![]() However taking into account the efficiency of the DC/DC at low currents, the current into the USB isolator will be even higher (43% higher assuming 70% efficiency).Īctually, I did some measurements with a Recom RKZ 0505S. The TS431 can handle this easily and the 75Ohm resistor is a 0.125W type, so 40mA are still OK (P=I 2R so 40mA*40mA*75 = 0.12W). ![]() ![]() So without any load (apart from the 5mA though the LED), the current drained could be as high as 40mA for a 2W type. So this is not a typical linear regulator approach.Īctually the current drained by the shunt regulator in an open load condition pretty much depends on the behavior of the DC/DC at low currents.įor a worst case assumption, let's assume a 10% scenario. It's important to note that the higher the load current, the closer the output voltage of the DC/DC gets to 5V. So with a current of 40mA and a voltage drop of 3.25V, this results in As a rule of thumb, the voltage drop on this resistor should be the desired voltage minus 2V at the expected maximum current. To limit the current through the shunt regulator, a serial resistor is needed. With a 22k and 20k resistor, it can be set to 5.25V It has an internal 2.5V reference and the output voltage can be configured with two resistors. Since Zener diodes are temperature dependent, not very precise and you can't get them for any voltage, a shunt regulator TS431CX is used. So the idea is to sink the current only when the output voltage exceeds 5.25V (maximum USB VBUS voltage). One idea would be to permanently sink 10% on the DC/DC output but firstly this would be a waste of energy and secondly this would also reduce the maximum current the slave could use. Without load or at very low load current, the specified 5V output might be exceeded by up to 100% - depending on the specific device. 40mA for a 2W device and a 400mA maximum continuous load current. Typically an isolated DC/DC only keeps its specified output voltage with a minimum of 10% load. Now this would be straightforward if these DC/DC converter modules would be regulated, but they usually aren't. To be able to supply the USB device, an isolated 5V to 5V 2W DC/DC converter is added which is able to deliver 400mA at 5V. It supports low (1.5MBit/s) and full (12Mbit/s) speed and can withstand a voltage difference of 2500V (rms) for one minute.įor transient voltage protection, TVS diode arrays are added at the input and the output in addition to the typical serial resistors. The decision regarding the isolator was pretty much straightforward, as the ADUM3160 digital isolator from Analog Devices is more or less the only easily available solution. The idea was to create a relatively cheap (material cost around 20€) and simple USB isolator which fits into a small and cheap off-the-shelf case and which is able to supply a slave directly. ![]() Typically, you need to connect a power supply on the USB slave side (behind the isolation barrier), which is not very handy. Also most of them can't supply a connected USB device. So there are good reasons to use an USB isolator - it's just that they are usually quite expensive. if a catastrophic short circuit happens in the measurement device, this could damage the PC. it can be necessary for measurement devices to use some potential in a circuit as reference instead of the common ground.īesides, the direct connection of 5V and GND lines between an USB device and your PC is also problematic in case of electrical faults of the USB device.Į.g. There are scenarios where this is not desired, e.g. So even if the USB device has an isolated power supply and would be isolated from ground, this isolation is bypassed by the USB connection. On a desktop PC, the GND signals of the USB ports are usually on the GND level of the electrical installation in your building. ![]() USB Isolator USB Isolator 2.5kV USB2.0 Isolator with 400mA supply and full speed (12Mbit/s) support Introduction ![]()
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