LC filter in a design, why not an RC filter?

[Mr RB]

There isn't a switching frequency! It's only to turn the power on/off to the analogue LM358.

Personally I think the cap and inductor are much too small, but the intent was probably to reduce the dead time after turn on (for power saving, since the battery is so small).

Regarding using the micro pins to make a buck SMPS, you don't need the pin diodes. A push-pull digital output driver works in buck as the pulldown FET operates as synchronous recitification. Many people don't know you can use a push pull digital driver as the FET and diode in a buck SMPS.

I did some playing with that idea a while back;

**broken link removed**
https://www.romanblack.com/smps/pic-smps.htm

 

[MrAl]

Hi there Roman,


Well, it is true that the buck can be made without a lower diode if other conditions are right, but the lower diode is usually included one way or the other anyway because there is a time when the left side of the inductor (in your drawing) can go negative and that's when the lower diode kicks in.
But even so, that only works when the components are sized normally. For this circut the component values are much lower than normal so we can not switch from the positive rail to the zero volt rail and get normal PWM. It works better to work it as a buck with lower diode and high impedance switch.

Of course that is still assuming they intended PWM in the first place, which you are suggesting that they are not and i think it is a good suggestion. They may have intended something else, and that something else doesnt even have to follow some good design rule as they might have thought it would work better that way even if it really doesnt. So the inductor may be for nothing really.
The only way to know for sure is to ask whoever designed it and find out their exact *reason* for making the circuit that way. Only then can we go over the design and see if their intent was met with proper design or not, and even if their intent was for something that really needed that much attention.

As to the other views, a typical series resistance value for the inductor would be maybe around 10 ohms. The time constant is therefore around 5us so they would have had to be satisfied with maybe a 10us delay in current rise. Not very much. Combined with the R in the chip itself this would have been even shorter. So it could be that they intended something that still isnt happening due to an incorrect choice of value.

 

[The Electrician]

Showing current at MCU pins [V1]

Sorry about delay in replying, working on a PCB artwork.

E

EDIT: Added MCU internal clamp diodes, 2nd image

Somehow your simulation is using a 47 millihenry inductor, not a 47 microhenry inductor. The ring frequency is much too low.

f=1/(2 Pi SQRT(L C))

 

[Mr RB]

...
So it could be that they intended something that still isnt happening due to an incorrect choice of value.

Yep, I agree 100% there!

I would bet the "intent" was to reduce surge current when the micro switches the power on for the LM358, but the author was not good on choosing the inductor value, and/or has made the inductor and cap very small to reduce the dead time from turning the power on to being able to use the LM358.

To TheElectrician; have you added the micro output pin driver resistance and seen how that affects the ringing? The pulldown FET Rds ON is normally about 30 ohms and the pullup FET Rds On is maybe 50 ohms.

 

[ericgibbs]

Somehow your simulation is using a 47 millihenry inductor, not a 47 microhenry inductor. The ring frequency is much too low.

f=1/(2 Pi SQRT(L C))

hi E,
Thanks for that.

As you can see I defined the inductor correctly as 47micro. I will run some more sims and see what LTS shows.

Perhaps its seeing 'm' in the suffix 'micro' as 'milli' in some way, no errors were reported.?

E

EDIT:
Re-run the Sim with the inductor value set as 47u, the frequency of the ring is approx 68Khz.

Will have to watch out in future for defining components with the full label ie: micro.

E

 

[MrAl]

Hi Eric,

First, thanks for doing the sims here.

I think now it would be good to see the output with higher frequency pulses. That should show the normal PWM type output.

Also, it pays sometimes to use use something like "47e-6" as then there's no doubt what value it is. I use this myself a lot because often i calculate the value anyway, and it comes out to an actual number rather than something like "47u" or "47m".
And yeah, "m" is a source of confusion among simulators. It's easy to make the mistake of say for a resistor R1 as "1M", and that does not make it a 1e6 Ohms resistor but rather a 0.001 Ohm resistor (ha ha). World of difference there too

So whenever an "m" or "M" is involved, turn on low heat and sim'mer rather than boil

My theory on this is that the people who design the simulators are more computer scientists rather than regular engineering scientists.

 

[ericgibbs]

hi Al,
Today tried further PWM sims for this circuit and I have not been able to produce a usable voltage for Vo.

Tried a range of frequencies using the MCU pin switching high/low and high/tristate.

IMHO its a simple DC on/off control for the circuits Analog supply, perhaps as Nigel suggested someone could disassemble the hex file or perhaps contact the designer.

With ref to the inductor value, I would have thought that the full label of 'micro' would have overridden the leading 'm' in the label, apparently not.

Normally I would have used the 'u' or 'μ' as the multiplier.

Eric

 

[Nigel Goodwin]

With ref to the inductor value, I would have thought that the full label of 'micro' would have overridden the leading 'm' in the label, apparently not.

Normally I would have used the 'u' or 'μ' as the multiplier.

I imagine the level of surge reduction is considerably less with u instead of m

 

[ericgibbs]

I imagine the level of surge reduction is considerably less with u instead of m

hi Nigel.

This sim is high/low from the MCU pins.

Without the presumed MCU internal output impedance of 60R the plotted 40mA becomes approx 2Amps.

The 47uH inductor is set for 0R self resistance.

The 3K load is to approximate the analog current draw at 3V.

E.

 

[MrAl]

Hello again,


Oh yes Eric i was wondering where that 3k 'load' came from. That was not the original assumption as i think it was much higher back then. With 3k load i dont see what any of this is doing for us

As to the small (very small) pun, i was trying to fit "sim'mer' into the sentence where that would stand for the person doing the simulation, but it didnt come out quite right (chuckle). ie "simmer"="simulation person"

 

[JimB]

As to the small (very small) pun, i was trying to fit "sim'mer' into the sentence where that would stand for the person doing the simulation, but it didnt come out quite right (chuckle). ie "simmer"="simulation person"

Don't fret MrAl, I saw your pun immediately.
JimB