Merde with SMPS

[Douglas Beeson]

Hi all,

So I decided to leap with both feet into the deep cold pool of switched-mode power supplies, surface-mount no less, and now have something that doesn't work as advertised. I could use some help debugging it.

I am trying to get 5V out with two AA cells as input. My IC of choice is the TI TLV61220.

Here is the schematic: https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?dl=0

and here is the board layout: https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V conversion, but I have unsoldered those to better isolate the problem with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other two SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be useful?

Thanks!

--
Douglas Beeson <c.difficile@gmail.com>

 

[George Herold]

On Monday, November 10, 2014 9:54:14 AM UTC-5, Doug Beeson wrote:

Hi all,

So I decided to leap with both feet into the deep cold pool of switched-mode power supplies, surface-mount no less, and now have something that doesn't work as advertised. I could use some help debugging it.

I am trying to get 5V out with two AA cells as input. My IC of choice is the TI TLV61220.

Here is the schematic: https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?dl=0

and here is the board layout: https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V conversion, but I have unsoldered those to better isolate the problem with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other two SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be useful?

Thanks!

--
Douglas Beeson <c.difficile@gmail.com

Your schematic is mirrored. It says the minimum switching current is 200mA So try loading the output? (but I don't know this IC.) Have you tried running it first from a bench supply? (rather than a battery.)

George H.

 

[George Herold]

On Monday, November 10, 2014 11:14:34 AM UTC-5, Doug Beeson wrote:

On Mon, 10 Nov 2014 07:32:50 -0800 (PST)
George Herold <gherold@teachspin.com> wrote:

On Monday, November 10, 2014 9:54:14 AM UTC-5, Doug Beeson wrote:
Hi all,

So I decided to leap with both feet into the deep cold pool of switched-mode power supplies, surface-mount no less, and now have something that doesn't work as advertised. I could use some help debugging it.

I am trying to get 5V out with two AA cells as input. My IC of choice is the TI TLV61220.

Here is the schematic: https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?dl=0

and here is the board layout: https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V conversion, but I have unsoldered those to better isolate the problem with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other two SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be useful?

Thanks!

--
Douglas Beeson <c.difficile@gmail.com

Your schematic is mirrored. It says the minimum switching current is 200mA So try loading the output? (but I don't know this IC.) Have you tried running it first from a bench supply? (rather than a battery.)

George H.

Fixed the mirrored schematic. Sorry about that. There is some load already - about 5 mA in theory, but I haven't measured it.

I don't think the minimum load is anywhere near 200 mA. Figure 6 in the datasheet shows happy looking efficiency figures for Io from 0.1 mA all the way up to 100mA, with 5 V out, same as mine should be.

Doug, Yeah sorry about that. I replied in haste. The 200 mA is the smallest maximum current. Well try a fixed supply. (And then flip that polarized tantalum cap around so it's in the right way. :^)

One thing that bugs me: I used a copper pour all around the chip and the passives (dotted line on the board) that is tied to ground. Was that a good idea?

That shouldn't be a problem. (I had a ground plane spoil the (high) Q of an inductor, but I don't think that matters here.)

George H.

--
Douglas Beeson <c.difficile@gmail.com

 

[RobertMacy]

On Mon, 10 Nov 2014 07:54:10 -0700, Douglas Beeson <c.difficile@gmail.com>
wrote:

Hi all,

So I decided to leap with both feet into the deep cold pool of
switched-mode power supplies, surface-mount no less, and now have
something that doesn't work as advertised. I could use some help
debugging it.

I am trying to get 5V out with two AA cells as input. My IC of choice is
the TI TLV61220.

Here is the schematic:
https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?dl=0

and here is the board layout:
https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V
conversion, but I have unsoldered those to better isolate the problem
with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other two
SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be useful?

Thanks!

Dropbox crashed me ,....again! so can't see schematic you are using. TI's
website gave me the chip spec.

