Timeline for Bidirectional 5 V to 3.3 V level shifter
Current License: CC BY-SA 3.0
12 events
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| Apr 13, 2017 at 12:32 | history | edited | CommunityBot |
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| Dec 1, 2015 at 20:21 | comment | added | KyranF | glorious! time for some coffee -> the grinder just died, time to try some other drug | |
| Dec 1, 2015 at 20:19 | comment | added | got trolled too much this week | @KyranF: Yes, that's a good summary. Also, the switching speed was 50kHz and duty cycle of 0.5, in case someone wonders. | |
| Dec 1, 2015 at 20:12 | comment | added | KyranF | So from your experiements/simulations, you conclude that the FET vs BJT+Schottky have approximately the same operating power, and both systems need resistors anyway (so are equally as bad), but the BJT's base resistor (not needed in the FET setup) is an additional load, making it just that bit less power efficient? | |
| Dec 1, 2015 at 20:05 | comment | added | got trolled too much this week | With NXP's BSS138P I got 17.5uW when driving from the low side. And 22uW from the high side. (And no weird oscillations.) Curves look nice too. So this is indeed better than the BJT disspation-wise... But still the resistors dominate (at mW) and they have the same dissipation as for a BJT setup assuming same values. That BJT base resistor (which is not needed for a FET) also blows some 400uW. BSS138P has much lower gate capacitance than AO6408 (but higher Rdson). Basically you can't have low Rdson and low gate capacitance at the same time. The latter seems more important in this application. | |
| Dec 1, 2015 at 19:43 | comment | added | got trolled too much this week | With an AO6408 (which has built-in LTspice model, and opens well at 1.5V [10A]) I got positive dissipation of some 600uW from the high side and 460uW from the low side. And surprisingly the FET has significantly worse-looking curves when driven from the high side. The body diode is really slow for some power FETs, because it's a PIN diode. So the BJT+Shottky, surprisingly fares better than low Rdson FET. This with the same 4.7K pull-ups. The choice of FET looks critical here, but I don't have to investigate further today. | |
| Dec 1, 2015 at 19:42 | comment | added | got trolled too much this week | @KyranF: I tried using ON's BSS138LT1 model, but that gives negative average power disspation in LTspice (of -63uW)... so I think their model may be problematic. Garbage in... garbage out. Also it started to oscillate (at much higher frequency than the signal) when driven from the high side. It's a subckt model rather than simple MOS model, so it's hard to figure out what's going on. [continued] | |
| Dec 1, 2015 at 19:04 | comment | added | got trolled too much this week | @KyranF: I've looked at it in simulation: the transistor and the diode dissipate uW of power while the resistors dissipate mW. Actually the BJT dissipates around 100uW, the diode 1uW. FET or BJT won't matter here... resistors having path to ground for static dissipation is what matters. This is with 1Meg input impedances. | |
| Dec 1, 2015 at 18:11 | comment | added | KyranF | Thanks for the links. I suspect the BJT approach would be very high power in comparison during the times when the base is saturated. The constant load of pull up resistors is indeed worrying for any design, especially battery operated designs. | |
| Dec 1, 2015 at 18:08 | comment | added | got trolled too much this week | @KyranF: I haven't compared them, but if you're worried about that either is going be not rosy because of the pull-ups. The low power CMOS versions look like m.eet.com/media/1103155/Fig2.gif That's from eetimes.com/document.asp?doc_id=1231111 which is good article comparing various approaches. | |
| Dec 1, 2015 at 18:04 | comment | added | KyranF | What is the expected current draw/power consumption of this method in comparison to the FET-based method? | |
| Dec 1, 2015 at 17:07 | history | answered | got trolled too much this week | CC BY-SA 3.0 |
