Electrical Engineering

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rdtsc

13:17

Trap for young players, volume 46: when a logic analyzer is showing all kinds of asynchronous data at power-up (which doesn't seem to be any standard protocol), don't assume it's really data. First check for slow rise-time with a scope!

Bartek Banachewicz

@Shalvenay yeah, I'm gonna do it asynchronously

the device is the AS5043 rotation sensor

ams.com/as5043

it has 16 bit of "configuration" that can be programmed temporarily and then subsequently "burned" in with one-time process

the OTP requires a higher (~7.5V) voltage on the PROG line. my issues are

a) I'm not sure how to build such a programer. I've constructed a prototype with two voltage sources and two PNP transistors connecting both collectors to the output. I forgo that for the PNP I need a driver if the voltage is higher, so I'm adding that next - not sure if that's a good approach. Will post schematic soon.

b) I don't understand the actual OTP process; I've managed to temporarily program the unit (simply shifting in the data) and it worked - but now to actually burn it, am I supposed to repeat the same with HV? the diagram just isn't clear to me

1 hour later…

Bartek Banachewicz

14:23

WRT that driver circuit, my analog knowledge is very lacking

I've been looking at those 2, or rather 3

Q: Is one circuit better than the other to drive the PNP transistor?

Of the two circuits, would you prefer one over the other?

Bartek Banachewicz

14:46

I have to admit I have rather hard time understanding the difference between the 3 - but it also doesn't sound that crucial

2 hours later…

Bartek Banachewicz

16:52

actually the 10n cap is worrying too

not sure what the load is supposed to be there

with 1k5 the 10n is making the signal... not so square even at 1ms pulses

not sure how im supposed to generate 2us ones with a 10n cap connected, id need to wait ages for it to discharge

3 hours later…

rdtsc

19:39

I think they're assuming you'll have a particularly stiff 7V supply. It does have to supply >100mA while programming. Add a 0.1µF cap to the 7V rail and that should charge the 10n pretty quickly.

They show a 10µF here - what they probably don't say is that it is likely a ceramic type with low ESR. I'd add a 0.1µF to it anyways for better high-frequency response, since that's the concern (poor transient response at high frequency.)

That should work... could always try a representative circuit in a simulator such as LTspice.

Bartek Banachewicz

20:34

here's what im getting right now

this is the driver circuit i made

the scope pic is at 5V btw, but it works the same at 7.5

i don't particularly like either output but the HV one is significantly worse

oh and their simplified schematic only shows one PNP, but I presume that's an oversimplification?

1 hour later…

rdtsc

21:44

Are these 8550SS-C-BP transistors?

Looking for their datasheet

Bartek Banachewicz

@rdtsc i think so, they're just random PNPs i had laying around

i thought for this purpose it really won't matter, i think they're rated to like 1A and 100MHz

i altered the circuit according to this answer

A: Is one circuit better than the other to drive the PNP transistor?

In most applications it doesn't matter. The first circuit will turn off the PNP a little faster, which would only matter in high speed switching, like if the load was being PWM controlled. Here is yet another topology to consider: This only works if V+ is about a Volt or more higher than the...

> The first circuit will turn off the PNP a little faster

but observed no such change

the voltage after R3 seems to be high for way longer (and again, I understand next to nothing about transistors...)

they're labeled "8050SS D 06F"

rdtsc

Looks like unisonic.com.tw/datasheet/8550S.pdf

Bartek Banachewicz

they seem to have been produced by a lot of companies with roughly similar parameters

I wouldn't know if it's a problem with my transistor selection or the circuit or...

the one thing I'm fairly confident about (and you can actually see) is the 2us control pulse I'm generating

rdtsc

Nothing too unusual about them. Hmm "Emitter-Base Breakdown Voltage BVEBO (IE=-100μA, IC=0) --> 5V". That line says that for Q2, in it's "off" state, with 5v or more between it's base and emitter, it will leak 0.1mA or more. Not sure how much that is affecting your circuit, but a 2µs delay definitely seems excessive. For a 100MHz part, it should be much faster than that.

Bartek Banachewicz

that's what i thought - but what I'm observing is the blue line

i don't understand what's causing it to stay open for so long

that offset stays constant at about 1.8us btw

i mean i guess that's probably expected

so for some reason it needs an extra 1.8us to get back off

I am 99% sure this is a simple mistake

eh time to post a question I gues

rdtsc

22:05

Could try a push-pull or totem-pole output stage. That should be able to drive PROG as intended. But that means even more transistors, heheh.

Bartek Banachewicz

@rdtsc but why does it behave like that?

rdtsc

Well, one reason is that turning on Q1 or Q2 "charges up" C3. Then when they are turned off, what is there to discharge C3? Nothing... it gradually loses charge. But this doesn't explain the 2µs delay.

Bartek Banachewicz

@rdtsc I have a resistor across PROG to discharge it and simulate the "load". That's why it tapers off like that, which makes sense, but it doesn't explain why the pulse is much longer than the control one.

rdtsc

Is the blue trace the base of Q3 or Q2? Ahh crud gotta run.

Bartek Banachewicz

22:26

@rdtsc base of Q3

whatever is happening at the base of Q2 just follows that

posted Q anyway

Q: Transistor turns off only after a delay

I'm building an IC programmer (for the AS5043 sensor) that requires two voltage levels; 5V and 7.5V. I decided to control those with two PNP transistors. I'm controlling them from a 5V microcontroller. The programming scheme requires a 2us pulse for each bit. For the 7.5V line, I made an NPN driv...

transistors npn pnp programmer

umfh

i tried with the 5V line where I just have one transistor

and uh that one also has a delay

a bit smaller it would seem but still

and actually, i think i just found a dupe

Q: How do I make the turn off and turn on time equal in a NPN transistor?

I have a simple NPN switch, see the diagram. I feed a 100KHz square wave (TTL) to the base of this transistor and it turns on very very fast (a few nSec) but it doesn't turn off as fast, it almost takes 2uSec for it to turn off. (I am looking at the collector of this circuit). The diode is a la...