Usually an upconverter shorts the inductor to GND building up a flow of
current through it, THEN opens the switch to GND causing the outpuit end
of the inductor to pop up to almost any voltage it wants to go to. At
least the inductor pops up to 5V plus Vbe drop, or so [looks like TI put a
low drop switch in there to improve efficiency]. When all the current is
exhausted from the inductor, its output leg goes to the same voltage as
the input, usually with a lot of high frequency [scarey looking] ringing.
So the voltage at the inductor's output should go to GND, pop high for a
bit, then ring down to the battery voltage coming in. Then cycle repeats.
*if* something doesn't go right, all this action gets fouled up. for
example, the switch to GND never lets go of the inductor, not good. It is
very important to match the inductor, the capacitor [the FIRST capacitor],
and the load so that the chip is happily running in its 'sweet' spot.


Or, there's an open in the resisttor feedback causing the chip to think
the output is ALWAYS too much and thus voltage IN goes straight thru and
no switching occurs at all.

I agree put a nice minimum load on the output, switchers historical abhor
no load.

When you get it running we can all help addressing how to make the output
as quiet as a battery.

 

[Douglas Beeson]

On Mon, 10 Nov 2014 07:32:50 -0800 (PST)
George Herold <gherold@teachspin.com> wrote:

On Monday, November 10, 2014 9:54:14 AM UTC-5, Doug Beeson wrote:
Hi all,

So I decided to leap with both feet into the deep cold pool of switched-mode power supplies, surface-mount no less, and now have something that doesn't work as advertised. I could use some help debugging it.

I am trying to get 5V out with two AA cells as input. My IC of choice is the TI TLV61220.

Here is the schematic: https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?dl=0

and here is the board layout: https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V conversion, but I have unsoldered those to better isolate the problem with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other two SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be useful?

Thanks!

--
Douglas Beeson <c.difficile@gmail.com

Your schematic is mirrored. It says the minimum switching current is 200mA So try loading the output? (but I don't know this IC.) Have you tried running it first from a bench supply? (rather than a battery.)

George H.

Fixed the mirrored schematic. Sorry about that. There is some load already - about 5 mA in theory, but I haven't measured it.

I don't think the minimum load is anywhere near 200 mA. Figure 6 in the datasheet shows happy looking efficiency figures for Io from 0.1 mA all the way up to 100mA, with 5 V out, same as mine should be.

One thing that bugs me: I used a copper pour all around the chip and the passives (dotted line on the board) that is tied to ground. Was that a good idea?

--
Douglas Beeson <c.difficile@gmail.com>

 

[Tim Wescott]

On Mon, 10 Nov 2014 09:54:10 -0500, Douglas Beeson wrote:

Hi all,

So I decided to leap with both feet into the deep cold pool of
switched-mode power supplies, surface-mount no less, and now have
something that doesn't work as advertised. I could use some help
debugging it.

I am trying to get 5V out with two AA cells as input. My IC of choice is
the TI TLV61220.

Here is the schematic:
https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?dl=0

and here is the board layout:
https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V
conversion, but I have unsoldered those to better isolate the problem
with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other two
SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be useful?

Thanks!

Everything looks kosher to me, but the fact that the output voltage goes
up with the other converters disabled indicates that you may be
overloading the battery. What's the supply voltage with the one converter
running? You may be sucking the battery dry right off the bat.

How much current are you asking from the thing?

Barring weak batteries, and given that I get the idea that you're new to
surface mount, I suspect wiring errors. Go over the thing with a strong
magnification or an assembly microscope if you have one, and look for
plain ol' solder bridges, bad joints, and whatnot. Going over it with a
continuity checker is probably a good idea, too.

If you do as suggested and put it on a good stiff bench supply, then one
or more wiring errors may cause a smoke signal to be sent up, or at least
cause some informative heating of parts (just lay your finger down on the
circuit -- if you snatch it back in pain, the shiny spots on your finger
will indicate the hot components).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Douglas Beeson]

On Mon, 10 Nov 2014 09:12:25 -0700
RobertMacy <robert.a.macy@gmail.com> wrote:

On Mon, 10 Nov 2014 07:54:10 -0700, Douglas Beeson <c.difficile@gmail.com
wrote:

Hi all,

So I decided to leap with both feet into the deep cold pool of
switched-mode power supplies, surface-mount no less, and now have
something that doesn't work as advertised. I could use some help
debugging it.

I am trying to get 5V out with two AA cells as input. My IC of choice is
the TI TLV61220.

Here is the schematic:
https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?dl=0

and here is the board layout:
https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V
conversion, but I have unsoldered those to better isolate the problem
with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other two
SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be useful?

