Transcript pptx
EECS 373 On Operational Amplifiers and Other Means of Manipulating Voltage and Current Schedule updates Op-amps Topic talks**** Short (50 min?) lecture: Design Expo stuff: poster, video etc. information 10:30am-1:30pm • I’ll have office hours and a review session between the last day of class and the final. – Exam review, likely Sunday late afternoon. • HW6 will be assigned on April 3rd. – Practice final—can’t assign until last topic talk is done. Once you’ve done your topic talk… • Send me your slides – Pdf is fine, but I’d prefer power point (and both is better yet). • Don’t believe I have the two from Tuesday yet. Other Administrivia? • And yes, Merriam-Webster accepts that as an English word. Analog—the bane of the computer engineer • In embedded systems, you often need to deal with voltages and currents. – Often the outputs you have don’t match the inputs you need. • Generally the current or voltage is too small • Sometimes the voltage is too big. • Sometimes the values are in the wrong range. • Today we’ll touch on some ways of manipulating these values. – This is intended to give you some idea what options are out there • Often the details are tricky and/or annoying. • We expect you may need to ask for help… Examples of where you might have problems. • You have 5V for power, but some devices need 3.3V for power. • You are using a device that generates too little current for your ADC, you may want to amplify the current but hold the voltage constant. • You may be using a UART or other serial bus where one device wants 3.3V or 1.7V and the other wants 5V. • You may be driving a motor that needs 12V@1A but you can only drive 5V@5mA, what do you do? What are DC converters? • DC converters convert one DC voltage level to another. – Very commonly on PCBs • Often have USB or battery power • But might need 1.8V, 3.3V, 5V, 12V and -12V all on the same board. – On-PCB converters allow us to do that Images from http://itpedia.nyu.edu/wiki/File:V_reg_7805.jpg, http://www.electronics-lab.com/blog/wp-content/uploads/2007/10/p1000255.JPG DC converters • Probably the most common problem is dropping power from 5V to 3.3V. – Often because we’ve got a device that wants 3.3V as Vcc and everything else wants 5V. • The generic term for a device used to change voltages is “DC converter” – But when dropping it is sometimes called a “voltage regulator”. – Specific types are called “linear regulator”, “Low Dropout (LDO)”, and “Switching regulator” Different types of DC converters Linear converters Switching converters • Simpler to design • Low-noise output for noisesensitive applications • Can only drop voltage • Can be significantly more complex to design – And in fact must drop it by some minimum amount – The larger the voltage drop the less power efficient the converter is • (All voltage dropped is converted to heat). – **Don’t use for project** • Can drop voltage or increase voltage – “buck” and “boost” respectively • Generally very power efficient – 75% to 98% is normal Linear regulator 1. Input 2. Ground 3. Output Linear regulators and capacitors • Specification for regulators almost always include required capacitors – If you don’t have them, your output may get noisy and cause all kinds of problems including reseting your chips. • Too big is better than too small. – May have required capacitor types (ceramic, etc.) • Be sure to check (most linear regulators don’t…) Op-amps • We’ll briefly talk about using op-amps to do a few basic things. – Current buffering – Threshold detection – Etc. • I’m not going to talk much about single supply vs. dual supply. – https://www.researchgate.net/post/what_is_the_difference_between_sin gle_supply_opamp_and_dual_supply_opamp is helpful though. Very idealized basics http://www.electronics-tutorials.ws/opamp/opamp1.gif?81223b Voltage comparator • Can change the ground input to any voltage. – Often just goes down to ground rather than –Vcc. Voltage follower • How does this work? And a bunch of others Current to voltage and back. Where would we have used a current-to-voltage device? Additional reading • http://research.cs.tamu.edu/prism/lectures/iss/iss_l5.pdf – Used for the last 4 slides. Serial bus issues • There are devices that can convert data between voltage levels automatically. – TXS0102 – It uses two separate power supply rails, with the A ports supporting operating voltages from 1.65 V to 3.6 V while it tracks the Vsupply, and the B ports supporting operating voltages from 2.3 V to 5.5 V