Hello Uwe, Tony and others, First, thank you for your reactions.
Tony wrote: > What DC voltages are available? What DC voltages are needed? > Do you need isolation? What are the power requirements? See below. Uwe wrote: > ad 1.) instead of the diodes you could build a transistor > circuit which senses the external supply. If you want to go > for the diode solution, you could use 2 or more diodes in > series with the battery to favour the external supply - the > of course your circuit would have to be able to run from > 9V-2*Vd when driven by battery... The battery voltage is more critical than the supply voltage. The battery voltage drops when the battery is loaded. > ad 2.) the NatSemi's simple switchers are really simple. > However without you telling us a bit more about your needs > (input voltage, output voltage, output current, size) it is > hard to tell what would be the right choice... I can still choose the input voltage. Depending on the configuration, I need several voltages First supply 5V, 2A (no battery backup required) Second 5V, 30mA typical, 80mA max. (battery operated when power fail) Third min. 7.5V output voltage, around 300mA typical, taken over by the battery when power fail If the primary power fails the battery takes over for maximum of 10 seconds. Then the device is switched off completely. It seems logical to me to derive the second 5V voltage with a linear regulator from the third voltage. It will also have to operate from the battery, so these two things can be combined. I can use the 2 diode trick if I raise the voltage to 9.5V in stead of the needed 7.5V without loosing battery power. However, 7.5V at the output is still sufficient to operate the circuity behind the diodes. Al extra power will be exchanged into heat. Lowering the voltage below 9V means I have to design a transistor circuit to switch off the battery when the primary power is available to prevent unwanted battery usage. I'm thinking of a primary powersupply of 5V, 3A. This solves the 5V, 2A supply which does not need the battery backup and I don't have to use any components on the PCB (only a transorb-like device). Then use a step up converter to create the 9.5V. Kind regards, Elbert Smit. -- Author: Elbert Smit INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