Thanks!


Dropbox crashed me ,....again! so can't see schematic you are using. TI's
website gave me the chip spec.

Usually an upconverter shorts the inductor to GND building up a flow of
current through it, THEN opens the switch to GND causing the outpuit end
of the inductor to pop up to almost any voltage it wants to go to. At
least the inductor pops up to 5V plus Vbe drop, or so [looks like TI put a
low drop switch in there to improve efficiency]. When all the current is
exhausted from the inductor, its output leg goes to the same voltage as
the input, usually with a lot of high frequency [scarey looking] ringing.
So the voltage at the inductor's output should go to GND, pop high for a
bit, then ring down to the battery voltage coming in. Then cycle repeats.
*if* something doesn't go right, all this action gets fouled up. for
example, the switch to GND never lets go of the inductor, not good. It is
very important to match the inductor, the capacitor [the FIRST capacitor],
and the load so that the chip is happily running in its 'sweet' spot.


Or, there's an open in the resisttor feedback causing the chip to think
the output is ALWAYS too much and thus voltage IN goes straight thru and
no switching occurs at all.

I agree put a nice minimum load on the output, switchers historical abhor
no load.

When you get it running we can all help addressing how to make the output
as quiet as a battery.

Thank you for the great info. I did a quick capture of the output (near the output capacitor) and graphed it in Excel:

https://www.dropbox.com/s/1vfj4ghipmzl6yd/Vout%20Capture%20PNG.png?dl=0

The output looks regular to me but seems to have a lot of ripple. Is 100mV on 2.5V OK?


--
Douglas Beeson <c.difficile@gmail.com>

 

[Douglas Beeson]

On Mon, 10 Nov 2014 13:19:15 -0600
Tim Wescott <seemywebsite@myfooter.really> wrote:

On Mon, 10 Nov 2014 09:54:10 -0500, Douglas Beeson wrote:

Hi all,

So I decided to leap with both feet into the deep cold pool of
switched-mode power supplies, surface-mount no less, and now have
something that doesn't work as advertised. I could use some help
debugging it.

I am trying to get 5V out with two AA cells as input. My IC of choice is
the TI TLV61220.

Here is the schematic:
https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?dl=0

and here is the board layout:
https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V
conversion, but I have unsoldered those to better isolate the problem
with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other two
SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be useful?

Thanks!

Everything looks kosher to me, but the fact that the output voltage goes
up with the other converters disabled indicates that you may be
overloading the battery. What's the supply voltage with the one converter
running? You may be sucking the battery dry right off the bat.

The batteries were fresh the other day when I started debugging and gave a clean 3 V. Now they're down to about 2.75V.

I don't have a bench supply but I can wire up a LM317-based thing pretty quickly and give it a try.

How much current are you asking from the thing?

In theory, should be about 5mA (a PIC and an LT1720 plus a few things that want nano-amps. But I can't figure out how to actually measure it!

Barring weak batteries, and given that I get the idea that you're new to
surface mount, I suspect wiring errors. Go over the thing with a strong
magnification or an assembly microscope if you have one, and look for
plain ol' solder bridges, bad joints, and whatnot. Going over it with a
continuity checker is probably a good idea, too.

If you do as suggested and put it on a good stiff bench supply, then one
or more wiring errors may cause a smoke signal to be sent up, or at least
cause some informative heating of parts (just lay your finger down on the
circuit -- if you snatch it back in pain, the shiny spots on your finger
will indicate the hot components).

Ha! I did notice that the TLV61220 gets a little warm (actually got very hot before I desoldered the other two SMPS), but nothing else seems overly hot.

But I will triple check the wiring.

Thanks, Tim.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

--
Douglas Beeson <c.difficile@gmail.com>

 

[RobertMacy]

On Mon, 10 Nov 2014 12:45:29 -0700, Douglas Beeson <c.difficile@gmail.com>
wrote:

...snip..
Thank you for the great info. I did a quick capture of the output (near
the output capacitor) and graphed it in Excel:

https://www.dropbox.com/s/1vfj4ghipmzl6yd/Vout%20Capture%20PNG.png?dl=0

The output looks regular to me but seems to have a lot of ripple. Is
100mV on 2.5V OK?

