>>Hope this helps... Wow, it sure does. Thanks for detailed explanation. Here is another, related question: I have two enclosures making up my device - one powering the other through a cable that also carries serial Tx/Rx. The first is powered from a wall transformer. Can I supply regulated 5V to the remote device (6-12 ft), or am I better off sending the higher, unregulated voltage to the remote device and give it its own, second 5v reg? The remote unit draws 50ma typical, 120ma in surges. --BobG Details of what I am doing if you are curious: Wall Transformer: Main Enclosure: LCD Enclosure: 12V unregulated: -> --------12V-----> 5V@250ma 5V@100ma 3V@40ma -12V@2ma My breadboarded prototype (which works) uses a 12v unregulated wall transformer and I have lots of regulators around. The 12v wall wart connects to the main enclosure where a 5V linear reg supplies about 250ma. This enclosure also has a 3V linear reg that drops the 5V down for about 50ma at 3v. A twisted pair cable takes the +12V unregulated and serial signals to the second enclosure that is basically a serial LCD. In fact the prototype uses a serial LCD I bought from www.earchLCD.com. It's a nice product, but too expensive for this project and with a lot of extra features I am not using. This serial LCD board has another 5v linear reg to supply the logic (100ma) and an LTC1044A converts the +12 to -12V for the 2ma Vee for the LCD. Since I am making a new, leaner serial LCD board from scratch, I figured might be able to reduce the components. Also, the more important thing I am trying to solve with changing the power supply method is the heat generated by the 5V linear reg in the main circuit. It has well over 7 volts of head voltage @ 1/4 amp, so it tends to get hot. I want to put it in an enclosure without a fan. So, I figured that I could use a 5V transformer (over-rated so the reg sees at at least 5+(its drop out) at full load) and use only one 5v reg in the main enclosure. I could send the regulated 5V over the serial connection to power so that the display enclosure's would not need another. Then at the display enclosure boost and invert the +5v to -12v. The -12V really needs to be set between -10V and -8V to adjust the contrast for a particular LCD. The data sheet for the LCD suggests a simple voltage divider with a pot since the current is so small. I am thinking of putting in a variable reg so that the adustment will be more stable for the end user. -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Dave Dilatush Sent: Sunday, July 22, 2001 7:14 PM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [OT]: How to reduce power consumption for battery operated circuits? BobG wrote... >New, Related Question: >Do you have to be more concerned with EMI when you use a switching DC-DC >convert / regulator? Yes, you do; possible interference to sensitive circuits in your own device is always an issue, as are possible radiated and/or conducted emissions that could interfere with other devices. The LT1931 device you linked to below operates around a megahertz, which could cause a lot of interference with nearby radios. >I am considering this linear device >http://www.linear.com/prod/datasheet?datasheet=569&product_family=power >In a nutshell its 5V in, -12V @150ma out. Its overkill for what I need (2ma >at -12V for an LCD), but the chip is only a couple bucks, and I am afraid of >discrete components:) It's overkill, and at the very low currents your application requires, it won't be very efficient either. Take a look at the efficiency curve for the "+5V to -12V Inverting Converter" circuit (I assume this is the one you were thinking of); see how the efficiency drops off sharply at low output currents? >From what I see on a quick read of the datasheet, it looks like this device was optimized to give high output power in a VERY small circuit footprint; hence, the high operating frequency to make the inductors small. It doesn't seem to be targeted at low current applications. >The data sheet actual has a sample board layout, warning, "If you don't lay >out your PCB carefully, its not going to work". And they REALLY mean it, too: if you have a layout that doesn't have the right ground topology (i.e., the right arrangement of the current paths in the circuit) the chip won't work. >Is this normal? Unfortunately, yes. >Are switching regulators trickier to work with? Sometimes yes, sometimes not so tricky. At best, they take considerably more care to design with than the trusty ol' 7805 linear regulator. At worst, they are unbelievably nightmarish to design. For applications requiring a couple of watts of power, there are many IC switching regulators that are fairly easy to design with. National Semiconductor's "Simple Switcher" chips are the ones I've used most often, and they have the design procedure down to a cookbook approach where you just plug some numbers into a few formulas, look up a few component values in a table, and you're done. Component selection, particularly filter capacitors, is critical, and so is circuit layout; but other than that it's straightforward. For applications requiring more power, design becomes progressively more and more difficult as the power increases. Above a few dozen watts, things get tricky; above a few hundred, it becomes "don't try this trick at home". Above a few thousand watts, it gets real scary. For lower-power applications, the situation gets even trickier; designing a switching regulator to convert 10 milliwatts at high efficiency can be a real challenge because every piddling little power loss within the circuit has to be minimized. In any case, yes; switching regulators are generally more difficult to design with than linear regulators. >I gather >that a DC-DC convert is similar to a switching regulator; the difference >being that the output of the DC-DC convert will vary with changes in Vin. Is >that correct? Yes and no. In some cases (such as the LT1931 you cited), the terms are used interchangeably. Here's a suggestion, for whatever it's worth: The easiest way to generate a negative voltage for your LCD bias would be with one of the RS232 interface chips (like a MAX232A) that generate their own + and - RS232 levels using capacitive charge pumps. Just ignore the RS232 transceiver circuits (ground the unused inputs or connect them to Vcc, whichever results in the least current drain) and use only the power section. The efficiency will be a lot better than the LT1931, and so will the price tag. The only drawback is that you won't get a full -12 volts out of it, only around -9V to -10V. Would that be enough? Hope this helps... Dave -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics