μblog: engineering from the trenches

This week we have a four-channel bridge-tied load driver used to power a stepper motor inside an old CDROM drive. The schematics for the device are provided in the datasheet and the huge drive transistors can be seen on the right side of the integrated circuit.

( ba5983fp.PDF )

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From a design standpoint, the only complaint I have about my Shuttle small form factor desktop is the extremely fragile on-board fan controllers. All three have died over the past two years after being used to regulate the single on-board fan with respect to the system temperature. The last one gave up the ghost a few days ago as my machine suddenly shut down and a red light came on inside.

Figuring that I should just go build a more substantial fan controller myself, I promptly went to the lab to look for a thermistor. Unable to find one, I settled for the next best thing: a diode. As a review, a diode is typically created by placing a volume of p-type silicon next to a volume of n-type silicon. At the interface of the two materials, the excess holes on one side and excess electrons on the other side pair up creating a depletion layer which is free of mobile charge, but has a net positive charge on the former n-type side and negative charge on the former p-type side. To complicate things, hole-electron pairs are constantly generated in the depletion region, among other places, due to thermal agitations proportionally to the junction temperature. The net uncovered charge in the depletion region generates a local electric field which forces the holes to the p-type side and electrons to the n-type side typically resulting in a measureable reverse current (since forward biasing on the diode would be conventional flow from the p to n-type side). A typical reverse current versus temperature plot is illustrated above for a 1N4148 diode.

I decided to power my device off the +12V bus in the PC using a power connector that I removed from a broken CDROM drive. I put the 1N4148 diode (in reverse) in parallel with a 100pF capacitor and then in series with a 10MOhm resistor between +12V and GND. This signal was then buffered and fed into the inverting input of a summing 0p-amp circuit. The non-inverting input was connected to a reference voltage set by a 5kOhm potentiometer between +12V and GND. An improved design would have a 20kOhm resistor between one side of the potentiometer and the +12V rail for better dynamic range. The gain of this circuit was about 100 V/V to give the proportional controller increased bandwidth. Next, the output of this stage was connected to the ADJ pin of a LM317 positive voltage regulator. Because of the LM317 design, Vout will always be ADJ + 1.25V, so the regulator would supply adequate current to the fan based on the ADJ voltage. Finally, the current return path from the fan goes through a pair of large, forward-biased diodes to facilitate a ~1.25V drop to compensate the 1.25V minimum output voltage of the LM317. This is so that when the voltage coming out of the summing amplifier is 0V, and then Vout is 1.25V, no current flows through the fan since the other side of the fan requires a voltage of greater than 1.25V for current to flow. On a final note, care should be taken in the selection of the op-amps employed here. It is important to pick CMOS devices that have rail to rail output so that zero current through the fan is possible.

After building the circuit, I tested the output with a DMM and applied various amounts of heat using a hot air gun. For the final install, I simply stuck the diode inside the CPUs heatsink/heatpipe assembly, connected the device to power and the output to the fan. I used some diagnostic software that can read mainboard temperatures to set the threshold on the potentiometer to something sensible. If you end up building this and think that the fan approaches full speed too quickly (slowly), then simply change the 50kOhm feedback resistor to something smaller (larger). Lastly, don’t forget that he heatsink clip on the LM317 is tied to Vout, so don’t let it touch the case of the PC.

( 1n4148_1n4448_5.pdf )

( temp-fan.pdf ) <– schematic!

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I have been getting quite a few suggestions in regards to chips for IC Friday. While I really appreciate the reader input, the chips are often quite rare and will take me some time and effort to obtain. To make matters easier for myself and quicker for those wanting to see die shots, I propose that I will accept chips that readers want to see in the mail. Send the ICs that you want to see opened to the address at the bottom and I will go through them as they arrive. Please understand that this is a destructive process and you will not get the chips back, therefore, do not bother spending extra money on protective packaging. Finally, if anyone wants to see extra pictures of any chips already displayed, send an email or leave a comment. I keep all of them so extra images shouldn’t be too hard.

Nick Chernyy, ICF
212 EESB
University Park, PA 16802
U.S.A.

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Today’s chip is a multi-mode CDMA low-noise amplifier/mixer (not a RS-232 tranciever as the name suggests). The source of the chip is a Motorola V60 handset. It is somewhat interesting that the chip has both round and square coils on the surface, if any IC designers are reading this and know the rationale behind it, please leave a comment.

( max2323-max2325.pdf )

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It is now official:

To our valued OrCAD Layout customers:

As demonstrated in the OrCAD® product 16.0 release, Cadence® continues to invest in providing a fully scalable PCB design solution for our customers – one that grows with you as your PCB designs grow in complexity. We’ve all seen the PCB design landscape change dramatically in recent years. In order to help customers meet current market demands and maximize productivity, Cadence continues to leverage the power of its proven Allegro® PCB technology within our OrCAD product line. This allows Cadence to offer customers unique suites and technology bundles that address current and future design challenges.

