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| USB-UART bridge IC - powering VDDIO when VDD unpowered |
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| HwAoRrDk:
I am wanting to incorporate a USB-to-UART bridge IC into a project design, and the one I will probably use is the Holtek HT42B534. This ICs has several power pins. One is the main VDD, intended to take power from the 5V USB VBUS. Another is the core voltage, V33O, which as I understand it is only made available externally for decoupling and as a convenience for any other parts that need a 3.3V supply. The last is VDDIO, which is used to power the UART RX/TX pins at a voltage of the user's choice (i.e. whatever your micro is running at). I have doubt about the relationship between VDD and VDDIO, and whether how I have things set up will work properly or cause problems. Basically, I'm unsure as to whether it's okay to have VDDIO powered when VDD is not. My design is such that the USB is not the main source of power, and has it's own 5V supply. Currently, I have VDD fed from the USB VBUS, and VDDIO from the board's 5V rail (i.e. not from USB). But I'm not sure if this is a good idea. The Holtek datasheet linked above has an application example (page 10) showing the main circuitry powered from 3.3V battery, supplying the chip's VDDIO, and the VDD coming from VBUS. However, they have a diode between VDDIO and VDD to supply VDD when the USB is disconnected. Should I do the same? But one confusing thing here though is that the specs say that VDDIO must be no higher than VDD, but with the example circuit VDD will clearly be below VDDIO due to the diode's forward voltage drop. ??? |
| Ian.M:
They're using a 1N5817 1A Schottky diode. Its Vf drop at under 20mA will be a couple of hundred mV at most, far less than the voltage at which the silicon PN base junctions of any parasitic transistors present in the interface between the core and the I/O will start to conduct significantly and possibly trigger latchup. The datasheet is less than clear on this point as they don't quote an abs. max. voltage rating for Vddio - based on what's typical for MCUs and complex logic etc. from various manufacturers, there's typically 0.3V margin outside the rails. N.B. you'll also need a 1N5817 in series with USB Vbus so it cant back-power the USB port on the host if the host is off or in power-saving. |
| HwAoRrDk:
Ah, okay, thanks. :-+ So, out of curiosity, are you saying that if a diode was used that had a higher forward voltage, it might be troublesome, but a schottky is fine? I guess I could use a common-cathode schottky pair in SOT23, like a BAT54C - common-cathode to VDD, one anode to VBUS, other to 5V rail. Ah! And I realise as I type that this will essentially be an OR-ing arrangement. My understanding of the back-feeding with USB power is that it's generally not an issue when connecting to desktop/laptop PCs, as they commonly feature back-feed protection, but other devices may not, and who knows what someone may plug in, so probably prudent to always avoid back-feeding where possible. :) |
| HwAoRrDk:
Actually, I'm just thinking: why power the USB-UART bridge IC from VBUS at all? Is there a need to do that? Can I not just leave the VBUS pin on the USB connector unconnected, and power the HT42B534's VDD pin from my internal 5V rail? Then no extra diodes needed at all. Edit: Ah, if I do that I'm going to need a bigger 5V regulator. The SOT89 I was planning for will be marginal on power dissipation with an extra 16mA. :( Could I do the opposite, and power VDDIO from VBUS? The only problem I can imagine with that arrangement is if: a) USB and rest of my circuit don't share a ground (they will), or b) 5V levels from VBUS and my regulator vary too widely, and I get excess current flow through the UART pins on the HT42B534 and/or my microcontroller. For the latter, could put series resistors on RX & TX to mitigate this possibility, though, right? |
| Peabody:
If you power everything from the project's supply, you would need to determine the idle state of the UART outputs when USB is not connected - to make sure they won't confuse your processor. In theory they should all be high. But I think this would be a good idea if the power usage isn't a problem. It eliminates having to deal with what happens when you apply voltage to devices which aren't powered up. Everything comes up at the same time, so protection diodes don't come into play. |
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