Product How-to: Digital isolators offer easy-to-use isolated USB option
Transparent USB isolator requirements
Figure 3: ADuM4160 block diagram
A USB isolator system must satisfy several requirements to fully achieve ‘transparent’ operation:
- It must drive UD+, UD-, DD+, and DD- in the same manner as a standard USB transceiver, and in fact contains two USB-compliant transceivers, one on each side of the isolation barrier. (Figure 3).
- It must manage bi-directional communication on the USB cable, by ensuring that its transceivers transmit and receive at appropriate times, and accurately reproduce all driven and idle states. To accurately reproduce the idle states it must include a pullup resistor on its upstream side to mimic the state of the pullup resistor attached to the downstream peripheral. It may also include pulldown resistors on its downstream side. The bus must be monitored for signals that indicate idle bus, start-of-packet, and end-of packet to appropriately respond to those conditions.
- Signal isolator components inside the USB isolator must communicate D+ and D- data back and forth across the isolation. If the signal isolators are unidirectional (as is generally the case), the USB isolator system needs multiple isolation channels, some transmitting in a downstream direction, and others transmitting in the opposite, upstream direction.
- The signal isolators must run fast with accurate timing, to support the desired USB signaling speeds, and conform to USB requirements for propagation delay and timing error.
- Each side of the USB isolator should support power provided by 5V or 3.3V supplies. If 5V power is provided, the isolator should derive a 3.3V regulated supply suitable for powering that side’s USB transceiver. If 3.3V power is provided, the isolator can use it to directly power the USB transceiver and bypass its regulator.
A transparent USB isolator realization
The Analog Devices ADuM4160 USB digital isolator2 meets all requirements, and is integrated into a 16-lead SOIC package. A block diagram is shown in figure 3. It contains a pair of USB transceivers, five channels of iCoupler®-based digital isolation, control logic, and two ‘smart regulators’. It also includes a 1.5kΩ upstream pullup resistor, and 15kΩ downstream pulldown resistors.
Its USB transceivers are controlled by a simplified controller, which doesn’t need to fully decode and analyze the packets to support the isolation function. Instead it can monitor UD+, UD-, DD+, and DD- for signals that indicate idle bus, start-of-packet, and end-of packet, and use them to correctly enable or disable USB transmitters while ignoring packet content. When transmitting a packet downstream from host to peripheral, the upper two isolation channels in figure 3 are active, as are the upstream USB receiver and downstream USB transmitter. Data is copied from UD+/UD- to DD+/DD-. When the packet ends, the end-of-packet sequence is detected and all USB transmitters disabled, allowing the bus to reach the idle state. If the peripheral subsequently starts transmitting a packet upstream, the USB isolator detects the start-of-packet sequence, enables the third and fourth isolation channels and upstream USB transmitter, and copies data from DD+/DD- to UD+/UD- until the packet ends. Then the bus is again returned to idle with all transmitters off, awaiting new data.
The ADuM4160 uses a fifth isolation channel to communicate the status of a control line on the downstream side3, which activates a pull-up resistor integrated into the upstream side. This allows the downstream port to control when the upstream port attaches to the USB bus. The pin can be tied to the peripheral pull-up, a control line, or the VDD2 pin, depending on when the initial bus connect is to be performed. Attaching the pin to the peripheral pullup enables its state to be mimicked by the upstream pullup, and the ADuM4160’s pulldowns mimic the state of those attached to the host. All active and idle states are copied from one side of the isolation to the other.
The isolation channels are digital isolators using chip-scale transformers to achieve isolated communication. The individual channels can each operate beyond 100 Mbps, easily supporting 12Mbps USB ‘full speed’ data. Integrating all channels together in a single chip enables tight control of timing, giving low timing error that meets USB timing requirements. Overall propagation delay through the ADuM4160 is equivalent to the delay through a standard USB hub. Quiescent power consumption is below USB limits for idle buses.
The smart regulators support the power supply options mentioned above in requirement 5, without requiring explicit user control4. To power one side of the USB isolator from 5V (for example the upstream side), the 5V supply is connected to the appropriate VBUS pin (e.g. VBUS1), while VDD1 is not connected. When sensors detect that voltage is applied to VBUS1 but not VDD1, they activate the 3.3V regulator to power VDD1.
To instead power one side of the USB isolator from 3.3V (for example the downstream side), the 3.3V supply is connected to both VBUS2 and VDD2. When sensors detect applied voltage at both pins simultaneously, the on-chip regulator is disabled in order to directly use the externally provided 3.3V.
The ‘transparent’ USB isolator in which isolation conceptually bisects the USB cable is very easy to use with USB hardware that was originally designed for non-isolated applications. This contrasts with alternatives which place the isolation inside the host or peripheral hardware, requiring substantial hardware modification and sometimes degrading USB performance. The transparent concept is very challenging to implement using discrete components like off-the-shelf general purpose isolators. However, recent integrated solutions like the ADuM4160 overcome the challenges in a single convenient package, greatly simplifying the addition of isolation in USB applications.
1: Hauck, Lane, “Isolating USB”, EDN magazine, July 2006.
2: Information on ADuM4160, iCoupler digital isolators, and other Analog Devices products can be found at www.analog.com/icoupler
3: US patent #8432182
4: patent pending
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