SparkFun Forums

[roach]

Can anyone tell me the voltage tolerance of a microSD/transflash card (or better yet, point me a t a datasheet)? I'm going to be using a LiPoly 3.7V battery, and I want to make sure I don't fry the thing (doubtful, but I want to be certain).

[ohararp]

They run off 3.3V and require 3.3V logic (its really probably a 2.8V threshold). You would interface to them exactly the same as you would a larger mini or SD sized card. Definitely, regulate the input into this card. A buck boost controller is even better since you will be able to use the battery when the voltage falls below 3.3V. the MCP1252/3 is great for this.

[roach]

Just found the blazingly obvious link to the SD card specification on the Sparkfun site, right after posting that...

According to the Physical Layer Specification (here in PDF), under "System Features", it notes a max operational voltage of 3.6V. This sheet is for regular SD, though, not MicroSD. Does that make a difference?

[ohararp]

When I first started playing with Sd cards I had the SFE breakout boards for Sd and MicroSD. Once I had the setup down for the SD cards I merely pulled the breakout board out of the breadboard I was using and plugged in the microsd card and everything worked just fine. Very compatible unlike so many other designs out there. Remember lipos make 4.2V at full charge and should be cutoff at 3V. This makes for a unique situation in regards to obtaining 3.3V. You will need a buck/boost converter to get the full life out of the battery. Don't forget to use a protection circuit module too!

[roach]

You will need a buck/boost converter to get the full life out of the battery. Don't forget to use a protection circuit module too!

Okay, well I have no idea what that is, so I looked itup on wikipedia. IT still makes not sense to me, except for this part:

One drawback of this converter is that the switch does not have a terminal at ground; this complicates the driving circuitry. Also, the polarity of the output voltage is opposite the input voltage.

So I guess I'd need some sort of inversion circuitry?

[ohararp]

Battery actually outputs 4.2V to 3V. You need 3.3V. The converter runs in buck mode when supply voltage is above 3.3V. The converter runs in boost mode when the supply voltage is below 3.3V. If you only used a "tradition regulator" 78XX you would be limiting the amount of run time from the battery since you would not be able to power your 3.3V circuit when the battery voltage was in the this lower 3.3V-3V range.

Most regulators run in what could be considered buck mode whereby they must drop the supply voltage to the regulated voltage. I hope that makes sense. Checkout the LIPO backpack at my site (under blog link). This design has a MCP1252-33x50 from Microchip that is configured to output 5V and 3.3V and will do what you need while supplying 120mA.

[roach]

This looks great. I just checked the datasheet. If it could source more than 120 mA, I'd use it for the whole circuit, not just the microSD.

Thanks Ryan!

[ohararp]

I've found a few other circuits that can supply a bit more amperage but not much. Maybe 150 mA or so. They are pretty cheap $1.05 from Micrhochip Direct ($.40 for parts) and I actually use three of them in my gps logger.

[ohararp]

Roach...check out these guys 1.8V to 5.5V selectable at 1A output.

TPS61030 from TI.

[roach]

Yeah, but those are boost-only (unless I'm misreading something).

Upon further contemplation, I realized: I have no friggin' clue what kind of current I can expect my circuit to draw. I'm using:

- LPC2148 uC (100mA MAX. What's typical?)
- MicroSD/Transflash card (100mA MAX. Again, what's typical?)
- Vs1002 (50 mA, I think?)
- MAX1555 (Nothing from the battery)
- OLED display from sparkfun (80mA, I think?)

Presumably, not all of this will be drawing current at the same time, but still. Just adding it all together is about 330mA. Does that mean I need to source that much current at 3.3V? I'm a little murky on the electrical stuff...

[ohararp]

I was hoping you would "gloss" over that title. Apparently these act both ways! Ha! Anyways, your best bet is to design a pcb such that you provide enough current and then actually measure the ouput of the devices (data sheets are never right). On my gps datalogger that is what all the red jumper are for since it is really easy for to hook up a voltmeter and measure the actual current used by these devices. Also, the converter are not 100% effiiecient and ther are some losses asociated with stepping up voltage (ie device might require 100 mA but the converter is only 80% efficient (really bad by the way) and the device really draws 120 mA (according to the battery)).

[roach]

bah. Add to all this the fact that the only battery that will fit in this design is a replacement iPod Nano LiPoly, 330mAh, and it looks like I'll only have enough juice for a single hour of play time...

... Guess it's back to the drawing board.