V bus. The 0.5 Ω resistor not only limits peak currents to a reasonable value during the blind time, it also (following that condition) limits IOUT to 500 mA maximum, as required by a USB port.

Editor's Note : Other stories at Planet Analog related to USB protection include:

About the author
Luciano Bordogna was born in Monza, near Milan, (Italy). He received an electronics degree in 1980 and worked on very-high-frequency circuits at Alcatel (in Vimercate). After that, he designed electronic circuits for destructive and non-destructive tests for construction material such as concrete, iron, and bituminous ores for about four years. He then worked for Philips Medical System on X-ray equipment for three years, after which he joined a company working in the industrial electronics and motor drivers. He joined Maxim in 1995 as inside application engineer and then as a field application engineer covering OEM customers in Italy.

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