I'm tinkering with the ATtiny 2 series (ATtiny 3224). I'm also starting to teach myself about switch mode power and surface mount design. I'm planning to drive two Toshiba SSM6K804R MOSFETs. They are logic level MOSFETs with a 4.5V drive. They have a gate capacitance of 1110pF (1nF).
Even though these are logic level MOSFETs, I think I still need an external gate drive for them for crisp switching above a few kHz. I don't need to try and charge the gate directly with the ATtiny as I would need a 2-300ohm resistor in series to protect the microcontroller and that will slow everything down. Also need a pulldown for the gate.
Can someone please recommend a simple IC or maybe a packaged transistor pair that I can slam the gates with?
Questions/comments/advice greatly appreciated!!
A push-pull BJT configuration seems like it could be appropriate, if your application will be unbothered by limiting the Vgs to a max of 5.5v (the 3224's max supply voltage). The MOSFET in question seems to be good for Rds(on) below 20 mOhm at that voltage, so it seems fine for at least a handful of amps.
If you are doing higher frequency switching than a few kHz, then you probably want to use a driver IC. And especially so if you have a higher voltage bus than the 5v needed by the ATTiny.
I'll be reading up on push-pull. Thank you!
Main power source is a single cell lithium, so 3.7ish volts. Supply for the ATtiny is a 5V driver IC (I've already got it working).
The convertor I want to build will be inverting as the load is basically a pure constant resistance. (14.5w 1.6 ohm typical). I think that means I sidestep all the problems with a voltage higher than my control.
I'm hoping to use the window mode of the ADC and the programmable logic (CLC) to actually control the convertor mostly independent of the CPU.
I suppose it's worth mentioning that the 3224 datasheet suggests that it's perfectly capable of running at 3.7 volts directly, since it has a wide voltage range from 1.8v to 5.5v absolute. But I imagine you have other ICs that need the 5v supply.
I'm not entirely familiar with what you mean by a "convertor", but I presume it's some sort DC to AC function. Given the output loading, 3 amps through that Toshiba MOSFET seems reasonable enough, with the power dissipation being something like 0.3-0.5 W max, depending on switching speed.
Plan is for the ATtiny to actually mostly be asleep on the battery voltage. The 5V driver IC has an enable pin and goes into bypass mode when it's not enabled. Only when it wakes up will it turn on the 5V.
I want the 5V for the 4.5V gates on my MOSFETs, the OLED display (IIRC, it will also run on 3.3V), and the 20mhz for PWM and ADC.
DC/DC convertor. I'm hoping I can make a synchronus inverting buckboost supply in the neighborhood of 25w/6V max.
After a little more thinking about the problem you're trying to solve, an op-amp voltage buffer with an off the shelf DC-DC converter for the high voltage rail might be the simplest overall circuit. Unfortunately the ATtiny you're using doesn't have a digital to analog (DAC) output, so it's hard to set the voltage reference with that setup. You could probably rig up something with a PWM channel and a capacitor on the output, and adjust the gain on the op-amp feedback so you can reach the max output voltage you want (6V ?)
Unless of course the goal is actually to learn how to build your own variable output buck-boost converter, in which case, all the power to you!
Edit: I just realized 25W/6V means you'd be delivering over 4 amps... Those would not be very cheap op-amps. Have you measured the actual load you're driving? This could end up very similar to a 25w class D audio amplifier circuit depending on your load impedance.
I'm looking at the FAN3224, a gate driver capable of several amps at 5V to run my MOSFETs. I'll probably play with direct driving them with the ATtiny at first just to familliarize myself.
The original circuit is running some sort of bridge configuration. I've used a 1.6ohm load for years, with wattage set to 14.5 or 14.8. However, the orignal is capable 25w in the lowest output version. I'm not sure what resistance range it's capable of pushing that 25w.
And, yes, the goal is to build a variable DC supply. I'm finding it really interesting. I'm looking at the synchronus inverting buckboost. I think my final version would use half the components of the original.
If I understand this stuff correctly, (real big if), with the convertor pushing negative voltage, I sidestep high side drive problems. My load is dumb resistance so it doesn't matter how noisy or negative its power is.
So the ATtiny has an ADC capabale of running in windowed mode. A lot of the peripherals are able to run independently of the CPU, you can link them up with onboard programable logic called CLC. So, using logic I can tie the ADC to my PWM at pretty fast speeds. It also has a selection of internal voltage references. I'm planning to use some sort of voltage divider for feedback to the ADC, using the logic to vary my duty cycle when the voltage goes out of bounds.
Before I get that far out in the weeds, I need to play with mosfets, pwm, an inductor, and my new scope.