SX 48 max power draw?

CounterRotatingProps

[font=Arial,Bold][font=Arial,Bold]Some N00B questions for a remote telemetry application... I·need to determine·worst-case for the·power supply (batteries and solar charger).··Have the ancillary chips figured out, but need to get a grip on the SX itself.
1. Does the table below tell me what the max power consumption *could* be on the '48?· --- specifically the "total power dissipation" entry below?·· (Including·pulling the max·current from the SX pins - within the pins' and chips' tolerences of course).· Y/N?
2. Would this reflect full speed (say 75mHz) in a tight loop? Y/N?
3. What would you search for here in the forums to get some info on batteries and solar chargers ? --- I imagine this is a pretty common discussion.
Thanks much for your assistance - Parallax Rocks!·
- Howard in Florida

Table 18-1: Absolute Maximum Ratings (beyond which permanent damage may occur) 
...

 
[b]Total power dissipation[/b] 1 W at 70 °C; [b]1.5 W at 25 °C[/b] 

Maximum current out of Vss pins 180 mA at 70 °C; 300 mA at 25 °C 

 
Maximum current into Vdd pins 180 mA at 70 °C; 300 mA at 25 °C

From the SX48 data sheet ( http://www.parallax.com/Portals/0/Downloads/docs/prod/datast/SX48BD-Data-v1.5.pdf·)

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CounterRotatingProps

Not intending to thread bump, please let me ask this in another, clearer way (My apologies, it's been a long while since I've looked at a datasheet):

Are the voltage regulators on the two versions of SX protoboards rated at an estimated reasonable power draw, say at a %50 duty cycle ... meaning that could I use the v.reg ampacity rating as a reasonable guess what my SX chip would need for power (at very worse case) ?

thanks much for the assistance,
Howard

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Chris Savage

Hello,

The regulator’s ability to provide a certain amount of current depends on several factors, the most important of which are input voltage and thermal relief. The SX Proto Boards has a heat sink on the regulator, so it will provide more current than our boards without a heat sink at the same input voltage. With an input voltage of 7.5V you can probably expect to get at least 400mA if not 500mA from that regulator before heat starts to become an issue. As for the original question, you do have to add the combined current drawn from the I/O pins for max current draw, however the faster the clock on the SX the more current (power) it will draw. That said, if you draw max current from the I/O pins with the SX running @ 50 MHz, you would still have plenty of available current left from the regulator. Take care.

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Chris Savage
Parallax Engineering

CounterRotatingProps

As for the original question, you do have to add the combined current drawn from the I/O pins for max current draw

Hello Chris, thanks for the reply.

Interesting - so I did misunderstand the datasheet -· I thought total power would include the max for total I/O pins. But, to repeat what you say,

Real total power = datasheet "total power dissipation" (of 1.5 Watts at 25 deg. C) + combined current draw from all I/O pins.· (Plus accounting for the temp and clocking as you mention.)··

Right,·so I need to look up max on each pin and max pin total limit.· wait a sec... ;-)

• Maximum allowable sink current per I/O pin 45 mA
• Maximum allowable source current per I/O pin 45 mA
• Maximum allowable sink current per group of I/O pins between Vdd pins 50 mA
• Maximum allowable source current per group of I/O pins between Vdd pins 50 mA

OK, really pushing it = add 45mA to the total?· Correct?

Two more questions, please:

How much power difference does it make to clock it at say full tilt 75 mHz versus running it sub megaHertz?

Does turbo / non-turbo·mode affect power or just instructions per clock tick?

Thanks again for the assist,

- Howard







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PJMonty

CounterRotatingProps,

The part of the datasheet you are looking at tells you how much current can flow though the chip before it melts basically. What you're looking for is the supply current to the chip. Look at the next table in the datasheet (18-2) under "Supply Current, active". If the SX is sourcing current on any I/O pins, then the supply current will go up from the datasheet value. If you try to source enough current, the chip will die beause you have exceeded the limits spec'ed in table 18-1.

The best thing to do is build a circuit and test it. Datasheet specs for things like supply current can't possibly reflect all possibilities, so you'll need to verify with real world testing anyway.

Thanks,
PeterM

CounterRotatingProps

Thanks for the reply, Peter!

How odd, for some reason, table 18-2 slipped my notice. Now it makes more sense ! Yes, indeed ... sure don't want another toaster ... and am breadboarding it here soon. Just wanted to make sure this was on the right track.

thanks again!
- H

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