Here’s an example of one IC + external components: bq25886, a Boost-Mode Battery Charger (nothing fancy nor overly specialized as a component or as an application IMO).
Take a look at section 11.1. What kind of autorouter could satisfy those requirements?
The switching node rise and fall times should be minimized for minimum switching loss. Proper layout of the components to minimize high frequency current path loops is important to prevent electrical and magnetic field radiation and high frequency resonant problems. Here is a PCB layout priority list for proper layout. Layout PCB according to this specific order is essential.
Put SYS output capacitor as close to SYS and GND pins as possible. Ground connections need to be tied to the IC ground with a short copper trace connection or GND plane.
Place PMID input capacitor as close as possible to PMID pins and PGND pins and use shortest copper trace connection or GND plane.
Place inductor input terminal to SW pins as close as possible. Minimize the copper area of this trace to lower electrical and magnetic field radiation but make the trace wide enough to carry the input current. Minimize parasitic capacitance from this area to any other trace or plane.
Decoupling capacitors should be placed on the same side of and next to the IC and make trace connection as short as possible.
Route analog ground separately from power ground. Connect analog ground and connect power ground separately. Connect analog ground and power ground together using thermal pad as the single ground connection point. Or using a 0-Ω resistor to tie analog ground to power ground.
It is critical that the exposed thermal pad on the backside of the device package be soldered to the PCB ground. Ensure that there are sufficient thermal vias directly under the IC, connecting to the ground plane on the other layers.
Via size and number should be enough for a given current path.
Last night I ran some simulations of the basic linear power supply. Microchip (supports this free version of Simplis/Simetrix simulator) did not include a 7805 linear reg model, so I used an LDO they had. This all ran well before I introduced the undesirable trace resistance. With the added trace resistance, the LDO model oscillated and I could not easily fix that. (The 7805 regulator did not oscillate in the real world.) I think I can import a 7805 SPICE model from somewhere. I have done this sort of thing previously but I am not a simulation expert and I don’t remember what I did…may need to figure it out again. I think I had to fix some minor issues in the text file to get an imported model to run in Simetrix.
LM7805 has a darlington NPN transistor on it’s output, and this is an emitter follower.
Low Drop regulators have an PNP transistor on their output to make the voltage differential lower. This is a completely different setup, with different rules for loop stability and you can not expect to have sensible results if you arbitrarily exchange them in simulations (or real life).
For example:
Low Drop Regulators tend to have some issues with high capacitive outputs, and often need a minimum ESR to work properly. They work OK with electolytics, but can oscillate with Tantalums or high value ceramics or polymer Elco’s.
Yes I am generally familiar with much of what you are saying. But…the original hardware did not oscillate, and behavior I am trying to reproduce is not related to instability. There is a (2 * AC Line frequency) current component in the DC output of the bridge rectifier. This causes a corresponding voltage drop waveform in copper traces which the current passes through. In a good design, the regulator should nicely reject this ripple and the output voltage ripple should be in the low mV peak-peak or less. But a bad pcb layout can greatly increase the output ripple, especially at full load current. So in my simulation if I get oscillation that is something I need to fix…
It is funny how sometimes something which ought to be easy turns out not to be; and sometimes vice versa. I spent a little while today trying to find an easy behavioral SPICE model for a 7805 regulator…have not found it yet. So instead, I took a different Microchip LDO model, rated for 150 mA. Then my simulation circuit added a PNP booster transistor. In the “clean” design, the 200 mSec simulation ran in maybe 2 seconds and worked nicely, producing a clean 5V output. Then I inserted the 10 mOhm trace resistance, and the simulation run is maybe 60% finished after about 4 hours! I am expecting another dismal failure and I would quit it if anything were pressing, but nothing else is urgent and I am being patient for later tonight to see what garbage the simulation will produce. I will then bid goodnight to my darlington.
It depends upon the LDO. Most of the newer types are designed to work with ceramic output capacitors. I think that a 7805 regulator (which is not an LDO) is NOT designed to work with a very low ESR output capacitor. But one of the things I tried was adding some ESR in series with my output capacitor. This did not seem to help.
LM7805, ua7805, KA7805 and a whole bunch of others are all different internally, though they have similar functionality. The big Ceramics and very low ESR polymer caps did not even exist when the 7805 variants were designed, but the emitter follower topology is much less prone to oscillation.
