Power circuit input noise - Page 1
Forum section: electronics / projects

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 05, 2013, 03:46:18 pm
content: 1) I have Power management board which accepts power from a battery (Li ion) and an external 5V wall wart adaptor. When the adapter supply is there it charges the battery and drives the stack up board connected to the power management board. in absence of adaptor the device is powered from the charged battery.2)Power management board generates 3.3V which is usd to power a stack up board.3) The stack up board has an FTDI chipset for USB serial communication. On plugging and unplugging the adaptor I notice that the FT232 chip freezes and refuse to communicate. Please not that the FTDI is powered from USB and has no relation to the power rails of Stack up 3.3V generated from power management board.4) I believe there is surge in the board during insertion and removal of the adaptor which causes the FTDI to hang.5) I have tried putting resistance in the path of interface b/w FTDI chip(especially RXD signal) and the 3.3V signals. But no improvements were observed.6) I tried to implement an LC filter at the o/p pf 3.3V LDO (l=680nH and 10uF cap. This too did not help me resolve the issue.7) I tried a common mode choke at the power entry point of 5V adaptor but it did not help , but I am not sure whether I implemented the right scheme.How to avoid such problems??? I am desperate to resolve the issue.....7) How to make the system immune to such surges.

author: Paul Price
author_rank: Super Contributor
timestamp: June 05, 2013, 11:26:05 pm
content: I think the power supply is interfering with the chip.You can easily make a "soft-start" circuit. Try the circuit shown attached. It will gently raise the voltage up over a few seconds, if this is really the trouble.

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 06, 2013, 02:42:24 pm
content: hi,I have added a "soft start" in the circuit (soft start for 1.5s approximate). Still I could hang the FT232 by insertion of adaptors after several tries.There was already a soft start for 13ms approx. I increase it to 330ms and later to 1.5s. Please refer to the circuit below. (R39 and R41 increased to 1M each)."From Paul's circuit I have added a 47uF Electrolytic capacitor at the o/p of 3.3V (of Power management mode). There was already ceramic capacitors in the range of 4.7Uf , 10uF etc in parallel." This also yielded no result.What if I cutoff 5V(adaptor path) from powering the stack up board?? In this case battery will power the Stack up board and Adaptor will just recharge the battery. I believe the low impedance path of battery will absorb the Surge noise from insertion.

author: Paul Price
author_rank: Super Contributor
timestamp: June 06, 2013, 03:02:41 pm
content: I am totally confused by you circuit as shown, the battery to charge is not in this circuit.

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 06, 2013, 04:14:50 pm
content: Hi Paul,Battery charger comes after the D5VIN.5VIN in the input entry path for the 5V Supply.The switches Q6 and Q7 are ON/OFF switches to control the 5VIN (Basically cut off 5V supply if an over voltage condition is detected).D5VIN is applied to a li-ion charger chip input and also powers the 3.3V LDO.Please refer to the images. VBAT Header is the battery header.NOTE SW_D5VIN is the same D5VIN routed through a mechanical switch.In schematics its hard to define but by cutting of layout tracks I can prevent the LDO powering from the 5V supply but alway powerd by the battery.

author: Paul Price
author_rank: Super Contributor
timestamp: June 06, 2013, 04:38:52 pm
content: What if I cutoff 5V(adaptor path) from powering the stack up board?? In this case battery will power the Stack up board and Adaptor will just recharge the battery. I believe the low impedance path of battery will absorb the Surge noise from insertion.Give that a try.Check the voltages on the power and signal pins of FDTI chip and stack up board before and after the power supply is plugged in. There seems they might be somehow connected to the 3.3V or other points you are not aware off.

author: C
author_rank: Super Contributor
timestamp: June 06, 2013, 06:55:05 pm
content: You might want to do a simple testsCould be a ground loop problem.Try connecting1. Just the ground lead from your external 5V wall wart adaptor2. a non grounded external 5V#1 checks for ground loops while #2 checks switch from batt.C

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 07, 2013, 02:01:09 am
content: Hi C,PLease Could you please clarify,1) Where should I Connect ground leads from 5V supply??If you could elaborate a little more, I did not understand the idea.RegardsRakesh M.

