A quiet SEPIC negative supply

W

Winfield Hill

Guest
I needed to add a -3.3V to -6V supply to my RIS-796A
250A pulser, analog version. The PCB didn't have much
space, so I implemented a simple inverting switcher
with an LM2840 buck converter. See circuit and layout
on file U7=LM2840_sch+pcb.jpg in the DropBox folder:

https://www.dropbox.com/sh/rfqohfvubj9pddh/AAC9dSrWbeJmY9iq_rODYonIa?dl=1

There's also a sub-folder with relevant datasheets.

The circuit works OK, although it's a bit fragile, with
ICs failing instantly when I miss-probe. The L1 ME3215
inductor has 1R5 resistance, and can deliver over 100mA,
more than needed. But it has a horrible 200mV output
ripple, forcing me to add an LC output filter stage.

Checkout the other file U7=AP3012_sch.jpg It's my idea
for a simple SEPIC inverting converter, using a boost-
switcher and a coupled inductor. The inductor feeds a
continuous current into the output cap, for low ripple,
so maybe the LC filter can be eliminated. Now L1 is an
LPD3015, with 2R8 ohms, but it can still deliver 100mA.
The coupled inductor is only 3x3mm, and I hope the parts
will fit into the same space as the original circuit.

I thought about an LMR64010 for the boost IC, but its
current-mode sense resistor is only 0.05 ohms, which
doesn't bode well for operation at currents like 20mA.
I had a hard time finding modest-current switchers in
sot-23 packages, until I discovered the AP3012 by BCD,
which has a 0.2-ohm sense resistor. Also, they only
cost 32-cents! That's 10x less than the $3.32 LT8330
I also considered. BCD was purchased by Diodes, Inc
about 7 years ago. and their parts are well stocked.


--
Thanks,
- Win
 
Winfield Hill wrote...
I needed to add a -3.3V to -6V supply to my RIS-796A
250A pulser, analog version. The PCB didn't have much
space, so I implemented a simple inverting switcher
with an LM2840 buck converter. See circuit and layout
on file U7=LM2840_sch+pcb.jpg in the DropBox folder:

https://www.dropbox.com/sh/rfqohfvubj9pddh/AAC9dSrWbeJmY9iq_rODYonIa?dl=1

There's also a sub-folder with relevant datasheets.

The circuit works OK, although it's a bit fragile, with
ICs failing instantly when I miss-probe. The L1 ME3215
inductor has 1R5 resistance, and can deliver over 100mA,
more than needed. But it has a horrible 200mV output
ripple, forcing me to add an LC output filter stage.

Checkout the other file U7=AP3012_sch.jpg It's my idea
for a simple SEPIC inverting converter, using a boost-
switcher and a coupled inductor. The inductor feeds a
continuous current into the output cap, for low ripple,
so maybe the LC filter can be eliminated. Now L1 is an
LPD3015, with 2R8 ohms, but it can still deliver 100mA.
The coupled inductor is only 3x3mm, and I hope the parts
will fit into the same space as the original circuit.

I thought about an LMR64010 for the boost IC, but its
current-mode sense resistor is only 0.05 ohms, which
doesn't bode well for operation at currents like 20mA.
I had a hard time finding modest-current switchers in
sot-23 packages, until I discovered the AP3012 by BCD,
which has a 0.2-ohm sense resistor. Also, they only
cost 32-cents! That's 10x less than the $3.32 LT8330
I also considered. BCD was purchased by Diodes, Inc
about 7 years ago. and their parts are well stocked.
Click to expand...

OK, after moving parts around, I actually ended up with
a Cuk rather than a SEPIC converter. See schematic:
U7=AP3012_neg-Vee_CUK.JPG Cuk converters naturally
create negative outputs, but one must add an opamp to
provide positive FB-pin feedback. My simplification
was to move the converter's ground to the -Vee output:
U7=AP3012_neg-Vee_CUK_1. It's slightly weird in that
the input inductor L1A gets 20% of its energy from the
output rail, but hey! Here's a alternate view:
U7=AP3012_neg-Vee_CUK_2.JPG Now to fit it onto the
PCB. There's a question of the converter's built-in
loop stability, with the 4th-order Cuk configuration,
but slowing with a larger inductor should handle that.



--
Thanks,
- Win
 
On 9 Mar 2020 08:25:12 -0700, Winfield Hill <winfieldhill@yahoo.com>
wrote:

was: A quiet SEPIC negative supply

Winfield Hill wrote...
Winfield Hill wrote...

