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# How do you implement offset voltage in SMPS Load share circuitry?

#### cupoftea

Hi,
The two Bucks shown share current due to the sharing circuitry. Each needs the 0.1V offset voltage to be used (shown in dotted rectangle). What would you think is the lowest component count way of producing this offset voltage? Was thinking “current source to put current through resistor” but it’s a lot of components. Do you have a better way?

AYK, the offset voltage is to stop the set output voltage rising to maximum when the two bucks just happen to share current perfectly anyway.
(LTspice and jeg attached)

#### Attachments

• 2 BUCKs pllel_with offset.zip
4.3 KB · Views: 20
• 2 bucks in pllel.jpg
160 KB · Views: 27

#### cupoftea

Now as above, but this time with three Bucks in parallel...so...

Hi,
The attached shows a discrete version of the UCC29002 Load share controller. (LTspice and jpeg)

There’s a “share bus”, which simply shows a voltage representing the current output of the highest current stage. All stages compare this with their own current level. There is a series 50mV offset voltage added to each stage’s (current measurement) voltage value, so the one with highest current definitely does not ever try and reduce its own current. This does mean all the stages can in fact never have exactly equal current, but it gets close enough.

Anyway, do you know how to best do the circuitry for the 50mV series offset?....as shown it’s a bit much.

#### Attachments

• 3 bucks _offset cct.jpg
153.3 KB · Views: 19
• 3 bucks _offset sources.jpg
142.2 KB · Views: 21
• 3 BUCKs pllel_with offset source.zip
6.3 KB · Views: 17
• 3 BUCKs pllel_with offset cct.zip
7.4 KB · Views: 17

#### dick_freebird

Is the pin impedance / current such that a simple resistor

#### cupoftea

Thanks, you mean the offset produced by say the "middle" resistor of a chain of resistors spanning the 5V rail?
I also was considering that, but I think this would give a too_variable reference over the load range of the SMPSs.