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ADuM4160BRIZ Datasheet(PDF) 10 Page - Analog Devices |
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ADuM4160BRIZ Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page ADuM4160 Rev. C | Page 10 of 16 APPLICATIONS INFORMATION FUNCTIONAL DESCRIPTION USB isolation in the D+/D− lines is challenging for several reasons. First, access to the output enable signals is normally required to control a transceiver. Some level of intelligence must be built into the isolator to interpret the data stream and determine when to enable and disable its upstream and down- stream output buffers. Second, the signal must be faithfully reconstructed on the output side of the coupler while retaining precise timing and not passing transient states such as invalid SE0 and SE1 states. In addition, the part must meet the low power requirements of the suspend mode. The iCoupler technology is based on edge detection, and, therefore, lends itself well to the USB application. The flow of data through the device is accomplished by monitoring the inputs for activity and setting the direction for data transfer based on a transition from the idle (J) state. When data direction is established, data is transferred until either an end- of-packet (EOP) or a sufficiently long idle state is encountered. At this point, the coupler disables its output buffers and monitors its inputs for the next activity During the data transfers, the input side of the coupler holds its output buffers disabled. The output side enables its output buffers and disables edge detection from the input buffers. This allows the data to flow in one direction without wrapping back through the coupler making the iCoupler latch. Logic is included to eliminate any artifacts due to different input thresholds of the differential and single-ended buffers. The input state is transferred across the isolation barrier as one of three valid states, J, K, or SE0. The signal is reconstructed at the output side with a fixed time delay from the input side differential input. The iCoupler does not have a special suspend mode, nor does it need one because its power supply current is below the suspend current limit of 2.5 mA when the USB bus is idle. The ADuM4160 is designed to interface with an upstream facing low/full speed USB port by isolating the D+/D− lines. An upstream facing port supports only one speed of operation, thus, the speed related parameters, J/K logic levels, and D+/D− slew rate are set to match the speed of the upstream facing peripheral port (see Table 10). A control line on the downstream side of the ADuM4160 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. PRODUCT USAGE The ADuM4160 is designed to be integrated into a USB peripheral with an upstream facing USB port as shown in Figure 4. The key design points are: 1. The USB host provides power for the upstream side of the ADuM4160 through the cable. 2. The peripheral supply provides power to the downstream side of the ADuM4160 3. The DD+/DD− lines of the isolator interface with the peripheral controller, and the UD+/UD− lines of the isolator connect to the cable or host. 4. Peripheral devices have a fixed data rate that is set at design time. The ADuM4160 has configuration pins, SPU and SPD, that determine the buffer speed and logic convention for each side. These must be set identically and match the desired peripheral speed. 5. USB enumeration begins when either the UD+ or UD− line is pulled high at the peripheral end of the USB cable, which is the upstream side of the ADuM4160. Control of the timing of this event is provided by the PIN input on the downstream side of the coupler. 6. Pull-up and pull-down resistors are implemented inside the coupler. Only external series resistors and bypass capacitors are required for operation. USB HOST ADuM4160 VBUS1 VBUS2 DD+ DD– GND1 DD+ 3.3V VDD2 DD– PIN MICRO- CONTROLLER POWER SUPPLY PERIPHERAL Figure 4. Typical Application Other than the delayed application of pull-up resistors, the ADuM4160 is transparent to USB traffic, and no modifications to the peripheral design are required to provide isolation. The isolator adds propagation delay to the signals comparable to a hub and cable. Isolated peripherals must be treated as if there were a built-in hub when determining the maximum number of hubs in a data chain. Hubs can be isolated like any other peripheral. Isolated hubs can be created by placing an ADuM4160 on the upstream port of a hub chip. This configuration can be made compliant if counted as two hub delays. The hub chip allows the ADuM4160 to operate at full speed yet maintains compatibility with low speed devices. COMPATIBILITY OF UPSTREAM APPLICATIONS The ADuM4160 is designed specifically for isolating a USB peripheral. However, the chip does have two USB interfaces that meet the electrical requirements for driving USB cables. This opens the possibility of implementing isolation in downstream USB ports such as isolated cables, which have generic connections to both upstream and downstream devices, as well as isolating host ports. |
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