This is what a datasheet of a Motorola LM317 says about output capacitance:
Although the LM317 is stable with no output capacitance, like any feedback circuit, certain values of external capacitance can cause excessive ringing. An output capacitance (CO) in the form of a 1.0 μF tantalum or 25 μF aluminum electrolytic capacitor on the output swamps this effect and insures stability.
More generally I simply do not trust simulators for stuff like this. How do you verify if the spice model you use is even close to the real thing on these parameters?
I Once spent over an hour to find a working model of a NE555 for simulation in KiCad / NgSpice. The youtube tutorial I found was: Install this commercial Simulation program, and then steel the spice models from it, which I did not find acceptable.
I agree again. And I will plead ignorance regarding differing internal architecture of the 7805 variants. But I think that any of them (if they do not oscillate) would produce high 2x mains frequency ripple if connected as the subject matter was connected. Just don’t call any of these LDO’s (Meaning low drop out (linear regulator). I am not saying that you did but some people do so.) These devices typically require Vin-Vout > 2.0V so they are the definition of a (NOT low dropout) linear regulator.
I have come to disdain the NE555. Any time I consider using one, I find I can do better with 1 or 2 comparators.
In our first product (1988) I used NE555 as a DCDC controller to convert 5V to VPP (12/21/25V) voltage for EPROM programming.
In my opinion it’s a brilliant IC. Of course it contains not a lot more than two comparators.
Why product design industry prefer manual routing over autoroute for designing Pcb layout?
Look at it like this, anyone can press the autoroute button, not many can lay out a design well.
If you are wanting to be employed in the role of PCB layout, what can you offer that a computer can’t
EDIT: It looks like the forum website has issues down for a while, up, down again?
I guess that your voltage conversion might have used the NE555 to drive a charge pump? Or did it work as a controller in the strict sense…driving an external power switch?
I recently used an inverting low side gate driver as a power oscillator to drive a charge pump producing -5V from +12V. This oscillator would free run or synch to an incoming 300 KHz clock. Perhaps a similar “trick.”
Even when it was up I could’t login - Unknown error was reported.
It was driving external BD137 NPN as switch in clasical step-up. As I remember there were two possibilities (I have read about) to give feedback to it - to pin 5 or to reset pin. I don’t remember which way I used.
I have now attached a zip file which includes a simulation of the bad layout. In the end I constructed my own discrete linear regulator to get to my “point”. I think the performance of this sim is reasonably close to the hardware. The zip includes my Simetrix schematic and a .pdf printout of schematic, waveforms, and explanation. The one key thing I did not provide is any sort of layout image, but I think you might be able to reverse engineer this as I can. Open to any further questions; if anyone is interested!
The core subject is pcb layout, and the hardware issue at hand does not significantly depend upon the internal design of the regulator. I was unable to find an available regulator model which did not oscillate (the original hardware did not oscillate.) But even though the internal architecture of my regulator model is different, its behavior resulting from pcb resistance in the ground leg as shown duplicates that which was seen in hardware with a 7805. So I think I have demonstrated the point I was trying to make. The lesson is that pcb trace resistance can be a problem even with a simple pcb design such as this.
By the way…I do not want to try to manually duplicate the 7805 internal schematic in Simetrix. I might violate a node limit for my free simulator version, but mainly do not want to put that much effort into something which misses the point. However if you can direct me to a suitable SPICE model for a 7805 regulator (probably a behavioral model rather than device level) I can try to import & run it. I will update my post if I am successful at doing this.
I use PSpice very rarely and practically only to simulate passive filters behaviour. Don’t know where to look for 7805 models.
The key difference between your model and 7805 is that there is NPN Darlington used as an voltage follower. So if you do such follower driven from the second transistor of differential pair it would be closer to 7805. I think you can replace the current source used in real 7805 with ideal current source (current - don’t know - may be 1mA, may be 0.1mA).
I am surprised that you are doing anything in this subject. Until we don’t know exactly what was the circuit and what was the problem I think it is waste of your time.
I disagree. The initial subject concerns autorouting, which could easily produce a bad layout such as my example illustrates. My point is that even a very simple electronic circuit can be screwed up with a poor pcb layout. If I could easily use a 7805 model, that would be somewhat preferable. But that part of the issue does not seem to be worth a lot of time. I think almost any working 3 terminal regulator design can illustrate my point.
I am expecting another dismal failure and I would quit it if anything were pressing, but nothing else is urgent and I am being patient for later tonight to see what garbage the simulation will produce. I will then bid goodnight to my darlington.