I needed to add a -3.3V to -6V supply to my RIS-796A
250A pulser, analog version. The PCB didn't have much
space, so I implemented a simple inverting switcher
with an LM2840 buck converter. See circuit and layout
on file U7=LM2840_sch+pcb.jpg in the DropBox folder:

https://www.dropbox.com/sh/rfqohfvubj9pddh/AAC9dSrWbeJmY9iq_rODYonIa?dl=1

There's also a sub-folder with relevant datasheets.

The circuit works OK, although it's a bit fragile, with
ICs failing instantly when I miss-probe. The L1 ME3215
inductor has 1R5 resistance, and can deliver over 100mA,
more than needed. But it has a horrible 200mV output
ripple, forcing me to add an LC output filter stage.

Checkout the other file U7=AP3012_sch.jpg It's my idea
for a simple SEPIC inverting converter, using a boost-
switcher and a coupled inductor. The inductor feeds a
continuous current into the output cap, for low ripple,
so maybe the LC filter can be eliminated. Now L1 is an
LPD3015, with 2R8 ohms, but it can still deliver 100mA.
The coupled inductor is only 3x3mm, and I hope the parts
will fit into the same space as the original circuit.

I thought about an LMR64010 for the boost IC, but its
current-mode sense resistor is only 0.05 ohms, which
doesn't bode well for operation at currents like 20mA.
I had a hard time finding modest-current switchers in
sot-23 packages, until I discovered the AP3012 by BCD,
which has a 0.2-ohm sense resistor. Also, they only
cost 32-cents! That's 10x less than the $3.32 LT8330
I also considered. BCD was purchased by Diodes, Inc
about 7 years ago. and their parts are well stocked.

OK, after moving parts around, I actually ended up with
a Cuk rather than a SEPIC converter. See schematic:
U7=AP3012_neg-Vee_CUK.JPG Cuk converters naturally
create negative outputs, but one must add an opamp to
provide positive FB-pin feedback. My simplification
was to move the converter's ground to the -Vee output:
U7=AP3012_neg-Vee_CUK_1. It's slightly weird in that
the input inductor L1A gets 20% of its energy from the
output rail, but hey! Here's a alternate view:
U7=AP3012_neg-Vee_CUK_2.JPG Now to fit it onto the
PCB. There's a question of the converter's built-in
loop stability, with the 4th-order Cuk configuration,
but slowing with a larger inductor should handle that.

OK, I threw in the towel, the Cuk converter simply had
too many parts. Realizing my max -Vee load was 30mA,
and that -5V, more or less would be fine, a charge-pump
converter was an obvious choice. See how simple it is,
schematic and PCB layout in this file in the folder:
U7=TPS60403_charge-pump.JPG Some parts have always
been off to the left. I called the node -4.5 volts.
Click to expand...

The filtering is appropriate. Charge pumps can be noisy.

RCs make good filters, especially with a lot of uF of low-ESR ceramic
or polymer cap. RCs don't resonate!



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
 
was: A quiet SEPIC negative supply

Winfield Hill wrote...
Winfield Hill wrote...

I needed to add a -3.3V to -6V supply to my RIS-796A
250A pulser, analog version. The PCB didn't have much
space, so I implemented a simple inverting switcher
with an LM2840 buck converter. See circuit and layout
on file U7=LM2840_sch+pcb.jpg in the DropBox folder:

https://www.dropbox.com/sh/rfqohfvubj9pddh/AAC9dSrWbeJmY9iq_rODYonIa?dl=1

There's also a sub-folder with relevant datasheets.

The circuit works OK, although it's a bit fragile, with
ICs failing instantly when I miss-probe. The L1 ME3215
inductor has 1R5 resistance, and can deliver over 100mA,
more than needed. But it has a horrible 200mV output
ripple, forcing me to add an LC output filter stage.

Checkout the other file U7=AP3012_sch.jpg It's my idea
for a simple SEPIC inverting converter, using a boost-
switcher and a coupled inductor. The inductor feeds a
continuous current into the output cap, for low ripple,
so maybe the LC filter can be eliminated. Now L1 is an
LPD3015, with 2R8 ohms, but it can still deliver 100mA.
The coupled inductor is only 3x3mm, and I hope the parts
will fit into the same space as the original circuit.

