Guys - I need to clear some things up. Here goes. Get a coffee and sit down..
Firstly a current mode controller regulates output CURRENT, not voltage (ironically). Its control algorithm is designed to regulate output CURRENT, that is turned into a voltage by the load (say, a resistor). It measures this output voltage and creates an error signal (using the error amplifier) and the output of this error amplifier modulates the CURRENT (not output voltage). Think of the output voltage as being a mere consequence of the control mechanism. The chip is changing its output current, not output voltage. Yes it is measuring the output voltage, but it is changing the output current (to change the output voltage), not the output voltage itself. The output voltage is just a byproduct.
There will be a difference in error amplifier characteristics due to offset voltages and reference tolerances. This is what accounts for the non perfect sharing of current. Because the current mode controller outputs CURRENT (I think I mentioned that before), if one phase has a lower reference than the other phase, it will provide more current than the other phase, but it wont provide all the current (while the other phase provides none). If the reference voltage of one phase is 2% low, this phase will produce, roughly, 2% more current.
So with this in mind:
"Current output regulated converters" can be paralleled and share current as per there regulation...but "current mode" converters ( which are setup to regulate an output voltage) do not inherently share current with each other when paralleled.......they can be influenced to do so as in the explanations by Simon above, ...as in Simon's tying together of the ITH pins........but current mode converters, per se, (which regulate an output voltage) do not "inherently" share output current if put in parallel.
Current mode converters DO share current, since they are outputting a current, not a voltage, as explained above. The tying together of the ITH pins ensures the controllers have the same error voltage, so hence output the same current.
Point is that since a CM controller limits PW when it see's the current get to a set threshold level (the control signal), every converter was putting out the same current.
Yes - kind of... the high side FET drive terminates when the current limit is reached. If you go through the maths, the duty cycle of the PWM is determined by the input
utput voltage ratio and not the controller. The controller settles down to this duty cycle because of the currents in the external inductor, not because it explicitely wants to.
Maybe I should have said they share current inherently if they are using the same control level and are tied together in the same loop. I didn't mean you can pick two random power supplies and strap the outputs together.
This is correct. You need 2 controllers that have the same transfer function from the FB pin to the current sense trip threshold. If you use 2 different controllers, you can adjust the current sense resistor to ensure they both have the same transfer function, but obviously it is easier if you use 2 controller and design both controllers with the same external components.
Thanks, I appreciate why LTC3774 has two error amplifiers, but what I don’t see is, why, when it is only regulating one output voltage, does it not disable one of the error amplifiers? After all, if you want to regulate one output, then you only need one error amplifier. The LTspice simulation called “Paralleled SMPS’s_1” in the first post of this thread shows this.
This is because each converter is controlling its output CURRENT, not voltage. An increased output voltage is just a byproduct of an increased output current. Thus they do share current very well.
Also, the LTC3774 claims to be able to control up to 12 sync bucks all feeding into one output load. What is it about the LTC3774 that makes it so suitable for this multi buck paralleling when other similar linear.com chips are not having such a glorious feature boast?
ANY current mode controller can be paralleled in as many phases as you want. They only difference with the LTC3774 is that you have a phase shifted output clock that means each controller turns on its high side FET at a different point in time so you don't end up with huge gulps of current on the input. With a 12 phase dc/dc converter, you can have 13 phases, it is just that phase 13 will be in phase with phase 1. You can design a 12 phase circuit using 6 dual phase controllers, but if you don't have the phase shifting feature of the LTC3774, the odd phases will be switching on together and so will the even phases. The LTC3774 shifts the drive to the high side FET.
The basic current mode design REQUIRES two error amplifiers because they are measuring two different things. One is output voltage and the other is inductor current. The output of these error amps feeds into a summing node and whichever error amp gets to it's threshold first cuts off the pulse which is to say turns off the FET switch
No - there is only one error amplifier and this controls the trip threshold of the current sense circuit to control the output current. Sometimes this is explained as having an outer feedback loop (to measure the output voltage) and an inner feedback loop (to measure the inductor current), but there is only one error amplifier. When the output voltage reaches regulation, the ITH pins drops, throttling back the inductor current (to zero), thus keeping the output voltage in regulation
No, you would need one "master" controller measuring the output voltage and sending a control voltage level to all the subordinate converters
No - in a true multiphase buck converter, they are all masters. The LT3791 circuit below is different because this is a buck boost controller. You need a master slave configuration to ensure current is shared when Vin = Vout
The attached from linear.com shows a very differently organised way of paralleling SMPS's into the same single output, using LTC3791...
Where did you get this from? I was in that presentation and this info is confidential (!)
I hope the above helps
Simon