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#apertus IRC Channel Logs

2020/05/01

Timezone: UTC


02:00
metal_dent[m]
Good morning!
02:07
Bertl_oO
morning metal_dent[m]!
02:28
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03:00
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05:45
Bertl_oO
off to bed now ... have a good one everyone!
05:45
Bertl_oO
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07:01
BAndiT1983|away
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BAndiT1983
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BAndiT1983|away
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BAndiT1983
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10:40
se6ast1an
good day
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11:37
Bertl_zZ
changed nick to: Bertl
11:37
Bertl
morning folks!
11:39
se6ast1an
good day
11:42
BAndiT1983|away
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12:14
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13:04
vup
Bertl: on the micro r3 we will have two usb ports
13:04
vup
any recommendations on how to connect their vbus together?
13:04
vup
is there a better solution than just using two schottky diodes?
13:06
Bertl
vbus in or out and what is the purpose of the ports?
13:08
vup
vbus in
13:09
vup
they are used as the power source
13:09
vup
(and for data output)
13:09
Bertl
so the idea is to feed power from both sources?
13:09
vup
kind of
13:10
Bertl
any specific reason for that?
13:10
vup
switching between one of them would be enough
13:10
vup
well if we have two usb ports it would be nice, if either could be used for powering the camera
13:11
Bertl
how much power are we talking about?
13:12
vup
3A @ 5V
13:12
Bertl
you are negotiating that with the USB port? :)
13:12
vup
no :)
13:13
vup
but usb type c allows hosts to provide up to 3A without negotiation
13:13
Bertl
well, you could use P channel mosfets instead of shottky diodes
13:14
vup
does that actually help? the ones i found has a Rds(on) of ~50mΩ, which is still a 150mV voltage drop at 3A
13:16
Bertl
a quick check @ mouser gives starting at 1.5mOhm
13:17
Bertl
how much PCB space do you have available?
13:18
vup
unclear
13:18
vup
but shouldn't be too tight currently
13:20
Bertl
https://www.mouser.at/datasheet/2/115/DMP1005UFDF-1142690.pdf
13:52
vup
would you use a ideal diode controller with that or build one discretely?
14:09
Bertl
neither, just connect the gate to GND use the source on the input and the drain as output
14:10
Bertl
if you want to make the inputs selectable, add a dual NPN stage as low voltage 'enables'
14:31
vup
wait I thought mosfet's provide no inherent reverse current protection?
14:31
vup
if I just connect the gate to GND and connect both usb ports, won't one feed the other?
14:37
Bertl
drain to source happens via the body diode, which has a forward voltage of 0.8V
14:38
Bertl
assuming that the 5V supplies are within the spec this should prevent any reverse current
14:38
Bertl
but if you want to play it safe, you can add a controller or discrete circuitry as well
14:42
vup
ah right as long as both supplies are 5V is should work
14:42
vup
I think thats a safe assumption
14:43
Bertl
5V +/- 0.7 or so
14:43
Bertl
well, let's say |V1-V2| < 0.7 :)
14:43
vup
yeah
14:50
Bertl
but you can use a comparator (or dual comparator) to shut off any of the two mosfets
14:51
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15:00
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15:21
megora
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15:45
Bertl
off for now ... bbl
15:45
Bertl
changed nick to: Bertl_oO
16:14
vup
Bertl_oO: Hmm I am still not convinced what you propose will actually work. To reiterate, I have two usb ports and want to prevent current from going into the usb port.
16:14
vup
You propose connecting a p channel mosfet with gate tied to GND, source connected to vbus and drain connected to the shared 5V net to each of the two vbus's.
16:14
vup
Now suppose both ports are connected and port 1 has a voltage of 4.9V and port 2 a voltage of 5V.
16:14
vup
Both mosfets will be on (as Vgs will be > Vth), I would think current would flow from port 2 to the shared 5V net. And now because the mosfet of port 1 will also be on, won't the current flow through that mosfet aswell?
16:44
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17:06
BAndiT1983
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BAndiT1983|away
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21:10
panintended
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BAndiT1983
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21:42
se6ast1an
off to bed
21:42
se6ast1an
good night
21:43
panintended
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22:07
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22:07
comradekingu
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22:50
Bertl_oO
vup: yeah, you're right ... doesn't work without additional active circuitry
22:51
Bertl_oO
some kind of comparator which turns off the mosfet when the output voltage is above the input level
22:51
vup
ok makes sense
22:53
Bertl_oO
it works quite fine as reverse polarity protection because the gate is at a higher level in this case
22:53
Bertl_oO
but it doesn't help with competing inputs
22:53
vup
yeah
22:53
vup
i got confused by that at first aswell
23:28
vup
well the FPF1048 seems like a neat solution
23:30
Bertl_oO
indeed