I thought about an LMR64010 for the boost IC, but its
current-mode sense resistor is only 0.05 ohms, which
doesn't bode well for operation at currents like 20mA.
I had a hard time finding modest-current switchers in
sot-23 packages, until I discovered the AP3012 by BCD,
which has a 0.2-ohm sense resistor. Also, they only
cost 32-cents! That's 10x less than the $3.32 LT8330
I also considered. BCD was purchased by Diodes, Inc
about 7 years ago. and their parts are well stocked.

OK, after moving parts around, I actually ended up with
a Cuk rather than a SEPIC converter. See schematic:
U7=AP3012_neg-Vee_CUK.JPG Cuk converters naturally
create negative outputs, but one must add an opamp to
provide positive FB-pin feedback. My simplification
was to move the converter's ground to the -Vee output:
U7=AP3012_neg-Vee_CUK_1. It's slightly weird in that
the input inductor L1A gets 20% of its energy from the
output rail, but hey! Here's a alternate view:
U7=AP3012_neg-Vee_CUK_2.JPG Now to fit it onto the
PCB. There's a question of the converter's built-in
loop stability, with the 4th-order Cuk configuration,
but slowing with a larger inductor should handle that.
Click to expand...

OK, I threw in the towel, the Cuk converter simply had
too many parts. Realizing my max -Vee load was 30mA,
and that -5V, more or less would be fine, a charge-pump
converter was an obvious choice. See how simple it is,
schematic and PCB layout in this file in the folder:
U7=TPS60403_charge-pump.JPG Some parts have always
been off to the left. I called the node -4.5 volts.


--
Thanks,
- Win
 
On 9 Mar 2020 08:54:13 -0700, Winfield Hill <winfieldhill@yahoo.com>
wrote:

jlarkin@highlandsniptechnology.com wrote...

On 9 Mar 2020, Winfield Hill wrote:

was: A quiet SEPIC negative supply [ snip ]

OK, I threw in the towel, the Cuk converter simply had
too many parts. Realizing my max -Vee load was 30mA,
and that -5V, more or less would be fine, a charge-pump
converter was an obvious choice. See how simple it is,
schematic and PCB layout in this file in the folder:
U7=TPS60403_charge-pump.JPG Some parts have always
been off to the left. I called the node -4.5 volts.

The filtering is appropriate. Charge pumps can be noisy.

RCs make good filters, especially with a lot of uF of
low-ESR ceramic or polymer cap. RCs don't resonate!

Good point. But the 330uH and 10uF resonate frequency
is 2.77kHz, for an inductor reactance of 5.74 ohms.
But with the LB3218's DC resistance of 7 ohms, the
resonate Q is less than 1. The LC should attenuate
well at the 250kHz frequency. The 15-ohm Zout makes
a 450mV drop at 30mA, hence the -4.5V label. So this
is strictly a low-current solution. Guess I'll play
with the Cuk negative supply circuit some time later.
Click to expand...

I'm in the process of going to rev B of this (10-layer!) board.

https://www.dropbox.com/s/830e4dhvgj3crfp/P5A_PCB.jpg?raw=1

Rev A works, but some circuits bounce other circuits on shared power
rails. LC filtering fixes switcher noise problems but can make a rail
soft for transient loads, which you may not have.

The new plan is to make the switcher voltages a little high and
scatter LDOs locally as needed. I'm enthusiastic about using some
LTM8078s in the main supply, which are claimed to be inherently quiet.
But still no Spice model!

8078 should do positive to negative, maybe positive to two negative
rails, maybe even positive to + and - rails.








--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
 
jlarkin@highlandsniptechnology.com wrote...
On 9 Mar 2020, Winfield Hill wrote:

was: A quiet SEPIC negative supply [ snip ]

OK, I threw in the towel, the Cuk converter simply had
too many parts. Realizing my max -Vee load was 30mA,
and that -5V, more or less would be fine, a charge-pump
converter was an obvious choice. See how simple it is,
schematic and PCB layout in this file in the folder:
U7=TPS60403_charge-pump.JPG Some parts have always
been off to the left. I called the node -4.5 volts.

The filtering is appropriate. Charge pumps can be noisy.

RCs make good filters, especially with a lot of uF of
low-ESR ceramic or polymer cap. RCs don't resonate!
Click to expand...

