Blah, that's far worse of a spike than I was planning for. My plan was just a guess and was based on if you exceeded 20V for more than around 5 to 10 mS, it would blow an upstream fuse. I'll have to re-evaluate each design after I get to absorb that spec. Which will take a couple days. I'm also sure it will result in more QUCS simulations. So stay tuned.
Has the "LM267X Made Simple (Version 6.0)" software been used to do some kind of verification? I see that software is noted in the datasheet for the switcher.
Upon a quick look, my primary concern would be ripple rejection. Do you know what kind of ripple to expect on the 6V? Also do you know what kind of ripple rejection to expect from the linear's? I know that these linear's often pass the ripple right on through, which can be a pain. I see those ferrites, but I also see a switching frequency of 260kHz. I would expect that to be below the ferrite cutoff frequencies, but above acceptable usable tolerances. I'm wondering what the expected ripple is at those points. Perhaps a simulation can help shed some light on the expected ripple, as that can be hard to predict via data sheets and pencil techniques.
I see page 2 of 3, are there other pages that can be shared? Often I need to see what other components are being connected to know what kind of tolerances and ripple are acceptable. Even if only a draft, it helps me understand what the requirements would be.
I've seen several "automotive" switchers that generate 2A. I guess that kind of current is useful for driving dashboard gauges directly. It might be worth considering a more powerful switcher if you plan for to drive dashboard devices at some point.
kb1gtt wrote:Has the "LM267X Made Simple (Version 6.0)" software been used to do some kind of verification? I see that software is noted in the datasheet for the switcher.
No. I have a problem with installation of this software and plan to test it later.
Upon a quick look, my primary concern would be ripple rejection. Do you know what kind of ripple to expect on the 6V? Also do you know what kind of ripple rejection to expect from the linear's? I know that these linear's often pass the ripple right on through, which can be a pain. I see those ferrites, but I also see a switching frequency of 260kHz. I would expect that to be below the ferrite cutoff frequencies, but above acceptable usable tolerances. I'm wondering what the expected ripple is at those points. Perhaps a simulation can help shed some light on the expected ripple, as that can be hard to predict via data sheets and pencil techniques.
Maybe I will build this circuit on the PCB and do the test.
This figure is not promise anything good at 260 kHz.
Screen Shot 2013-12-18 at 14.56.29.png (31.31 KiB) Viewed 21082 times
I see page 2 of 3, are there other pages that can be shared? Often I need to see what other components are being connected to know what kind of tolerances and ripple are acceptable. Even if only a draft, it helps me understand what the requirements would be.
I finally got a chance to read through ISO 7637, here's my summary of the major concerns to keep in mind when making a design.
Pg 10 includes a suggested wiring network used to simulate the vehicles wiring when the vehicle. It includes a typical inductive and capacitance properties of ha wire harness.
Pg 12 specifies a power supply with resistant only properties of .01 ohm or less impedance under 400Hz.
Pg 14 specifies -V transients caused by inductive loads. It notes a peak of -100V with a total time of up to 2mS.
Pg 15 specifies +V transients caused by parallel load switching. This spike can get up to 37V for .5mS and happens regularly.
Pg 16 specifies a DC motor disconnect. It's decay can take up to 2 seconds.
Pg 21 specifies an alternator, or battery disconnect. Spikes up to +87V with a maximum time of 400mS.
There's lots of other good information in there, but those are the high lights I see in terms of making sure you uphold a good design.
