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buck converter for negative voltages

Started byWinfield Hill <hill@rowland.harvard.edu>
First post2017-02-01 12:57 -0800
Last post2017-02-01 15:04 -0800
Articles 14 — 5 participants

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  buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-01 12:57 -0800
    Re: buck converter for negative voltages John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-02-01 13:35 -0800
      Re: buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-01 14:02 -0800
        Re: buck converter for negative voltages Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-02-01 14:16 -0800
          Re: buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-01 14:27 -0800
            Re: buck converter for negative voltages Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-02-01 14:39 -0800
              Re: buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-02 11:33 -0800
                Re: buck converter for negative voltages Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-02-03 09:44 -0500
        Re: buck converter for negative voltages John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-02-01 14:38 -0800
          Re: buck converter for negative voltages Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-02-01 14:43 -0800
            Re: buck converter for negative voltages John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-02-01 15:35 -0800
              Re: buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-01 18:25 -0800
    Re: buck converter for negative voltages Winfield Hill <hill@rowland.harvard.edu> - 2017-02-01 13:36 -0800
    Re: buck converter for negative voltages Wanderer <wanderer@dialup4less.com> - 2017-02-01 15:04 -0800

#445658 — buck converter for negative voltages

FromWinfield Hill <hill@rowland.harvard.edu>
Date2017-02-01 12:57 -0800
Subjectbuck converter for negative voltages
Message-ID<o6ti3m0312e@drn.newsguy.com>
Suggestions for buck converter ICs for negative voltages? 
Controller ICs (external MOSFETs) would be nice.


-- 
 Thanks,
    - Win

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#445665

FromJohn Larkin <jjlarkinxyxy@highlandtechnology.com>
Date2017-02-01 13:35 -0800
Message-ID<3pk49ct21gjuhka45gqe30g5c7s7f48upg@4ax.com>
In reply to#445658
On 1 Feb 2017 12:57:26 -0800, Winfield Hill <hill@rowland.harvard.edu>
wrote:

>Suggestions for buck converter ICs for negative voltages? 
>Controller ICs (external MOSFETs) would be nice.

I can't recall ever seeing a real negative-to-negative switcher!
People sometimes hack a positive controller.

Hey, here's one:

http://www.linear.com/product/LT8709



-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

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

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#445667

FromWinfield Hill <hill@rowland.harvard.edu>
Date2017-02-01 14:02 -0800
Message-ID<o6tlu5051p@drn.newsguy.com>
In reply to#445665
John Larkin wrote...
>
> Winfield Hill wrote:
>
>> Suggestions for buck converter ICs for negative voltages? 
>> Controller ICs (external MOSFETs) would be nice.
>
> I can't recall ever seeing a real negative-to-negative
> switcher!  People sometimes hack a positive controller.
>
> Hey, here's one:
> http://www.linear.com/product/LT8709

 Yep, good find.  Whew, that IC is a bit complicated.
 LTC also suggests the positive boost-converter hack,
 in design note DN1022.


-- 
 Thanks,
    - Win

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#445669

FromLasse Langwadt Christensen <langwadt@fonz.dk>
Date2017-02-01 14:16 -0800
Message-ID<13132ace-e417-4daf-ba61-243365f1c77e@googlegroups.com>
In reply to#445667
Den onsdag den 1. februar 2017 kl. 23.03.00 UTC+1 skrev Winfield Hill:
> John Larkin wrote...
> >
> > Winfield Hill wrote:
> >
> >> Suggestions for buck converter ICs for negative voltages? 
> >> Controller ICs (external MOSFETs) would be nice.
> >
> > I can't recall ever seeing a real negative-to-negative
> > switcher!  People sometimes hack a positive controller.
> >
> > Hey, here's one:
> > http://www.linear.com/product/LT8709
> 
>  Yep, good find.  Whew, that IC is a bit complicated.
>  LTC also suggests the positive boost-converter hack,
>  in design note DN1022.
> 

if you can figure out a way to flip the feedback or live with the output
being relative to input, wouldn't any synchronous buck work?

