Buck voltage controlled by an MCU

Is it possible to use the voltage divider in the feedback loop of an adjustable buck converter to continously change the desired output voltage over time ..?

How fast could the feedback voltage be allowed to change without compromising stability ..?

(example chip:

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Could the MCU then be interfaced to the voltage divider for tps54283 page 42, figure 45. By connecting an transistor in parallel over the R4 resistor (or R10). And control the transistor base via an RC filter connected to one of the MCU digital outputs. The MCU could then output PWM to regulate the output voltage level. An alternative is to use an one chip D/A instead of the RC/PWM setup to control the transistor base.

Any thoughts on this? (I did some searches. But no real good info turned up) I suspect there's some real gotchas here :-)

The original idea turned up when looking at how buck converters are designed and looking for a way to control DC motor speed without burning away excess power or getting lot's of EMI (from PWM). The initial thought was to simple use an inductor to rid of the EMI. Then by controlling a switch transistor the power level could be controlled by an MCU. But then there's already chips doing this, namely buck converters. So by controling the buck converters feedback voltage divider one can control the dc motor speed. And a transistor in parallel over the "top" resistor used in the linear region would allow adjustment over the voltage divider. An D/A could then be used to control the transistor base. A simple D/A could be accomplished by using an RC filter wired to one digital output from the MCU. The problem seems to be how to control the buck converter feedback mechanism and still have voltage control loop work without the MCU.

Reply to
sky465nm
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Sure.

That predominantly depends on how much capacitance you hang onto its output.

If you need to be able to drop all the way to zero the only practical method is to sink a current into the FB node. This has to come from a source that is a tad above the ref level so you can fool the switcher into thinking its output is regulated while in reality it sits at zero. So you'll have to make a current source that is controlled by your uC.

It's no big deal but you'll have to ask yourself what the advantage will be. The only advantage I see is that you can put safety hooks in place that catch when the uC goes on the fritz for some reason. For example a voltage limit or a current limit.

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Regards, Joerg

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Joerg

10 output volt/seconds? for a 0.5A 6V DC motor? Guess the difficulty is getting the capacitance of a dc motor ;)

Any suggested way to create an mcu controlled current source?

If the supplied voltage is within the tolerated levels for a dc motor and the output stage of an ordinary buck converter is used. Which in essence is a inductor in series with the positive rail to the applience. And some capacitors in parallell. And a back-emf protection diode aswell. This is then driven by a transistor in series with the voltage source positive rail that is switched with pwm signal from an MCU. Would the same functionality be achived?

Quick schematics:

MCU.. | --Transistor------------Inductor--------- | | | | | Volt Diode --- --- Motor Source | --- --- | | | | | | -----------------------------------------

Reply to
sky465nm

10V/sec? No big deal. I've built stuff like that although the load wasn't a motor.

No, that of the switcher :-)

Yep: Pipe out a voltage. If the uC doesn't have a DAC you'll have to make do with a timer, PWM and RC lowpass. Now feed that into an opamp and then a transistor with one input tied to a current sense resistor in the emitter path. Which input depends on whether you'll pull up or down with the collector. For your job it'll be up, so pnp. That's it. You'll need the proper common mode ranges and a voltage shift up front because the uC most likely cannot go up to the motor rail in supply voltage. Basically "grunt work", good old discrete design.

^^^

Leave out the left capacitor, it'll fry the transistor. Usually the diode is also a transistor so in essence you have a totem-pole driver or what motor guys call half bridge and the high-low duty cycle determines the final voltage across the motor. That's how it's usually done, sans extra PWM chip. Use FETs for both transistors since they have built-in stiff substrate diodes. Or a staunch half-bridge with the diodes in there. Also, mind cross conduction, break before make and such.

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Regards, Joerg

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Reply to
Joerg

Maybe closer to 1V/0.1s ,possible 10x that ratio (faster). But not much more.

So only the switch transistor/fet sets the response limit?, I thought the control loop were the limit.

uC PWM -> RC lowpass -> OpAmp -> Transistor.

Guess the common mode range can be fixed with some clever resistor setup on the OpAmp.

Would this accomplish dc motor control without burning away excess power not used in the "applience" (dc motor). And eliminate the worst EMI?, esp if the cable to the actual motor is a few meters.

uC PWM | B/src --Transistor------------Inductor--------- | | | | Volt / \\ --- Motor Source Diode --- | | | | | -----------------------------------------

Reply to
sky465nm

No, I meant the capacitor at the far side of the inductor. If you make that smaller you yield faster response but more ripple. So there needs to be a compromise. But at your speeds it ain't rocket science, my last design had to perform a controlled ramp from 0V to about 100V in 600msec.

Yes. But with the usual direct approach below you don't need all that.

Sure. But you'll need an extra LC or two (small values) afterwards because the typical large inductor leaks a lot in the higher RF spectrum because of its size and build. Also, motor brushes create lots of EMI so usually you also need to filter at the motor side.

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Regards, Joerg

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Reply to
Joerg

Like this I suppose then?

Inductor1 > Inductor2 [Henry]

--Transistor------------Inductor1-----Inductor2----------- | | | | | Volt / \\ --- --- Motor Source Diode --- --- | | | | | | ----------------------------------------------------------

Thanks anyway for an insightful response.

Reply to
sky465nm

Yes, like that. If your ground plane is iffy you may want to have another small inductor in the return lead between the caps. Make sure none of the inductors gets too close to saturation. More hardcore cases may require a common mode choke as well.

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Regards, Joerg

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Reply to
Joerg

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