You are welcome. Graphed in Excel?! wow, you should get a copy of free
octave [Matlab clone]. I first started using it to replace Excel plotting
and calculations now use it to even analyze DSP before pouring the program
into concrete, to do complex communication systems analyses, to 3D plot a
myriad of data sets, to make Color Images to also plot 3D, make video
plots, and basically just use as one whiz bang calculator.

Remember dropbox crashes my systems so cannot retrieve images.

sounds like your FB path is not right. Did you 'measure' the resistors
before using? I've had some badly mismarked ones find their way into the
stock room. from memory most regulators use 1.2 Volts based upon what is
called the Widlar reference. About as constant as the energy gap in
silicon, or so. so the center tap of the resistor string is supposed to be
1.2 volts and then across the top resistor is supposed to 3.8 in your case.

From memory, linear supplies routinely yield 0.1% ripple while switchers
generally had 1% ripple which means at least 50mVpp at 5 Vdc.

You didn't say how much current the supply is requiring just to do 2.7
Vout.

 

[Tim Wescott]

On Mon, 10 Nov 2014 15:03:53 -0500, Douglas Beeson wrote:

On Mon, 10 Nov 2014 13:19:15 -0600 Tim Wescott
seemywebsite@myfooter.really> wrote:

On Mon, 10 Nov 2014 09:54:10 -0500, Douglas Beeson wrote:

Hi all,

So I decided to leap with both feet into the deep cold pool of
switched-mode power supplies, surface-mount no less, and now have
something that doesn't work as advertised. I could use some help
debugging it.

I am trying to get 5V out with two AA cells as input. My IC of choice
is the TI TLV61220.

Here is the schematic:
https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?
dl=0

and here is the board layout:
https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V
conversion, but I have unsoldered those to better isolate the problem
with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other
two SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be
useful?

Thanks!

Everything looks kosher to me, but the fact that the output voltage
goes up with the other converters disabled indicates that you may be
overloading the battery. What's the supply voltage with the one
converter running? You may be sucking the battery dry right off the
bat.

The batteries were fresh the other day when I started debugging and gave
a clean 3 V. Now they're down to about 2.75V.

I don't have a bench supply but I can wire up a LM317-based thing pretty
quickly and give it a try.

2.75 under load, or open-circuit? If it's open-circuit, then the cells
are pretty much toast.

How much current are you asking from the thing?

In theory, should be about 5mA (a PIC and an LT1720 plus a few things
that want nano-amps. But I can't figure out how to actually measure it!

Put a resistor in the power lead, enough to drop less than 100mV at your
predicted current (so, 10 ohms). Then measure the voltage drop across the
resistor.

Barring weak batteries, and given that I get the idea that you're new
to surface mount, I suspect wiring errors. Go over the thing with a
strong magnification or an assembly microscope if you have one, and
look for plain ol' solder bridges, bad joints, and whatnot. Going over
it with a continuity checker is probably a good idea, too.

If you do as suggested and put it on a good stiff bench supply, then
one or more wiring errors may cause a smoke signal to be sent up, or
at least cause some informative heating of parts (just lay your finger
down on the circuit -- if you snatch it back in pain, the shiny spots
on your finger will indicate the hot components).


Ha! I did notice that the TLV61220 gets a little warm (actually got very
hot before I desoldered the other two SMPS), but nothing else seems
overly hot.

"got very hot before I desoldered..."

Do you have an oscilloscope? Can you check the battery leads for
oscillation?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[George Herold]

On Tuesday, November 11, 2014 11:03:24 AM UTC-5, John Larkin wrote:

On Mon, 10 Nov 2014 14:45:29 -0500, Douglas Beeson
c.difficile@gmail.com> wrote:

On Mon, 10 Nov 2014 09:12:25 -0700
RobertMacy <robert.a.macy@gmail.com> wrote:

On Mon, 10 Nov 2014 07:54:10 -0700, Douglas Beeson <c.difficile@gmail.com
wrote:

Hi all,

So I decided to leap with both feet into the deep cold pool of
switched-mode power supplies, surface-mount no less, and now have
something that doesn't work as advertised. I could use some help
debugging it.

I am trying to get 5V out with two AA cells as input. My IC of choice is
the TI TLV61220.