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#445671

FromWinfield Hill <hill@rowland.harvard.edu>
Date2017-02-01 14:27 -0800
Message-ID<o6tncg0784@drn.newsguy.com>
In reply to#445669
Lasse Langwadt Christensen wrote...
>
> Den onsdag den 1. februar 2017 kl. skrev Winfield Hill:
>> John Larkin wrote...
>>>
>>> Winfield Hill wrote:
>>>
>>>> Suggestions for buck converter ICs for negative voltages? 
>>>> Controller ICs (external MOSFETs) would be nice.
>>>
>>> I can't recall ever seeing a real negative-to-negative
>>> switcher!  People sometimes hack a positive controller.
>>
>>  LTC also suggests the positive boost-converter hack,
>>  in design note DN1022.
>
> if you can figure out a way to flip the feedback or live with the
> output being relative to input, wouldn't any synchronous buck work? 

 The feedback is inverted in a *boost* converter, one
 just needs to translate it down to the negative rail.


-- 
 Thanks,
    - Win

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#445673

FromLasse Langwadt Christensen <langwadt@fonz.dk>
Date2017-02-01 14:39 -0800
Message-ID<43f62388-eb31-41b0-89e4-72f861ac6411@googlegroups.com>
In reply to#445671
Den onsdag den 1. februar 2017 kl. 23.27.44 UTC+1 skrev Winfield Hill:
> Lasse Langwadt Christensen wrote...
> >
> > Den onsdag den 1. februar 2017 kl. skrev Winfield Hill:
> >> John Larkin wrote...
> >>>
> >>> Winfield Hill wrote:
> >>>
> >>>> Suggestions for buck converter ICs for negative voltages? 
> >>>> Controller ICs (external MOSFETs) would be nice.
> >>>
> >>> I can't recall ever seeing a real negative-to-negative
> >>> switcher!  People sometimes hack a positive controller.
> >>
> >>  LTC also suggests the positive boost-converter hack,
> >>  in design note DN1022.
> >
> > if you can figure out a way to flip the feedback or live with the
> > output being relative to input, wouldn't any synchronous buck work? 
> 
>  The feedback is inverted in a *boost* converter, one
>  just needs to translate it down to the negative rail.
> 

afaict it is the same issue, you need to make the feedback relative to ground
instead of to the negative rail

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#445812

FromWinfield Hill <hill@rowland.harvard.edu>
Date2017-02-02 11:33 -0800
Message-ID<o701hv014n9@drn.newsguy.com>
In reply to#445673
Lasse Langwadt Christensen wrote...
>
>Den onsdag den 1. februar 2017 kl. 23.27.44 UTC+1 skrev Winfield Hill:
>> Lasse Langwadt Christensen wrote...
>>>
>>> Den onsdag den 1. februar 2017 kl. skrev Winfield Hill:
>>>> John Larkin wrote...
>>>>>
>>>>> Winfield Hill wrote:
>>>>>
>>>>>> Suggestions for buck converter ICs for negative voltages? 
>>>>>> Controller ICs (external MOSFETs) would be nice.
>>>>>
>>>>> I can't recall ever seeing a real negative-to-negative
>>>>> switcher!  People sometimes hack a positive controller.
>>>>
>>>>  LTC also suggests the positive boost-converter hack,
>>>>  in design note DN1022.
>>>
>>> if you can figure out a way to flip the feedback or live with the
>>> output being relative to input, wouldn't any synchronous buck work? 
>> 
>>  The feedback is inverted in a *boost* converter, one
>>  just needs to translate it down to the negative rail.
>
> afaict it is the same issue, you need to make the feedback
> relative to ground instead of to the negative rail

 Actually, besides level-shifting the feedback to the IC's
 FB pin, which is referenced to -Vin, it's necessary to
 invert the sign of the feedback.  That becomes clear when
 comparing buck-converter duty-cycle formula Vout/Vin = D,
 with a boost converter, Vout/Vin = 1/(1-D).  Also, it's
 obvious that when -Vout increases (more negative), FB
 must increase (more positive).

 The inversion occurs in the level-shift transistor, Q1.

