25 lines
1.2 KiB
Markdown
25 lines
1.2 KiB
Markdown
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v0
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first prototype, with pin header to debug
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the over-voltage protection is based on an design described by Texas Instruments, in Analog Engineer's Circuit: Amplifiers, SNOAA20, Overvoltage protection with comparator circuit.
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this design is not ideal for our case though.
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a Zener diode is used as reference voltage.
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it requires at least 1 mA to operate.
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this is already a lot for my application.
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but 1 mA is when VBUS is at it's minimum of 4.75V.
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at 35V this would result in 21 mA.
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this power is mainly dissipated by the resistor limiting the current going through the diode.
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the dissipated ~0.6W exceeds the 0.1W rating of the resistor, also heating up a zone of the board to 100 °C.
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the reference voltage is current dependent, thus the cutoff voltage goes above 5.5V.
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this is not too much of a problem because VBUS also exceeds this voltage, but it's not ideal.
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design errors:
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- the footprint for the USB-C plug has too small mounting holes
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- the LM393 is actually an open-collector comparator, but the circuit uses it as push-pull
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- the zener diode needs at least 1 mA to be used as voltage reference, not 10 µA (Ileakage and Ibias/Ikz are mixed)
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- the design does not respect the common-mode input-voltage range (up to Vcc-2)
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