LiPo 3.3V Linear Regulation - Chip Suggestions?

Jacob, I'm curious if you are seeing noise on a scope or are your ADC readings jumping around more than expected? If you are seeing it on the scope, at what point is the noise - on the Vcc line, on the bridge signals or on the output of the amplifier? If it is noise on Vcc, is it switching spikes or ripple or just an unsteady voltage?

Assuming it is noise on Vcc and you've tried increasing the inductor value and adding more bulk capacitance, I would try (1) adding an LC filter on the output of the TPS61200 (2) inserting a small resistor between Vcc and your analog circuit and change the 0.1uF cap value as needed to filter the noise and (3) use a small LDO just for the analog circuit. Those may not be easy to do depending on the board layout so I would start with the easiest to do.

Good luck!

Mike

Mike Lease wrote:

 

Jacob, I'm curious if you are seeing noise on a scope or are your ADC readings jumping around more than expected? If you are seeing it on the scope, at what point is the noise - on the Vcc line, on the bridge signals or on the output of the amplifier? If it is noise on Vcc, is it switching spikes or ripple or just an unsteady voltage?

 

See my previous reply to dmaruska, when I populated my PCB I checked each subsystem independently. The issue with the noise has been isolated to when I powered the circuit through the battery regulator, however, I did not check just the VCC line in circuit. My readings were on the output of the amplifier, I'll have to go and recheck Vcc on my scope. Perhaps I can compare Vcc and Vout at the same time. I'll try to post a screencap of my findings.

Mike Lease wrote:

 

Assuming it is noise on Vcc and you've tried increasing the inductor value and adding more bulk capacitance, I would try (1) adding an LC filter on the output of the TPS61200 (2) inserting a small resistor between Vcc and your analog circuit and change the 0.1uF cap value as needed to filter the noise and (3) use a small LDO just for the analog circuit. Those may not be easy to do depending on the board layout so I would start with the easiest to do.

I see what you are saying here, Just filter out any variations from the power supply with a low pass filter. My experience with these is just an RC filter, I'll have to look up more regarding LC design. Are there any drawbacks from filtering the Vcc line? Why wouldn't this be inherent to the chip design if not?

My room for options is limited but I might be able to find space, I am using a breakout board for the TPS61200 due to my ability to solder that size package. I'll have to use a small perf board to begin with and see if I can incorporate that with my next PCB layout.

In regards to item 3, This may be a part of the main purpose of the post. I'm having a tough time finding an inductorless LDO that doesn't have a dropout voltage of > 400 mV.

Thank you,

Jacob

Mike Lease wrote:

 

Jacob, I'm curious if you are seeing noise on a scope or are your ADC readings jumping around more than expected? If you are seeing it on the scope, at what point is the noise - on the Vcc line, on the bridge signals or on the output of the amplifier? If it is noise on Vcc, is it switching spikes or ripple or just an unsteady voltage?

 

See my previous reply to dmaruska, when I populated my PCB I checked each subsystem independently. The issue with the noise has been isolated to when I powered the circuit through the battery regulator, however, I did not check just the VCC line in circuit. My readings were on the output of the amplifier, I'll have to go and recheck Vcc on my scope. Perhaps I can compare Vcc and Vout at the same time. I'll try to post a screencap of my findings.

Mike Lease wrote:

 

Assuming it is noise on Vcc and you've tried increasing the inductor value and adding more bulk capacitance, I would try (1) adding an LC filter on the output of the TPS61200 (2) inserting a small resistor between Vcc and your analog circuit and change the 0.1uF cap value as needed to filter the noise and (3) use a small LDO just for the analog circuit. Those may not be easy to do depending on the board layout so I would start with the easiest to do.

I see what you are saying here, Just filter out any variations from the power supply with a low pass filter. My experience with these is just an RC filter, I'll have to look up more regarding LC design. Are there any drawbacks from filtering the Vcc line? Why wouldn't this be inherent to the chip design if not?

My room for options is limited but I might be able to find space, I am using a breakout board for the TPS61200 due to my ability to solder that size package. I'll have to use a small perf board to begin with and see if I can incorporate that with my next PCB layout.

In regards to item 3, This may be a part of the main purpose of the post. I'm having a tough time finding an inductorless LDO that doesn't have a dropout voltage of > 400 mV.

Thank you,

Jacob

Jacob, which regulator are you using? Your original post said a TPS61200 but that is a boost regulator. If your regulator has a pulse skipping mode to improve light load efficiency you should disable that if you can. It will improve efficiency but at the expense of higher ripple.

For an LC filter design I've used a program called Elsie that you can download from Elsie - electrical filter design program from Tonne Software, you should be able to use the free version for what you need to do. The noise from modern DC-DC regulators is low enough for digital circuits and many analog circuits so the additional parts usually aren't required. If you just filter the power to the analog circuit you should be able to get by with physically small inductor and cap.

As far as LDOs go, take a look at the TI LP2980 for just powering your analog circuit. The dropout should be below 100mV if that circuit draw is 10mA as indicated earlier.

One more thing to consider would be to use an RC filter on the amplifier output but put it right at the input to your micro. That assumes the frequency of the noise is much higher than the strain gauge transitions.

Mike

I like the idea of decoupling the power system by using a separate regulator for the analog side and the digital side, this will most likely fix the problem. I'm not sure why I haven't considered that before, I just never thought to run two regulators on one circuit design.

I think I've tried to put an RC filter before the MCU input before, not sure why it didn't make it to the PCB Design.

Thanks a bunch for the help!

Jacob G.

Jacob, I hope that works for you. Using a 2nd regulator does make sense sometimes. If your WiFi isn't always connected it might make sense to put it on the switching regulator and have everything else running off an LDO. That way you could shut off the switching regulator when the WiFi isn't needed but then you have to worry about the state of signals between the micro and the WiFi module when the module is turned off so it probably isn't worth the trouble unless you have a tight power budget.

The RC at the micro input would still be a good idea if the analog signal goes very far or is routed near the switching regulator.

Mike