.  gnd ----+---------+--------+--------,
.       ___|____     |       _|_       |
.      |        |   _|_      ---      R1
.      | boost  |   /_\       |        |
.      |        |    |        |        |
.      |     SW |----+-#####--+---+------- -Vout
.      |        |                 B    |
.      |     FB |----+----------C   E--'
.      |________|    |           Q1 pnp
.          |        R2            
.          |         |
.  -Vin ---+---------+-----


-- 
 Thanks,
    - Win

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#445935

FromPhil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Date2017-02-03 09:44 -0500
Message-ID<o724r7$e2q$1@dont-email.me>
In reply to#445812
On 02/02/2017 02:33 PM, Winfield Hill wrote:
> Lasse Langwadt Christensen wrote...
>>
>> Den onsdag den 1. februar 2017 kl. 23.27.44 UTC+1 skrev Winfield Hill:
>>> Lasse Langwadt Christensen wrote...
>>>>
>>>> Den onsdag den 1. februar 2017 kl. skrev Winfield Hill:
>>>>> John Larkin wrote...
>>>>>>
>>>>>> Winfield Hill wrote:
>>>>>>
>>>>>>> Suggestions for buck converter ICs for negative voltages?
>>>>>>> Controller ICs (external MOSFETs) would be nice.
>>>>>>
>>>>>> I can't recall ever seeing a real negative-to-negative
>>>>>> switcher!  People sometimes hack a positive controller.
>>>>>
>>>>>  LTC also suggests the positive boost-converter hack,
>>>>>  in design note DN1022.
>>>>
>>>> if you can figure out a way to flip the feedback or live with the
>>>> output being relative to input, wouldn't any synchronous buck work?
>>>
>>>  The feedback is inverted in a *boost* converter, one
>>>  just needs to translate it down to the negative rail.
>>
>> afaict it is the same issue, you need to make the feedback
>> relative to ground instead of to the negative rail
>
>  Actually, besides level-shifting the feedback to the IC's
>  FB pin, which is referenced to -Vin, it's necessary to
>  invert the sign of the feedback.  That becomes clear when
>  comparing buck-converter duty-cycle formula Vout/Vin = D,
>  with a boost converter, Vout/Vin = 1/(1-D).  Also, it's
>  obvious that when -Vout increases (more negative), FB
>  must increase (more positive).
>
>  The inversion occurs in the level-shift transistor, Q1.
>
> .  gnd ----+---------+--------+--------,
> .       ___|____     |       _|_       |
> .      |        |   _|_      ---      R1
> .      | boost  |   /_\       |        |
> .      |        |    |        |        |
> .      |     SW |----+-#####--+---+------- -Vout
> .      |        |                 B    |
> .      |     FB |----+----------C   E--'
> .      |________|    |           Q1 pnp
> .          |        R2
> .          |         |
> .  -Vin ---+---------+-----
>
>

I sometimes put cap multipliers inside the feedback loop of a 
positive-supply switcher, with an RC highpass to make the loop stable 
(the way you do with a slow buffer or a big C load).  (It helps the load 
regulation without wasting power on maintaining worst-case headroom all 
the time.) You can't do that when using a buck in inverting mode, for 
exactly this reason.

There are shunt references that work upside down, e.g. the LM385 and 
LM4041-ADJ, but no switchers that I know of.

Using one of those references instead of your PNP (with an RC highpass 
as above) would get rid of the V_BE error for fairly cheap.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

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#445672

FromJohn Larkin <jjlarkinxyxy@highlandtechnology.com>
Date2017-02-01 14:38 -0800
Message-ID<1fo49c1hih3gn6t2smovhpm56k9di3ao8k@4ax.com>
In reply to#445667
On 1 Feb 2017 14:02:45 -0800, Winfield Hill <hill@rowland.harvard.edu>
wrote:

>John Larkin wrote...
>>
>> Winfield Hill wrote:
>>
>>> Suggestions for buck converter ICs for negative voltages? 
>>> Controller ICs (external MOSFETs) would be nice.
>>
>> I can't recall ever seeing a real negative-to-negative
>> switcher!  People sometimes hack a positive controller.
>>
>> Hey, here's one:
>> http://www.linear.com/product/LT8709
>
> Yep, good find.  Whew, that IC is a bit complicated.
> LTC also suggests the positive boost-converter hack,
> in design note DN1022.

To make negative voltages at a watt or two, I often buy an isolated
potted dc/dc converter.

They are also handy for making, say, +-5 from a +12 or +24 wart.

For + to -, a Cuk is nice.

What sorts of i/o voltages and power do you need?