author: C
author_rank: Super Contributor
timestamp: June 07, 2013, 02:51:32 am
content: When you are connecting thatQuoteYou are getting 5 volts on two wires. You could also be creating a ground loop.If the negative wire of the wall wart is connected to it's power source ground(power line ground) then you could have a ground loop causing a voltage/current change witch is affecting USB chip.Ground loop would be as followsPower management board ground tostack up board ground toUSB cable ground topc ground topower line groundWhen you connect the wall wart you could addpower line ground towall wart 5 volt negative lead toPower management board groundYou now have a ground loop and any difference in potential will create a current flow in ground loopTest #2 supplies 5 volts to your Power management board with out the possible ground creation to power line and tests your circuit for problem with the transition from no external 5 volt power to being powered from external 5 volts. So by plugging and un-plugging the external supply to the Power management board you are testing if your circuit handles both going to external power and going back to internal power.Test #1 is testing for ground loop created problems. By not connecting the positive lead of the 5 volts power, the only thing your are doing is adding or removing a ground connection..The two tests breaks the problem down and will help identify what the problem is, so you know what needs fixing.Clear?C

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 07, 2013, 04:03:53 am
content: Hi C,Great!!!!1) I tried connecting the ground (-ve terminal of Wall wart supply) and left open the 5V +ve terminal. After 2 - 3 attempts of inserting adaptor I could recreate the problem.

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 07, 2013, 04:19:29 am
content: hi C,I tried connecting to a laptop's USB (not connected to mains) and tried to insert and remove the adapter. Even then the FTDI section hung.

author: C
author_rank: Super Contributor
timestamp: June 07, 2013, 05:09:36 am
content: If just plugging in and removing USB connection creates problems, then look at FTDI area. Do some looking at FTDI Chip Home Pagewww.ftdichip.com/?look for doc's and app notes for the chip your usingC

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 07, 2013, 05:18:35 am
content: Hi C,I just wanted to isolate the USB from mains power. For this purpose I connected USB to a laptop and started viewing data in the hyper terminal.Later i started inserting and removing the wallwart 5V adapter connected to the power management board. This resulted in the FTDI section hanging.

author: C
author_rank: Super Contributor
timestamp: June 07, 2013, 06:57:16 am
content: NO wall wart, just connecting and disconnecting USB cable. If you have problemsLaptop to FTDI connects usb power to FTDI area. and you have the FTDI getting connected over USB to laptop. You also have the changes with FTDI talking to your circuit.I would get this working first, look at link for ideas on what is wrong=========================laptop(not connected to mains)USB cable connected, just connecting and disconnecting wall wart. If you have problemsI would guess a power glitch, where voltage changes to FTDI or you are dropping power when switching between internal powerYou might think of what is needed. You are trying to use switches. One switch between external wall wart and common plus. Second switch between battery and common plus. If you have both switches off during the change only the cap's connected to common plus is your source of power for everything. What you need to happen is like changing a pot from one end to the other with the load connected to the wiper. Battery switch Resistance increases lowering current from battery while the external switch decreases resistance increasing current from external. If this transition is not smooth only the cap connected to common plus will save you.You may need a third switch to connect the external power to battery charger.C

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 07, 2013, 07:42:20 am
content: Result of Test #2Here i connected the 5V through a Keithley (5v isolated source) instead of the wall wart.I did create power cycle profile for 2s power off and 2s 5V power ON. I believe this would enable battery to take over for two second and be dormant for the next 2s.I ran this cycle 200 times. With the device communicating with PC .I did not face any problem of FTDI hanging. Also I observed the current IN and OUT which was normal.Note: - Power requirement of the stack up board is 30mA approximate and battery is charged at 200mA. So the total current intake by the system would be 230mA when battery is charging. This is what i observed during the power cycling.The battery and 5V are connected to a common point using Diode ORing.

author: C
author_rank: Super Contributor
timestamp: June 07, 2013, 08:16:26 am
content: You last post strongly suggests ground loop problemRepeat test of last post with pc connected to power line.try adding an removing the ground to Keithley (5v isolated source)5v isolated source to 5v UN-isolated source to 5v isolated sourceThink you will see your problem.If problem shows,You said FTDI is powered from USB. If you isolated everything from FTDI to your circuit you would break the ground loop and not have problem.Edit: no connection of FTDI's Power or Ground to Power management board's power or ground, with data passing over the isolation interface. This could be via optic couplers or one of the other isolator chips. Make sure what you do use will handle your max serial speed.C

