Propeller 2 core current draw (1.8v)
Tubular
Posts: 4,702
I'm looking into a regulator to suit the P2, to drop the 3v3 down to 1.8v for its core.
Anyone know what kind of worst case current draw I could expect on the 1.8v (core) rail?
I'm thinking something like a MCP1727 in a SOIC-8 pack to begin with.
Anyone know what kind of worst case current draw I could expect on the 1.8v (core) rail?
I'm thinking something like a MCP1727 in a SOIC-8 pack to begin with.
Comments
for the P2 chip realized that a more powerful regulator was required.
Looks quite good, reasonable price and margin, and has a proper Reset signal.
I would layout for the Adjustable version, initially, as often first silicon has different voltage sweet spots...
Looks to be an easy change.
I'm not really sure whether there are other regulators compatible in the same footprint - they seem to have subtle differences in soic8 pinout. But I'll go with it for now until advised otherwise.
I've pretty well finished a breakout board design and hope to send it off to the fab next week when they are back from new year holidays. Just the basics onboard - xtal, regulator, flash, prop plug
Hi BigFoot,
Did Chip already mention something about it? Perhaps I missed it.
I vaguely recall some mention of the leakage currents involved, but I'm more after the maximum operating current draw, which will depend on frequency of course. It'll also help me decide whether to hang the 1v8 regulator under the 3v3 one (likely) or run it separately from the 5v incoming...
I would also suggest doing an option for Xtal or Oscillator module, like a 5mm x 3.2mm FOX924B TCXO.- those are high precision, and come from 10MHz to 27MHz, which is outside the Prop 1 range...
That's a really nice part. I didn't know they did such nice things in such small packages. I'll add the supply rails to the other 2 pins.
As it happens I could only fit a 5x3.2 xtal near the prop2 anyway, plus there are also XI and XO pins broken out maybe 4cm away, with the ability to cut the tracks to the broken out pins if required.
Thanks for the advice.
Big foot may have been referring to this post: http://forums.parallax.com/showthread.php/125543-Propeller-II-update-BLOG?p=1162345&viewfull=1#post1162345
Assuming Chip's estimate is accurate, then a 1.5A regulator seems like a good start.
Yes, that is the post. He mentioned 1.5 Watts which is allot by today's standards and is going to make it difficult to use the chip
in battery powered applications. I have always thought that the P2 would be awesome to use for a quad copter and hope he was
talking about absolute maximum power consumption.
Ordinary USB can only supply 0.5A / 2.5 watts, not enough to power Prop2 without a switching reg. I can use a 1.0 Amp wall wart like used for the Pi, samsung phone chargers, ipads etc (1 amp /5 watts)
The MCP1727 still appeals, but its SOIC-8 package thermals at 150 C per watt are out of the question.
The DFN version of the MCP1727 (3x3mm, with exposed pad and vias underneath) is 41 degrees per watt, resulting in a 57 degree temperature rise for the 5v to 3v3 regulation, and 51 degree rise for the 3v3 to 1v8 regulator. That's burn territory so I'll try spreading the heat out with a pcb heatsink and see if we can get that well down.
And I guess start looking at suitable switcher solutions too
Note that current spec will be all COGs at full speed, which is a rare combination So an Analog regulator should comfortably deliver the peak, but typical use will have much lower average powers.
There is a little bit of voltage margin from 5V to 3v3, so adding (say) 2 x 1206 1 ohm (or less) resistors, spreads that thermal load a little and gives you a nice means to check the actual current.
The ideal here is a part that work from under 5V, and up to plug-pack regions, and has PowerGood, and avoids an external Schottly.
Dual would be nice, but they seem to be quite a jump in price.
The Richtek RT7250BZSP is ADJ 2A 8SOP, Stocked at DigiKey, 25 : $0.6244, and frees you to choose anything for the 3v3 side.
- so looks a good match for Prop 2 ?
that looks like a nice and well priced part.
For now I think I'll measure the temp rise on the MCP1727's by simulating the1.5 watt core loading with the board I just posted. I figure the power rail arrangement (node-gnd-3v) means 7805 switching regulator replacements (cui inc at up to 2 amps, but no 1v8, or recom 5v to 1v8) are easy to fit and add external smoothing caps to.
It would be easy enough to try a Richtek based plugin board in the same format, and then perhaps revise the main breakout board accordingly. I think that's how I'm going to tackle it.
Great work digging out these useful parts...
http://www.mouser.com/ProductDetail/Diodes-Inc/AP7175MP-13/?qs=sGAEpiMZZMv4Mmfa%252bK8JIj5e7r4%252b51XfB6ybjpS8c94%3d
33cents for 100 units (Wow!)
the AP7176B can handle Vin of 5.5v but a little harder to find at the moment.
I believe this is the correct spec for P1
It doesn't matter, as long as the I/O supply is within 50ms of the core 1.8V supply.
For our little module, we found a nice Micrel part that is only 2x2mm and uses one 1uH inductor and two 4.7uF 0603 caps to regulate 1.8V@2A:
The minimum current draw of the Prop2 will probably by about 1mA because of leakage.
Chip
Great, I need to make some final adjustments and that part looks good at around a buck. Are there any other hard and fast figures available yet (or maybe not so hard and fast).
http://www.mouser.com/ProductDetail/Analog-Devices/ADP5037ACPZ-3-R7/?qs=sGAEpiMZZMtitjHzVIkrqYThiPHWFX9axPSzmoXpPc8%3d
You're right about the ldo for analog, but that'd be at 3v3, so guess you'd have to feed both an ldo and a switcher off say a 3v7 or above rail...