This letter is intended to communicate some important developments regarding the future of Cadence OrCAD Layout. Cadence has begun the End-of-Life process for Cadence OrCAD Layout technology based products.

Please Note: Cadence OrCAD Capture, OrCAD Capture CIS, and PSpice® technology are all integral parts of Cadence’s long-term product strategy and are not affected by this notice.

Effective July 31, 2007, Cadence will no longer sell the following Cadence OrCAD Layout based technology products:

1. OrCAD Layout (PO1410)
2. OrCAD Layout Plus (PO1420)
3. OrCAD Unison PCB (PO1510)
4. OrCAD Unison Ultra (PO1530)
5. Layout Studio (PS1430)

We acknowledge that transitioning software systems is never easy and is often a juggling act between investing in learning new technologies and meeting current business priorities. EMA is committed to ensuring we do everything possible to help minimize the impact on you, wherever possible. To help ease the transition, Cadence is providing OrCAD Layout customers with multiple paths for migrating to new technology that leverages the power of Allegro PCB Editor. Learn more about the various transition path options by visiting http://www.ema-eda.com/orcadlayout.

The products entering End of Sale will be supported thru March 31, 2009. After that date, these products will no longer be supported for hot-fixes or support calls and will not be shipped on the OrCAD CD set.

If you have any questions, or would like to discuss these changes and how it may impact you, please contact your EMA Account Manager. You may also contact the EMA technical support team at 585-334-6001, Option 5, or by email at techsupport@ema-eda.com.

We remain focused on providing solutions to ensure your ongoing and future success!

Best regards,
Manny Marcano

President and CEO
EMA Design Automation

Many people saw this coming as virtually no new features have been added to Layout in the past four years, only bug fixes. Although Allegro PCB Editor is a little bit more pricey, I think its worth it, especially for high performance designs. Finally, the Layout site gives some instructions on migrating. Layout,… we have had some great times together:

• The great and unavoidable crashes that used to occur when the user would lock the (win32) workstation running Layout
• All the excellent times that Layout would close your design without saving if you hit CTRL-C twice instead of once
• Layout’s inability to recognize artwork that was placed on the Global Layer (0) when creating Gerber files

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It became apparent that the National Institutes of Health have a virtual carreer center that some decent guides to presenting and publishing. Although a lot of the information there geared to publishing in scholarly journals, the lessons can be taken and applied to many fields of technical writing. Furthermore, if you are a U.S.A. resident, this is your tax money at work, so you might as well reap the benefits.

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One of my colleagues was experiencing some problems with Microsoft Entourage and wanted to switch to Mozilla Thuderbird but had to also transfer the “local folders”. Entourage was crashing every time the email login was entered and would not permit us to copy the local files to an imap server in order to retrieve them inside Thunderbird. After much searching, I found that if the folders under Local Folders/Documents are simply dragged onto the desktop, or any other folder in that case, the come up as .mbox files and conform the mailbox standard. These .mbox files could then be directly copied to the Local Folders section of the Thunderbird profile (~/Library/Thunderbird/Profiles/x.default where x is something random). The .mbox suffix was then removed and a .msf file was created for each of the directory files using a script. The .msf file only display information and does not need to be specific for each file. Thunderbird was then restarted and all of the copied folders appeared under Local Folders.

This may seem trivial to most people that use Macs on a regular basis, but I figure I should document this for the one person who may end up in my position lacking substantial OS X experience.

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I am working on re-designing my isolated stimulator to work with current output instead of voltage output as it does at present. One of the associated problems is designing the output stage so that the potential between the two stimulating electrodes does not exceed a pre-set value governed by various safety and electrochemical limits. Without going into too much technical details, comparators are involved to create circuitry that will clamp the output voltage, even if it does not reach the prescribed current, at safe limits.

In the past, I have heard various views on using operational amplifiers as comparators. The general consensus is to avoid using them in that manner, however, some applications exist where cautious use can be accomplished without too much performance sacrifice. The AN-849 note, by James Bryant from Analog Devices Inc., gives a short overview of the pros and cons of this use and gives light to some subtleties that are not fully apparent.  One such issue that I did not think of from the start was including small amounts of positive feedback to increase stability.

P.S. Be sure to check “how to make sense of a data sheet“, also from Analog. The document is both informative and entertaining.

( an-849.pdf )

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This time, the IC is a straightforward logic level converter from TI: the GD75232.  The chip doesn’t have an on-board charge pump so the internal schematics (page 2 of datasheet) are similar to this design.

( gd75232.pdf )

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I recently found this guide from National Semiconductor going over the basics of various voltage regulator designs. They go over the basics as well as some high level schematics of these designs and a couple of application notes. I skimmed over it and found it to be a decent read, I may still disagree with their points regarding the utility equivalent series resistance in shunt capacitors so I will try to post on that topic when I find more substantial information.

( voltage-regulators.pdf )

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