Here is the schematic:
https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?dl=0

and here is the board layout:
https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V
conversion, but I have unsoldered those to better isolate the problem
with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other two
SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be useful?

Thanks!


Dropbox crashed me ,....again! so can't see schematic you are using. TI's
website gave me the chip spec.

Usually an upconverter shorts the inductor to GND building up a flow of
current through it, THEN opens the switch to GND causing the outpuit end
of the inductor to pop up to almost any voltage it wants to go to. At
least the inductor pops up to 5V plus Vbe drop, or so [looks like TI put a
low drop switch in there to improve efficiency]. When all the current is
exhausted from the inductor, its output leg goes to the same voltage as
the input, usually with a lot of high frequency [scarey looking] ringing.
So the voltage at the inductor's output should go to GND, pop high for a
bit, then ring down to the battery voltage coming in. Then cycle repeats.
*if* something doesn't go right, all this action gets fouled up. for
example, the switch to GND never lets go of the inductor, not good. It is
very important to match the inductor, the capacitor [the FIRST capacitor],
and the load so that the chip is happily running in its 'sweet' spot.


Or, there's an open in the resisttor feedback causing the chip to think
the output is ALWAYS too much and thus voltage IN goes straight thru and
no switching occurs at all.

I agree put a nice minimum load on the output, switchers historical abhor
no load.

When you get it running we can all help addressing how to make the output
as quiet as a battery.






Thank you for the great info. I did a quick capture of the output (near the output capacitor) and graphed it in Excel:

https://www.dropbox.com/s/1vfj4ghipmzl6yd/Vout%20Capture%20PNG.png?dl=0

The output looks regular to me but seems to have a lot of ripple. Is 100mV on 2.5V OK?

What are the time units?

Did you mention tantalum caps somewhere? The data sheet recommends
ceramics.

No I mentioned tantalum caps. It's just (for me) the most common mistake that get's by testing and then fails in the field... the d@mn things can last for hours/ days with a reverse voltage... I had one that was on a 5V supply and lasted for years. (35V I think) My only solution is a visual inspection.

George H.

Once you do get it working, using the +5 to drive the other switchers
may be a problem.


--

John Larkin Highland Technology, Inc

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Mon, 10 Nov 2014 14:45:29 -0500, Douglas Beeson
<c.difficile@gmail.com> wrote:

On Mon, 10 Nov 2014 09:12:25 -0700
RobertMacy <robert.a.macy@gmail.com> wrote:

On Mon, 10 Nov 2014 07:54:10 -0700, Douglas Beeson <c.difficile@gmail.com
wrote:

Hi all,

So I decided to leap with both feet into the deep cold pool of
switched-mode power supplies, surface-mount no less, and now have
something that doesn't work as advertised. I could use some help
debugging it.

I am trying to get 5V out with two AA cells as input. My IC of choice is
the TI TLV61220.

Here is the schematic:
https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?dl=0

and here is the board layout:
https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V
conversion, but I have unsoldered those to better isolate the problem
with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other two
SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be useful?

Thanks!


Dropbox crashed me ,....again! so can't see schematic you are using. TI's
website gave me the chip spec.

Usually an upconverter shorts the inductor to GND building up a flow of
current through it, THEN opens the switch to GND causing the outpuit end
of the inductor to pop up to almost any voltage it wants to go to. At
least the inductor pops up to 5V plus Vbe drop, or so [looks like TI put a
low drop switch in there to improve efficiency]. When all the current is
exhausted from the inductor, its output leg goes to the same voltage as
the input, usually with a lot of high frequency [scarey looking] ringing.
So the voltage at the inductor's output should go to GND, pop high for a
bit, then ring down to the battery voltage coming in. Then cycle repeats.
*if* something doesn't go right, all this action gets fouled up. for
example, the switch to GND never lets go of the inductor, not good. It is
very important to match the inductor, the capacitor [the FIRST capacitor],
and the load so that the chip is happily running in its 'sweet' spot.


Or, there's an open in the resisttor feedback causing the chip to think
the output is ALWAYS too much and thus voltage IN goes straight thru and
no switching occurs at all.

I agree put a nice minimum load on the output, switchers historical abhor
no load.

