DC/DC Converter in Noisy Environment - Page 1
Forum section: electronics / projects

author: martyn
author_rank: Newbie
timestamp: December 06, 2011, 10:38:34 pm
content: Hello all, first post!Right, I am making a wheeled robot (I am proficient in digital design, and layout, the mechanical ends and this is by far not the first electronics project of this kind I have done). I am using 2x Maxon motors with wheel encoders (get roughly 750 edges per wheel rotation, giving sub mm accuracy) plugged into a high end dspic etc. etc. The motors are driven from the battery (3.7V nominal lipo) and the back emf is blasted squarely back into the battery - no problems there.The issue is that in previous robots I have only used linear regulators, and was running from twice the voltage so getting 3.3V for the logic and 5V for the motor sensors was easy. This time I am using a low drop out regulator (50mV drop out at 200mA) to get 3.3V and a switch mode to get the 5V.As space is an issue I am looking to usehttp://www.ti.com/product/tps60130,but in general how do the step up DC/DC converters handle transients on the input voltage (within the operating spec)....Or is there a better way to get 200mA of old school 5V for the motor encoders!Your turn, thanks in advance!- Martyn

author: IanB
author_rank: Super Contributor
timestamp: December 06, 2011, 11:00:40 pm
content: If you didn't need 200 mA, I might have suggested just using a separate 9 V battery for the electronics and just using the lipo for the high power bits. But 200 mA is a heavy drain on a 9 V battery.

author: martyn
author_rank: Newbie
timestamp: December 06, 2011, 11:08:50 pm
content: 9V PP3 is pretty big, otherwise another lipo would do - the pcb is 60mm x 40mm: crammed IR sensors x4, dspic, 24F (data logging, 16MB flash attached), tri axis gyro and accelerometer plus the motor control circuitry and power circuits. Only two passives > 0608 and that's the low ESR cap for the on-board regulator on the dspic/PIC 24. No room for dedicated programming headers - so they are on a break out board with some debugging bits.The battery will only last 10 minutes, if I'm lucky.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: December 07, 2011, 12:33:40 am
content: The encoders are not available in 3.3v version?Or perhaps the 5V devices work properly when powered directly from the lipo?

author: sonicj
author_rank: Frequent Contributor
timestamp: December 07, 2011, 01:26:46 am
content: Quote from: Short Circuit on December 07, 2011, 12:33:40 amthats what i was thinking...are the encoders optical? hall? resistive? whats the model on the motor/encoder?TPS60130 looks sufficient to me, but im no expert.. what aboutTPS63002and some headlights?-sj

author: martyn
author_rank: Newbie
timestamp: December 07, 2011, 07:53:00 am
content: Quote from: Short Circuit on December 07, 2011, 12:33:40 amno 3.3V version avaliable i'm afraid:http://www.maxonmotor.com/medias/sys_master/8796909830174/ENC-MR-64imp-241057_11_EN_257.pdf4.75V is the lowest they are spec'ed to (too risky to try any lower). The reason they are 5V is so they are compatible with their own lines of motor driver products, its the same with Faulhaber - so I guess it's one of those industry standards which were never written down!

author: martyn
author_rank: Newbie
timestamp: December 07, 2011, 07:55:47 am
content: Quote from: sonicj on December 07, 2011, 01:26:46 amHall effect, they are built onto the motor, the combination is made to order.the TPS63002 is a bit overkillalthough anything that fits the bill without external components I will take a look atCheers,- Martyn

author: sonicj
author_rank: Frequent Contributor
timestamp: December 07, 2011, 12:58:51 pm
content: hmm... is the motor potted? looks like the hall effect is accessible from the exploded view.... maybe one sensor and 3 passives and double magents? could you not replace the hall sensor with a 3.3V compatible version?-sj

author: Short Circuit
author_rank: Frequent Contributor
timestamp: December 07, 2011, 01:56:36 pm
content: Quote from: martyn on December 06, 2011, 10:38:34 pmAs for the original question;This regulator uses pulse skipping if the output voltage gets too high. So effectively it also does downconversion.The performace graphs confirm this too, look at the graphs on page 12 of the datasheet. 5V output with input voltages ranging from 2.7 to 5.4V.

author: martyn
author_rank: Newbie
timestamp: December 07, 2011, 06:56:48 pm
content: Quote from: sonicj on December 07, 2011, 12:58:51 pmI think its a bit more than that part alone costs £60, there may be little to it but what is there is probably engineered to the micrometer!

author: martyn
author_rank: Newbie
timestamp: December 07, 2011, 07:01:48 pm
content: Quote from: Short Circuit on December 07, 2011, 01:56:36 pmShould be fine from that spec, I was just concerned with the noise on the input - what if the input jumps by 250mV (for instance) should I expect that hike to be attenuated somewhat - and if I start putting in transients on the input is there a risk the whole thing will become unstable (there probably is, but probably only if I choose a PWM frequency somewhere near or something that multiplies into the switching frequency).I have asked around a few places and most people say it should be fine - no one with any examples of it being fine, just a certain level of trust in DC/DC switching systems. So - bite the bullet time I suppose!

author: Short Circuit
author_rank: Frequent Contributor
timestamp: December 07, 2011, 07:59:20 pm
content: Input changes will be visible at the ouput, but with much lower level; see graph 21 op page 14.Even with input transients with equal frequency, there shouldn't be a problem;the device regulates based on output voltage, and that is not going to changesignificantly from one cycle to the next.

author: martyn
author_rank: Newbie
timestamp: December 07, 2011, 09:41:30 pm
content: Cheers for that. I have seen input noise put off a switch mode (strangley I saw it this morning, some 10kHz noise off a bench power supply was destabilizing a switch mode with a similar switching frequency).- Martyn

author: JimmyM
author_rank: Regular Contributor
timestamp: December 07, 2011, 10:42:12 pm
content: How about this little gem.http://www.dimensionengineering.com/lvboost.htm