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Options for switching a 12V load with a 1.8V logic signal

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magic:
That Zero999 circuit is actually quite neat :-+

It seems quite power efficient, MN4 may also serve as a synchronous rectifier and you don't really need the low voltage part if you can devote two MCU pins to alternately turn on MN1 or MN4.

edit
There is actually some shoot-through during state transitions :--
I think two changes are in order:
1. Put 1k(?) series resistors right at the drains of MP3 and MP4. The connection to the output coil of course bypasses this resistor.
2. Really drive the circuit from two MCU pins, and ensure some short dead time when both MN1 and MN4 are off.

T3sl4co1l:

--- Quote from: eddie1 on January 22, 2020, 07:37:49 am ---So this, if I understand correctly, is about making sure the inductor isn't overcharged beyond its rated current limit (or it becomes a fire hazard).
--- End quote ---

Well, usually the transistor dies in 10-100us (probably longer at this small scale, or unlimited if you use a somewhat over- or under-sized part, as the case may be).  Balls of wire and iron take seconds to ignite -- hence why dumb old fuses are adequate to protect wiring, transformers, etc.  But yes, that would eventually be a problem as well.



--- Quote ---Even if the load current and inductor size were such that it was not operating in DCM, does hysteretic control not effectively take care of this? The comparator isn't going to trip until the output voltage drops below a certain point, which presumably means there's not some abundance of current stored in the inductor.
--- End quote ---

How can it?  If the output stays low, the switch stays on, and on, and on...

A monostable timer, limiting maximum on-time, would be a nice addition.  That way you can make the assumption that Vin < Vin(max) and L >= Lmin, and get a maximum dI = Vin(max) dt / Lmin that's within the transistor/inductor ratings.

You still don't know how long to turn off for; if the load is shorted, and the low side switch is synchronous, it could be very long indeed (a few time constants of the inductor's tau = DCR / L).  If the low side switch is a diode, the diode drop is somewhat known (maybe 300-600mV for schottky), so you can expect the maximum off-time proportional to that and the supply voltage (using the equal-flux condition: the inductor spends as much time*voltage positive as negative).

Not that you'd likely use a synchronous switch here, but more for completeness.

We often diagram switching waveforms with certain ideals in mind, like assuming diode Vf = 0.  But that doesn't happen to be a good assumption in a case like this.  In this case (Vout ~ 0), Vf is significant, so we need to consider it.  The case Vf --> 0 is still of practical use when we employ a synchronous switch, however; it's not just a fantasy.



--- Quote ---This does make me question whether there's something basic in "buck converters 101" that I'm missing. The LTC3388 data sheet mentions ramping up the inductor current to 150 mA and then down to 0 mA, as if they're doing some kind of current monitoring similar to what you mention. (The LTC3388 uses hysteretic control.) The part I don't get is how they can do some kind of current control like this without affecting output voltage regulation. Surely if they keep the PMOS on until the inductor current reaches 150 mA even if the comparator is outputting 0 (voltage now in regulation), that's going to result in the output voltage overshooting the intended target?
--- End quote ---

They don't describe the "buck control" on the block diagram.  It may be an ordinary peak-current-mode synchronous control, perhaps extra source terminals on the transistors (thus tapping off some load current, to a sense resistor, to sense amps/comparators), perhaps sensing drain voltage (which has a gross Rds(on) tempco (which can be corrected for), but is practical in terms of thermal performance).

Or maybe it's something dumber, like a constant-on-time control or something.  Which is not uncommon among sync bucks: pulsing the inductor gives some dI, then you can watch just the low side switch's current to see 1. what peak current we got up to, and 2. where current crosses zero to turn off the sync diode.

Actual current consumption during operation might be much higher, but you don't notice it because inductor current dominates, of course; but then they disable everything inbetween, so the average consumption is almost nil.

You can't really do that with discretes, because few comparators/opamps are available with a bias control pin.  And most that do, are higher voltage, low performance vintage designs, like the LM13700 -- which is quite useful in itself, but isn't going to be setting any speed records here.  But in a monolithic design, it's a good plan, and probably what they're doing.



--- Quote ---For now, I'm thinking the best thing to do is use a higher-power, faster comparator with a hysteretic control algorithm (which means the switching speed won't be limited to the available clock). I can probe it and optimize it later, testing changes like dropping the comparator speed (and power). Since standalone comparator ICs are specced for lower overdrives, I ordered an ST TS3021 which gets to about 50 ns typical at 20 mV. (TI has a similar part with similar specs, but it requires a higher input voltage. I could use it via the charge pump doubler, but why bother when the ST part is similar and supports 1.8V directly.)

--- End quote ---

Yeah, that'll do.  Also MCP6561 family.

Here's an average current mode controller using discretes:
https://www.seventransistorlabs.com/Images/Flashlight2Sch.png
https://www.seventransistorlabs.com/Images/Flashlight2_Schematic.png



--- Quote from: magic on January 22, 2020, 08:36:55 am ---That Zero999 circuit is actually quite neat :-+

It seems quite power efficient, MN4 may also serve as a synchronous rectifier and you don't really need the low voltage part if you can devote two MCU pins to alternately turn on MN1 or MN4.

edit
There is actually some shoot-through during state transitions :--
I think two changes are in order:
1. Put 1k(?) series resistors right at the drains of MP3 and MP4. The connection to the output coil of course bypasses this resistor.
2. Really drive the circuit from two MCU pins, and ensure some short dead time when both MN1 and MN4 are off.

--- End quote ---

Yeah, doing it discrete, you want higher Rds(on), just overdriving a BSS84 directly will gulp 100s mA.

Note also that it's only latching while there's voltage available; in the discrete circuit, if both NMOS are off, the charge on either PMOS will eventually leak away.

The ring-of-two-inverters maintains state by itself, which may or may not be advantageous (if for some reason it's easier to generate brief pulses, that'll do; downside, it remembers what it was doing, which, if you didn't intend to say leave the switch in the 'on' position, well..).  The main thing about inverters is, you can get much smaller transistors in logic gates than discrete, which is probably going to be the decisive factor.  (If we could get 20V 20mA 50 ohm 4pF MOSFETs in singles, absolutely; if we're stuck between BSS84 and such, ehhh.)

Tim

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