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

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

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#445674

FromLasse Langwadt Christensen <langwadt@fonz.dk>
Date2017-02-01 14:43 -0800
Message-ID<5fd00670-12ef-403a-b2ba-53fa4659dfbc@googlegroups.com>
In reply to#445672
Den onsdag den 1. februar 2017 kl. 23.38.27 UTC+1 skrev John Larkin:
> On 1 Feb 2017 14:02:45 -0800, Winfield Hill <hill@rowland.harvard.edu>
> wrote:
> 
> >John Larkin wrote...
> >>
> >> Winfield Hill wrote:
> >>
> >>> Suggestions for buck converter ICs for negative voltages? 
> >>> Controller ICs (external MOSFETs) would be nice.
> >>
> >> I can't recall ever seeing a real negative-to-negative
> >> switcher!  People sometimes hack a positive controller.
> >>
> >> Hey, here's one:
> >> http://www.linear.com/product/LT8709
> >
> > Yep, good find.  Whew, that IC is a bit complicated.
> > LTC also suggests the positive boost-converter hack,
> > in design note DN1022.
> 
> To make negative voltages at a watt or two, I often buy an isolated
> potted dc/dc converter.
> 
> They are also handy for making, say, +-5 from a +12 or +24 wart.
> 

if you have +5V this look interesting: http://www.ti.com/product/LM27762

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#445680

FromJohn Larkin <jjlarkinxyxy@highlandtechnology.com>
Date2017-02-01 15:35 -0800
Message-ID<h0s49c14s085ue2fv4fgm26161mj1f188b@4ax.com>
In reply to#445674
On Wed, 1 Feb 2017 14:43:35 -0800 (PST), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>Den onsdag den 1. februar 2017 kl. 23.38.27 UTC+1 skrev John Larkin:
>> On 1 Feb 2017 14:02:45 -0800, Winfield Hill <hill@rowland.harvard.edu>
>> wrote:
>> 
>> >John Larkin wrote...
>> >>
>> >> Winfield Hill wrote:
>> >>
>> >>> Suggestions for buck converter ICs for negative voltages? 
>> >>> Controller ICs (external MOSFETs) would be nice.
>> >>
>> >> I can't recall ever seeing a real negative-to-negative
>> >> switcher!  People sometimes hack a positive controller.
>> >>
>> >> Hey, here's one:
>> >> http://www.linear.com/product/LT8709
>> >
>> > Yep, good find.  Whew, that IC is a bit complicated.
>> > LTC also suggests the positive boost-converter hack,
>> > in design note DN1022.
>> 
>> To make negative voltages at a watt or two, I often buy an isolated
>> potted dc/dc converter.
>> 
>> They are also handy for making, say, +-5 from a +12 or +24 wart.
>> 
>
>if you have +5V this look interesting: http://www.ti.com/product/LM27762
>

LT3580 is a fun chip. It's a flyback that will accept either a
negative or a positive feedback voltage.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

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

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#445704

FromWinfield Hill <hill@rowland.harvard.edu>
Date2017-02-01 18:25 -0800
Message-ID<o6u5bk012co@drn.newsguy.com>
In reply to#445680
John Larkin wrote...
>
> LT3580 is a fun chip.  It's a flyback that will accept
> either a negative or a positive feedback voltage.

 Yes it will, nice.  Too bad it's in such a small package.


-- 
 Thanks,
    - Win

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#445666

FromWinfield Hill <hill@rowland.harvard.edu>
Date2017-02-01 13:36 -0800
Message-ID<o6tkc302l1@drn.newsguy.com>
In reply to#445658
Winfield Hill wrote...
>
> Suggestions for buck converter ICs for negative voltages? 
> Controller ICs (external MOSFETs) would be nice.

 Ajmal Godil suggests using a positive boost converter,
 with a level-shifting transistor feedback trick.  Cute.
 http://electronicdesign.com/boards/turn-positive-buckboost-circuits-negative

 I think Ajmal is at Broadcom now.


-- 
 Thanks,
    - Win

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#445678

FromWanderer <wanderer@dialup4less.com>
Date2017-02-01 15:04 -0800
Message-ID<f3907e5d-e977-4683-af5e-9f7129ec7c78@googlegroups.com>
In reply to#445658
On Wednesday, February 1, 2017 at 3:57:45 PM UTC-5, Winfield Hill wrote:
> Suggestions for buck converter ICs for negative voltages? 
> Controller ICs (external MOSFETs) would be nice.
> 
> 
> -- 
>  Thanks,
>     - Win

I recall doing push-pull once. I used this Jim Williams AN-70 app note.

http://www.linear.com/docs/4159

The hard part was sourcing the transformer.

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