When you get it running we can all help addressing how to make the output
as quiet as a battery.






Thank you for the great info. I did a quick capture of the output (near the output capacitor) and graphed it in Excel:

https://www.dropbox.com/s/1vfj4ghipmzl6yd/Vout%20Capture%20PNG.png?dl=0

The output looks regular to me but seems to have a lot of ripple. Is 100mV on 2.5V OK?

What are the time units?

Did you mention tantalum caps somewhere? The data sheet recommends
ceramics.

Once you do get it working, using the +5 to drive the other switchers
may be a problem.


--

John Larkin Highland Technology, Inc

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Tim Wescott]

On Tue, 11 Nov 2014 08:03:23 -0800, John Larkin wrote:

On Mon, 10 Nov 2014 14:45:29 -0500, Douglas Beeson
c.difficile@gmail.com> wrote:

On Mon, 10 Nov 2014 09:12:25 -0700 RobertMacy <robert.a.macy@gmail.com
wrote:

On Mon, 10 Nov 2014 07:54:10 -0700, Douglas Beeson
c.difficile@gmail.com
wrote:

Hi all,

So I decided to leap with both feet into the deep cold pool of
switched-mode power supplies, surface-mount no less, and now have
something that doesn't work as advertised. I could use some help
debugging it.

I am trying to get 5V out with two AA cells as input. My IC of
choice is the TI TLV61220.

Here is the schematic:
https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?
dl=0

and here is the board layout:
https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V
conversion, but I have unsoldered those to better isolate the
problem with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other
two SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be
useful?

Thanks!


Dropbox crashed me ,....again! so can't see schematic you are using.
TI's website gave me the chip spec.

Usually an upconverter shorts the inductor to GND building up a flow
of current through it, THEN opens the switch to GND causing the
outpuit end of the inductor to pop up to almost any voltage it wants
to go to. At least the inductor pops up to 5V plus Vbe drop, or so
[looks like TI put a low drop switch in there to improve efficiency].
When all the current is exhausted from the inductor, its output leg
goes to the same voltage as the input, usually with a lot of high
frequency [scarey looking] ringing. So the voltage at the inductor's
output should go to GND, pop high for a bit, then ring down to the
battery voltage coming in. Then cycle repeats. *if* something doesn't
go right, all this action gets fouled up. for example, the switch to
GND never lets go of the inductor, not good. It is very important to
match the inductor, the capacitor [the FIRST capacitor],
and the load so that the chip is happily running in its 'sweet' spot.


Or, there's an open in the resisttor feedback causing the chip to
think the output is ALWAYS too much and thus voltage IN goes straight
thru and no switching occurs at all.

I agree put a nice minimum load on the output, switchers historical
abhor no load.

When you get it running we can all help addressing how to make the
output as quiet as a battery.






Thank you for the great info. I did a quick capture of the output (near
the output capacitor) and graphed it in Excel:

https://www.dropbox.com/s/1vfj4ghipmzl6yd/Vout%20Capture%20PNG.png?dl=0

The output looks regular to me but seems to have a lot of ripple. Is
100mV on 2.5V OK?

What are the time units?

Did you mention tantalum caps somewhere? The data sheet recommends
ceramics.

Once you do get it working, using the +5 to drive the other switchers
may be a problem.

Is he using the output of the one switcher to drive the input of others?
I had missed that. Yes, it can be an issue.

I recently helped a local colleague here in PDX with a problem he was
having with driving a switcher from a switcher -- he was having
intractable oscillation problems. Once I pointed out that an efficient
switching supply that's supplying constant power out has, BY NECESSITY, a
negative input impedance, his problem became much clearer to him.

He ended up just scraping the second switcher off the board, although I
still think that he could have tuned the first switcher into stability
with the second switcher as a load. He was uncomfortable with playing
games with the first switcher's compensation, though.

