Every 6.66 milliseconds (i.e. 150Hz) it will turn the LED on for anywhere between 1 microsecond and 256 microseconds.
A lot of modern LCD monitors pulse at 120Hz to reduce motion blur.
They don't pulse at 120Hz, they refresh at 120Hz. I'm sure their backlight PWM runs at many kHz.
Every 6.66 milliseconds (i.e. 150Hz) it will turn the LED on for anywhere between 1 microsecond and 256 microseconds.
Thanks all for your input! I hear what you're saying about the 150Hz PWM rate. I would've preferred 1kHz+, but there's a reality face when you want to scale down the 1A current to 1mA.
2) Turns out that the gate driver IC datasheet requests a 1uF film decoupling capacitor. These are bigger than I want on my PCB. How can I determine the impact of choosing a lower value? I'm aware that the goal is to as-quickly-as-possible push charge into the MOSTFET gate to turn it on, but 1uF seems like an over-abudance of capacity.
It starts to show some minor non-linearity, though. Flicker will be the real problem at such low duty cycle. You may want to up the freq even if that means that your minimum on-time is even lower and the low side control steps will be even more non-linear. You want to test that out in practice. At very low duty cycles, a large part of conduction period will be partially conducting i.e. switching loss but this doesn't need to be a problem.
To get a more exact idea of the waveform the USB port sees, I suggest you simulate this in Spice, approximating the USB cable and connector resistance and inductance, ESR of the capacitors, and so on.
A lot of modern LCD monitors pulse at 120Hz to reduce motion blur. If 120Hz flicker really is that noticeable to most users, that feature won't sell very well. (Where that typically becomes insufficient is in 3D mode where the glasses split that to 60Hz, a big reason why 3D TV failed.)
Thanks for the nods-of-approval for the 1uF MLCC, I'll switch to those. I'll also throw some across the LED's 100uF tantulum caps.
Its disappointing that it's not possible to calculate this analytically.
The non-linearity is material and it sucks, but the good news is that it seems like there's a delay in both MOSFET turn-on *and* turn-off, which will cancel out some of the non-linearity. At the end of the day I just want to get it built and start testing. Firmware is flexible and LED current resistors can be changed.
Across the LEDs? I hope that's a typo .
Still, remember to derate voltage, 10V or 16V part is needed for 5V bus, and look up the inrush current specification and max ripple current from the datasheet.
USB spec limits allowable bus capacitance (to limit inrush duration), if I recall correctly that's some 47µF, so you need to add a precharge circuit to be able to use more. It's getting some sort of PITA.
If you don't have precharge, you can't have much capacitance, and if you don't have much capacitance, the USB sees all those 1A peaks, in which case you would need to enumerate and ask for more current, or hope everything's OK.
Of course you can, but the equations get really complex when you include "everything", including cable ESR, ESL, capacitor C, ESR, ESL, and possibly C, ESR and ESL of another set of (different type of) capacitors. Spice handles coming up with all the equations and solving them for you. Tools are there to help.
Thanks for the nods-of-approval for the 1uF MLCC, I'll switch to those. I'll also throw some across the LED's 100uF tantulum caps.It starts to show some minor non-linearity, though. Flicker will be the real problem at such low duty cycle. You may want to up the freq even if that means that your minimum on-time is even lower and the low side control steps will be even more non-linear. You want to test that out in practice. At very low duty cycles, a large part of conduction period will be partially conducting i.e. switching loss but this doesn't need to be a problem.Yep. That's my thinking on the matter as well. The non-linearity is material and it sucks, but the good news is that it seems like there's a delay in both MOSFET turn-on *and* turn-off, which will cancel out some of the non-linearity. At the end of the day I just want to get it built and start testing. Firmware is flexible and LED current resistors can be changed.
Do you realize what will happen if Vin catches some ESD while the fuse is blown?