关键词: MAX22505, ±40V USB Protector
The MAX22505 is a highly integrated USB port circuit breaker. It automatically breaks the connection from a USB port when it detects voltages that could threaten system integrity. The MAX22505 also automatically reconnects the USB port when such threats are no longer present. This automatic operation is transparent to supported USB operation speeds and requires no configuration.
The MAX22505 is most useful protecting diagnostic USB ports on industrial PCs and programable logic controllers (PLCs). The MAX22505 excels at protecting against faults that can occur in 24VDC and 24VAC power bus systems.
The MAX22505 offers the following key benefits:
USB isolators are a more expensive technology, providing galvanic isolation between two domains, protecting against noise and ground loops, and typically meeting a safety or regulatory approval, such as the UL 1577 specification for optical isolators. Isolators permit continued operation of the USB interface despite ground shifts. In many industrial applications, this is overkill; protection against accidental connections to power buses is enough if communication continues once the fault is removed. A protector, such as the MAX22505, provides the following benefits compared to a USB isolator:
The MAX22505 supports 1.5Mbps low-speed, 12Mbps full-speed, and 480Mbps high-speed rates.
Yes, the MAX22505 is transparent in USB links, including USB OTG.
The MAX22505 protects all four standard USB lines, VBUS, GND, D+, and D-. A fault on any of these lines opens the circuit breaker, disconnecting all four lines simultaneously. Only when all faults end on all of VBUS, GND, D+, or D- lines, does the MAX22505 fully close the circuit breaker function, thereby reconnecting all four lines.
When open, the MAX22505 protects the D+ and D- signal lines from voltage differences up to ±40V. The VBUS and GND power lines are protected up to ±50V. The enhanced protection on the power lines provides enough margin against accidental connections to 24V AC power busses.
When not supplied with power, the MAX22505 circuit breaker function opens all connections with the same level of protection as if the MAX22505 were powered. All switches remain open.
The open-drain FLTB pin asserts active low when the circuit breaker function opens. If there is no fault, or if there is no USB bus connected, a pull-up resistor on this open-drain output then pulls FLTB inactive high.
Voltage drops on a USB cable can cause operation issues in high-speed (480Mbps) mode due to common mode shifts in the USB data signals D+ and D-. Low-speed and full-speed modes tolerate a greater common mode shift, compared to high-speed mode. When enabled, when there is a moderate common mode shift, and when the USB bus is in high-speed mode, the MAX22505 automatically changes from high-speed to low/full-speed mode. Should the common mode increase excessively, the MAX22505 opens the connection. High-speed operation resumes once the user removes then reattaches the USB device and there is no common mode shift on the link. This is an optional feature, enabled by ensuring that the HCMB pin is high at power-up. This indication is only checked at power-up.
The MAX22505 works well in both USB host and USB peripheral applications. One factor to consider in a USB peripheral application is whether the application is a self-powered or a bus-powered peripheral. The MAX22505 EV Kit demonstrates all possibilities—a USB host, a USB self-powered peripheral, and a USB bus-powered peripheral.
The MAX22505 EV Kit has three independent sections. Each section demonstrates the best way to protect a USB self-powered peripheral, a USB bus-powered peripheral, and a USB host.
The isolation switches built into the MAX22505 have an ABS MAX rating of 280mA through the VBUS and GND USB lines. That is more than enough in USB self-powered applications, which often need only 5mA or less from the USB host. However, bus-powered peripheral applications often need more than 280mA from the host. Also, host applications must provide at least 500mA to an attached peripheral. External switches supplement the power drive capability of the MAX22505 in the case of a high-power peripheral or in the case of a host. The MAX22505 already provides all the necessary control signals to properly drive these external switches.
This is why the high-power peripheral section of the EV Kit and the host section of the EV Kit have external switches, while the low-power peripheral section does not.
These inductors improve signal integrity. The MAX22505 appears slightly capacitive to the data signals, and these inductors cancel that slight capacitive effect. The following eye diagrams show the signal both without and with these inductors. A well-designed circuit and PCB works best with the recommended 20nH inductance.
Figure 1. Signals without series inductors (D+, top; D-, bottom).
Figure 2. Signals with series inductors (D+, top; D-, bottom).