Good point. But the 330uH and 10uF resonate frequency
is 2.77kHz, for an inductor reactance of 5.74 ohms.
But with the LB3218's DC resistance of 7 ohms, the
resonate Q is less than 1. The LC should attenuate
well at the 250kHz frequency. The 15-ohm Zout makes
a 450mV drop at 30mA, hence the -4.5V label. So this
is strictly a low-current solution. Guess I'll play
with the Cuk negative supply circuit some time later.


--
Thanks,
- Win
 
On Monday, March 9, 2020 at 9:26:05 AM UTC-7, jla...@highlandsniptechnology.com wrote:
8078 should do positive to negative, maybe positive to two negative
rails, maybe even positive to + and - rails.
Click to expand...

Do you need 40V in? If you could live with 20 max, an LTM8049 will do that.

-- john, KE5FX
 
On Mon, 9 Mar 2020 15:37:41 -0700 (PDT), "John Miles, KE5FX"
<jmiles@gmail.com> wrote:

On Monday, March 9, 2020 at 9:26:05 AM UTC-7, jla...@highlandsniptechnology.com wrote:
8078 should do positive to negative, maybe positive to two negative
rails, maybe even positive to + and - rails.

Do you need 40V in? If you could live with 20 max, an LTM8049 will do that.

-- john, KE5FX
Click to expand...

The box will run off a 24 volt wart. The max negative voltage I need
to make is -7.

The 8078 will work, and probably make me two negative voltages from
+24, like -2.5 and -7 maybe.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
On 9 Mar 2020 08:25:12 -0700, Winfield Hill <winfieldhill@yahoo.com>
wrote:

was: A quiet SEPIC negative supply

Winfield Hill wrote...
Winfield Hill wrote...

I needed to add a -3.3V to -6V supply to my RIS-796A
250A pulser, analog version. The PCB didn't have much
space, so I implemented a simple inverting switcher
with an LM2840 buck converter. See circuit and layout
on file U7=LM2840_sch+pcb.jpg in the DropBox folder:

https://www.dropbox.com/sh/rfqohfvubj9pddh/AAC9dSrWbeJmY9iq_rODYonIa?dl=1

There's also a sub-folder with relevant datasheets.

The circuit works OK, although it's a bit fragile, with
ICs failing instantly when I miss-probe. The L1 ME3215
inductor has 1R5 resistance, and can deliver over 100mA,
more than needed. But it has a horrible 200mV output
ripple, forcing me to add an LC output filter stage.

Checkout the other file U7=AP3012_sch.jpg It's my idea
for a simple SEPIC inverting converter, using a boost-
switcher and a coupled inductor. The inductor feeds a
continuous current into the output cap, for low ripple,
so maybe the LC filter can be eliminated. Now L1 is an
LPD3015, with 2R8 ohms, but it can still deliver 100mA.
The coupled inductor is only 3x3mm, and I hope the parts
will fit into the same space as the original circuit.

I thought about an LMR64010 for the boost IC, but its
current-mode sense resistor is only 0.05 ohms, which
doesn't bode well for operation at currents like 20mA.
I had a hard time finding modest-current switchers in
sot-23 packages, until I discovered the AP3012 by BCD,
which has a 0.2-ohm sense resistor. Also, they only
cost 32-cents! That's 10x less than the $3.32 LT8330
I also considered. BCD was purchased by Diodes, Inc
about 7 years ago. and their parts are well stocked.

OK, after moving parts around, I actually ended up with
a Cuk rather than a SEPIC converter. See schematic:
U7=AP3012_neg-Vee_CUK.JPG Cuk converters naturally
create negative outputs, but one must add an opamp to
provide positive FB-pin feedback. My simplification
was to move the converter's ground to the -Vee output:
U7=AP3012_neg-Vee_CUK_1. It's slightly weird in that
the input inductor L1A gets 20% of its energy from the
output rail, but hey! Here's a alternate view:
U7=AP3012_neg-Vee_CUK_2.JPG Now to fit it onto the
PCB. There's a question of the converter's built-in
loop stability, with the 4th-order Cuk configuration,
but slowing with a larger inductor should handle that.

OK, I threw in the towel, the Cuk converter simply had
too many parts. Realizing my max -Vee load was 30mA,
and that -5V, more or less would be fine, a charge-pump
converter was an obvious choice. See how simple it is,
schematic and PCB layout in this file in the folder:
U7=TPS60403_charge-pump.JPG Some parts have always
been off to the left. I called the node -4.5 volts.
Click to expand...

Maybe not perfect, but I was supprised how clean these were.

Cheers


Make
America
Germless
Again
 
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