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 07, 2013, 09:19:56 am
content: Hi C,You are bang On...1. I tried connecting isolated supply (Keithley supply) to the device.Connected a wall wart supply to a plug with the end o/p terminals unconnected.I plugged IN/OUT the wall wart adapter. This had no impact on device functionality......2. Then i connected the common ground of Keithley supply to the isolated ground.then tried to insert/remove wall wart adaptor to another plug...... This did hang the device......3. I tried connecting un isolated supply (Keithley supply with isolated ground connected to the common ground) to the device. 200times i ran the 2s ON OFF cycle. But this didnot hang the device......Does this suggest that I should use a quality Wall wart adapter???

author: C
author_rank: Super Contributor
timestamp: June 07, 2013, 09:55:44 am
content: Plugging and un plugging wall wart does two things power up | power down and ground |un-ground at same time.Your Keithley supply should be great while wall wart might not be. I would guess that your Keithley has a ground post that is connected to power line ground.With pc connected to power line.while Keithley test going power on/power offconnect and disconnect a wire from Keithley ground post to Keithley 5 volt negative lead..Just by the connect and disconnect of this wire, you should be able to create your problem.If you can not create the problem with this method then I would say bad wall wart.C

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 07, 2013, 10:15:47 am
content: Hi ,I could not recreate the problem by connect and disconnect of ground post to 5V negative lead while keithley ON/OFF the 5V source

author: C
author_rank: Super Contributor
timestamp: June 07, 2013, 10:35:46 am
content: I would say bad wall wart.could be a ground difference between it and pc or it goes crazy on powerup|connectionC

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 07, 2013, 11:22:12 am
content: mmmI tried a"V infinitywall wart 6V....4A from CUI Japan ". It has all the approvals CE, U/L,FCC ....This time I connected the ground to the CUI's wall wart adapter... tried the same insertion and removal...... device hung....

author: Paul Price
author_rank: Super Contributor
timestamp: June 07, 2013, 01:18:51 pm
content: looks like a ground loop problem. Increase the size of the grounding wires connecting the circuits. I still think you are missing some subtle circuit connection between these two assumed isolated circuits.

author: C
author_rank: Super Contributor
timestamp: June 07, 2013, 01:33:04 pm
content: You have tried many things,I would isolate everything from FTDI, done correctly this removes the possible ground loop, and removes effects to FTDI's Power.no connection of FTDI's Power or Ground to Power management board's power or ground, with data passing over the isolation interface. This could be via optic couplers or one of the other isolated chips. Make sure what you do use will handle your max serial speed.C

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 08, 2013, 06:22:19 am
content: Hiplease find the FTDI section below.VCC3.3 is the stack up board power which comes from J8 connector(Power management board).I had connected the reset pin of U10 to ground.Connected a 1K resistance across Pin no 5 of FTDI pin.Also removed the Diode D1.Ground is a common to all. The stack up board is a 4 layer design one digita ground plane (except Analog plane).I had performed all the tests in this condition as suggested by "Paul"

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 08, 2013, 06:36:23 am
content: Just a doubt...If its a ground loop then why the problem repeats when I device is connected to a laptop.I had tried device connected to laptop(USB) and only the wall wart connected to only to ground and +ve pin left unconnected.... Insertion and removal of wallwart hangs FTDI......

author: rakeshm55
author_rank: Regular Contributor
timestamp: June 10, 2013, 04:20:35 am
content: I am using a Wall wart adapter with 2 pins... wont it be isolated???

author: rakeshm55
author_rank: Regular Contributor
timestamp: July 01, 2013, 03:38:03 am
content: Hi,I contacted FTDI support team with the problem, they suggested to short the USB ground(Device ground) and the shield on my board. After this modification I am not able to hang the device .I have run the device for 3 days continuously (with a shielded USB cable of lenth 1.75m) and have tried inserting and removing the power adaptor, connecting drill machine to the same power line etc . The device didn't hang.By the way I am trying a lengthy USB cable 8m with shield .I didnt understand what principle is at work here??Support team of FTDI suggests a signal noise problem or an ESD problem.