Author Topic: What is this circuit doing?  (Read 685 times)

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Offline elimenohpeeTopic starter

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What is this circuit doing?
« on: January 03, 2020, 09:30:16 pm »
I developing an application where I need to current share a couple of external, stand-alone DC switch mode power supplies.  I'm planning to use TI UC2907 as a load share controller.  There aren't any application notes specifically on this chip, but there is a note on UC3907 with an approach that I want to use.  It looks like it ties into the sense feedback lines to drive/share the current between the supplies. 

In the application note it shows a rough schematic of the approach (see the attached image), but I'm a little confused what the highlighted portion is doing.  It looks like it's clamping the emitter of the transistor at the chip's reference voltage but I don't really understand why.  Does anyone have any idea what this portion of the circuit is doing?
 

Offline Mr Evil

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Re: What is this circuit doing?
« Reply #1 on: January 03, 2020, 11:15:16 pm »
From looking at the block diagram in the datasheet, pin 8 is a 1.75V reference voltage, so it will pull up the emitter of the transistor by a few hundred millivolts, preventing it from pulling down on the power supply's sense line until the output from the adjust amplifier rises above a threshold. Without spending too much time trying to work it out, it seems that this is because the output from the adjust amplifier is >0V under quiescent conditions, so R3 prevents it from constantly trying to adjust the power supply's output even when there is no current output from it.


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