Hi, El Tue, Dec 13, 2016 at 12:00:32PM -0800 Doug Anderson ha dit: > On Tue, Dec 13, 2016 at 9:19 AM, Mark Brown <broonie@xxxxxxxxxx> wrote: > > On Mon, Dec 12, 2016 at 01:15:02PM -0800, Matthias Kaehlcke wrote: > >> El Fri, Oct 28, 2016 at 07:15:21PM +0100 Mark Brown ha dit: > >> > On Mon, Sep 26, 2016 at 10:41:59AM -0700, Doug Anderson wrote: > > > >> > What you're describing to me is a discrete DCDC that has an input > >> > voltage that sets the output voltage which happens to be set with a PWM. > > > >> I experimented a bit with this. Besides the question of how to model > >> the passives I wonder how the two regulators would interact. The > >> correct thing seems to be to specify the input regulator as a supply > >> of the DCDC. dcdc->set_voltage breaks down a voltage transition into > > > > No, not unless the prior descriptions of the hardware have been wildly > > inaccurate - my understanding had been that the DCDC was a normal DCDC > > with an analogue input intended to be biased to set the output voltage > > (presumably in terms of a full rail supply) and that the PWM had been > > connected to this analogue input. If the PWM is supplying the DCDC then > > the hardware design just seems bizzare, I can't see how this would even > > work. > > Looking at one schematic, the discrete BUCK for at least one of the > rails is TPS65261RHBR, which appears to be described at > <https://store.ti.com/TPS65261RHBR.aspx>. Data sheet appears to be at > <http://www.ti.com/product/tps65261/technicaldocuments?HQS=TI-null-null-octopart-df-pf-null-wwe>. > > As you can see from the datasheet ("Adjusting the Output Voltage" > section), it is intended that you stuff a resistor to make a voltage > divider and that's how you select the output voltage. In our case the > PWM interacts here and allows you to make a more dynamic output > voltage. I've always thought about the input to the "FB" pin as > making an input voltage, but I guess it's not terribly simple since > the voltage divider ends up dividing between ground and the output > voltage. I also had put my mind on seeing the output of the PWM circuitry as an input voltage, but technically it isn't a supply of the buck regulator. It seems we could consider it a "control voltage" instead and thus avoid the recursive lock acquisition. Matthias -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
