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Need simple Buckboost current source controller IC for 2A ...nothing exists?

treez

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Hi,
We are seeking a Buckboost SMPS controller IC which allows a 2A current source into a 18-30V load, from an 18-30V input.
(ie as attached)
Preferably with the high side drive on the chip.
Do you know if there is anything offtheshelf?

LT8705 does the job but is very expensive and way over-spec'd for the simple task...

Surely this is a gap in the market for the semico's?

LTspice sim and PDF schem attached)
 

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  • Buckboost current source.pdf
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  • Buckboost battery charger.zip
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mtwieg

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Any reason you can't use a flyback?

Does the load need to be ground referenced?
 

treez

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Yes it needs to be gnd ref'd.
Prefer not flyback as just want simple offtheshelf inductors
 

BradtheRad

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For contrast and comparison with commercial IC...
Basic buck-boost controlled by op amp via hysteresis. Current is sensed as voltage on a low-ohm resistor (or else a few inches of wire).

18v supply. 30v battery (positive terminal is ground referenced). Average charge rate about 2 A. (I made a battery charger because your schematic appears to have a battery as the load.)

Uses high-side switch. In reality this can be a PNP biased by any workable arrangement.

buck-boost hysteresis-driven 18v charges 30v bat'y.png
 

treez

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Thanks, well, we just want simple and cheap, and dont want two inductors and a SEPIC cap, and the RC snubber across the SEPIC cap.
We also dont want an offtheshelf coupled inductor for SEPIC, as they have few second sources.
Two small FETs and a single inductor seems cheaper.
 

Easy peasy

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fair enough - complementary drive from a single IC is hard to find - we have done similar with a 555 for the 1st square wave and then another 555 to invert to get the complement to drive the other switch - you may need extra cktry to turn both switches off.
--- Updated ---

p.s. at low power a good schottky is 0.3 volts so you may be able to compromise there ?
--- Updated ---

- the classic buck-boost ( the original flyback ) gives a neg o/p per the dwg by Brad above,

I assume you want a pos o/p and a contiguous gnd line? the cacaded buck-boost - per your post #1 certainly gives that

as would a small flyback, - further, a sepic running at 400kHz would have small separate chokes ( for best operation ) a small linking cap, and can be run off a cmos 555 ( 3 x 3 mm ) - it is also inherently protected against switch failure ...
--- Updated ---

p.s. you don't need an RC snubber across the sepic cap - we have designed and built many, as long as the operating freq is well above the LC freq of the L's and linking C, and you damp the o/p caps ( electro ) - you'll be fine. Also good input cap(s) helps.
 
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mtwieg

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Thanks, well, we just want simple and cheap, and dont want two inductors and a SEPIC cap, and the RC snubber across the SEPIC cap.
We also dont want an offtheshelf coupled inductor for SEPIC, as they have few second sources.
Two small FETs and a single inductor seems cheaper.
Seems like you're only considering the complexity in terms of component count on the PCB. Four switch buck-boost looks nice from that perspective, but it requires a more sophisticated controller. For example it requires a much more complicated control algorithm in order to handle smooth transitions between buck/boost. Also requires charge pumps for high side drivers (or a controller which automatically refreshes bootstrap capacitors, again more complexity). No surprise that only LT offers a solution.
 

treez

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Thanks, page 20 of this is the nearest to what we are looking for...

...but the high current BJTs just dont seem to be available these days....all the high current ones are Darlingtons, and we definetely dont want darlington for page 20's external high side PNP Power transistor
 

FvM

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...but the high current BJTs just dont seem to be available these days....
No idea how you come to this conclusion. There's still a huge offer, just check distributors search machine, e.g. at Digikey.
 
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Easy peasy

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Also for the ckt on p.20 you can change the pnp to a P mosfet and similarly for the other - with simple turn off ckts for the fets - this should raise the efficiency and allow 100kHz operation is you so wish ...
--- Updated ---

For example:
1618870302622.png
--- Updated ---

And the P - fet drive:
1618871087860.png
--- Updated ---

The zener D2 is needed to keep the max drive to the gate within bounds.
 
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dick_freebird

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Are you convinced that there are -no- buck/boost chips
which pin out the error amp output / current setpoint node
in a current mode controller? Because if you have that point
available then you've got your voltage controlled current
source (small matters of accuracy, stability, repeatability
unit-unit aside). You put the error amp into noninverting
buffer mode (FB=EAOUT) and over-drive the VRef pin
(yeah, you need that accessible too) to set current.

I know when I was designing POL DC-DCs this was a thing
we did deliberately, to provide flexibility to the users.
 
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Easy peasy

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For a current source converter a Zeta ( inverse Sepic ) with its output choke - forms a very good current source ...

need a high side switch ...
 

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