I've driven switchers from switchers with good success - however, in my
case it was a bulk supply switcher that was powering EVERYTHING, including
a bias supply switcher that presented a very light load. Presumably the
positive impedance of the "everything" swamped out the negative impedance
presented by my bias supply.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[John Larkin]

On Tue, 11 Nov 2014 11:36:52 -0600, Tim Wescott
<seemywebsite@myfooter.really> wrote:

On Tue, 11 Nov 2014 08:03:23 -0800, John Larkin wrote:

On Mon, 10 Nov 2014 14:45:29 -0500, Douglas Beeson
c.difficile@gmail.com> wrote:

On Mon, 10 Nov 2014 09:12:25 -0700 RobertMacy <robert.a.macy@gmail.com
wrote:

On Mon, 10 Nov 2014 07:54:10 -0700, Douglas Beeson
c.difficile@gmail.com
wrote:

Hi all,

So I decided to leap with both feet into the deep cold pool of
switched-mode power supplies, surface-mount no less, and now have
something that doesn't work as advertised. I could use some help
debugging it.

I am trying to get 5V out with two AA cells as input. My IC of
choice is the TI TLV61220.

Here is the schematic:
https://www.dropbox.com/s/86y7n3doq8vk8s3/TLV61220%20schematic.pdf?
dl=0

and here is the board layout:
https://www.dropbox.com/s/sr9uefklgz4k7ac/TLV61220%20Layout.pdf?dl=0

The full board has two more SMPS ICs that do 5->15V and 15->-15V
conversion, but I have unsoldered those to better isolate the
problem with the 3-5V circuit.

I am getting about 2.7 V out, which is up from 1.8 V when the other
two SMPS ICs were in place.

I could provide some images from the oscilloscope. What would be
useful?

Thanks!


Dropbox crashed me ,....again! so can't see schematic you are using.
TI's website gave me the chip spec.

Usually an upconverter shorts the inductor to GND building up a flow
of current through it, THEN opens the switch to GND causing the
outpuit end of the inductor to pop up to almost any voltage it wants
to go to. At least the inductor pops up to 5V plus Vbe drop, or so
[looks like TI put a low drop switch in there to improve efficiency].
When all the current is exhausted from the inductor, its output leg
goes to the same voltage as the input, usually with a lot of high
frequency [scarey looking] ringing. So the voltage at the inductor's
output should go to GND, pop high for a bit, then ring down to the
battery voltage coming in. Then cycle repeats. *if* something doesn't
go right, all this action gets fouled up. for example, the switch to
GND never lets go of the inductor, not good. It is very important to
match the inductor, the capacitor [the FIRST capacitor],
and the load so that the chip is happily running in its 'sweet' spot.


Or, there's an open in the resisttor feedback causing the chip to
think the output is ALWAYS too much and thus voltage IN goes straight
thru and no switching occurs at all.

I agree put a nice minimum load on the output, switchers historical
abhor no load.

When you get it running we can all help addressing how to make the
output as quiet as a battery.






Thank you for the great info. I did a quick capture of the output (near
the output capacitor) and graphed it in Excel:

https://www.dropbox.com/s/1vfj4ghipmzl6yd/Vout%20Capture%20PNG.png?dl=0

The output looks regular to me but seems to have a lot of ripple. Is
100mV on 2.5V OK?

What are the time units?

Did you mention tantalum caps somewhere? The data sheet recommends
ceramics.

Once you do get it working, using the +5 to drive the other switchers
may be a problem.

Is he using the output of the one switcher to drive the input of others?
I had missed that. Yes, it can be an issue.

I recently helped a local colleague here in PDX with a problem he was
having with driving a switcher from a switcher -- he was having
intractable oscillation problems. Once I pointed out that an efficient
switching supply that's supplying constant power out has, BY NECESSITY, a
negative input impedance, his problem became much clearer to him.

He ended up just scraping the second switcher off the board, although I
still think that he could have tuned the first switcher into stability
with the second switcher as a load. He was uncomfortable with playing
games with the first switcher's compensation, though.

I've driven switchers from switchers with good success - however, in my
case it was a bulk supply switcher that was powering EVERYTHING, including
a bias supply switcher that presented a very light load. Presumably the
positive impedance of the "everything" swamped out the negative impedance
presented by my bias supply.

Soft-starting the secondary switchers can help a lot. Otherwise, as
you note, the negative Zin of the secondaries can bog down the
primary. Adding "real" resistance across the intermediate bus doesn't
help if the primary switcher is current limited trying to start up the
downstream ones.

I've had the same concern when powering a product from a switchmode
wall-wart; it works fine once it's up, but may never get there. Some
of the warts fold back, even worse.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com