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LM5145 Synchronous Buck DC-DC Controller current and voltage, FB control

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carpenter

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i need to have the option of MCU to control the power supply with any Buck DC-DC for example **broken link removed** output 0-20V 0-20A, 220 kHz.
This Controller have on FB 800mV Error amplifer and my idea is any as this
R2 R3 is standard voltage divider make from 20V 800mV for FB , FB LM5145 is center resistor divider R2/R3.
R1 and DAC 0-3.3 is used for voltage control. If DAC =0V is output voltage 20V, if is 3.3V is output voltage 0V.
Comparator U1 D1 and R4 (not necessary) us used for Current control
Comparator compare voltage from DAC with voltage from current shunt if is on output overcurrent set over D1 on FB voltage over 800mV .

it will work?
Which bandwith must have a differential amplifier at the shunt?
Otherwise I can use INA225

Full digital current control loop.
Use only voltage control and for control current use digital loop in MCU compare voltage measuren on shunt monitor with set current, if is ovecurrent reduce voltage over DAC on R1 .
INA225 have witch 100V/V bandwith 70kHz, ADC/DAC and MCU procesing I estimate below 5us i.e. 200kHz.
it's a big nonsense?



CC CV.png
 

Hi,

There are already discussions about "manipulating the FB" to adjust output voltage.
You can find all the necessary calculations there.

Just focus on voltage regulation:
Your circuit is correct: DAC, R1, R2, R3.
It seems your resistor values are correct, too.
But I recommend to make R1 value a bit smaller, because - even a rail to rail output - doesn't really go to the rails. There is always a dropout voltage. The datasheets show how much dropout voltage you can expect.
Use a standard resistor - maybe 10k - and do the fine tuning with software: A simple ADD and MULTIPLY on the DAC value will optimize the range.

Current control:
Here I doubt that your idea will work satisfactory.
A comparator switches ON or OFF.
Imagine: as long as there is no overcurrent condition, everything will work fine.
But on overcurrent condition
* the FB pin is pulled high...
* the switcher will stop to drive the output
* the output voltage drops
* the current drops
* the overcurrent condition disappears
* FB pin becomes normal
* the switcher will start to operate normally
* the output voltage will rise again
* the current will rise again
*... until the overcurrent condition will happen again
....then all starts from the beginning.
ON, OFF, with unpredictable timing, with unpredictable output voltage and current ripple...

Additionally:
* some switchers only allow a very narrow FB pin (over)voltage --> read datasheet
* you "switch" the FB pin voltage some how. This is not a usual operation of the whole feedback unit. Maybe this causes overdrive or instability within the FB circuit.

Therefore I recommend:
* either use a "regulating", analog overcurrent circuit: not switching ON/OFF. Then the FB pin voltage will still be regulated at about 0.8V, but the output voltage and the current will be reduced to the desired value.
* or use the "switching" method, but at the SHDN or ON, or OFF pin of the switcher, if there is any.

If you want the overcurrent feature fast enough to protect your load (low current overshot), then I assume the microcontroller digital loop will be too slow.

Klaus
 
Thank you for your response.
With voltage control, I was quite sure of DAC, because it's relatively simple and I can count it.
For current regulation, I agree with you on the one hand, on second hand.
In switching power supplly, the control is always non-continuous.
From this picture it will probably be more obvious.
The graph shows the gain of the feedback loop in the voltage control circuit.

This is all about amplifying the loop feedback in the current control circuit.
Unfortunately, I can not count it.
In any case, if you know about a sample solution by similar sources as well as current sources, I will be happy.

EA.png
 

Hi,

The error amplifier inputs are mixed up.
And the comparator is the usual PWM generator.
Nothing special, and the error amplifier output should be continous.

I just had the time to go through the datasheet.
As expected the abs nax input voltage on FB is 6V. Any overshot may kill the switcher.

But in any case the compensation capacitors will be over-charged in a way that it needs a lot of time to come back to normal operation.

As already mentioned: Use the EN pin to swith OFF the switcher on overcurrent condition. This doesn't harm the error amplifier signal that much

Klaus
 

I calculated with the comparator for +3.3V PS.

And what ?
Input Comp LM5145 is output of Error Amp, but too - input of PWM comp.
If we add on COMP over Diode other external Error Amp with compensation networks and as signal source, use Current shunt with current sense Amplifier.
What do you think
In the morning, I will try to specify the idea with the real values of the components

CCFB.png
 

Ok I look on use ILIM input and internal Ilim comparator.
This input is current input ,IRS ILIM source current, RSENSE mode is 100uA.
If is used Rsense current shunt in source of LSMosfet is her problem, becouse
in the calculation plays a role the peak-to-peak inductor ripple current and I thing this is not very stabble value.

Maybe I could use Current shunt on low side of output, but I hning this input is primary for protect of output MOSFET and not for "precise" output current control .Maybe I'm wrong

ilim.png

Another option is any as LM5117 but with externam Current monitor CM
CM is voltage output derived from output vurrent for example
CM= Iout * Rshut * A
A amplifi of current sense Amp

CM drive FB in CC mode
In CV mode drive FB CV circuit over BAT54, actually modifications of my first idea

111.png
 
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