Greetings All,
I am working on an ATTiny circuit design that runs off a coin battery (VCC) and has a VBUS USB connection (VDD) to program the Tiny. Looking at my schematic, do I need something (a diode I'm guessing) to isolate the VCC current from the VDD current on the power pin of the ATiny?
Two diodes can be used as a switches to isolate power supplies. A silicon diode will drop about 0.6 to 0.7 V, which can be a problem when using a coin cell. MOSFETs can also be used, with lower drop and higher circuit complexity.
A better idea is to use a programmer, like the Pololu AVR programmer v2.1, which detects and automatically adjusts the programming pin voltages to match the AVR MCU power supply.
I think a .7 drop would still be within tolerances for the ATTiny1614 (which is 1.8v-5.5v). For reasons of vertical space (no pin interface) and shape (no card-edge sticking out), I have to have the ISP built in. As far as a MOSFET, any recommendations on a good SMD one for this type of thing? Thanks!
Voltage tolerance is not the issue. It is the waste of coin cell power. I would just use two Schottky diodes.
Do you really expect to light up to 36 LEDs from a CR2477 coin cell, as suggested by your schematic?
Open up the UNO schematic and see how it uses a MOSFET to select the different power sources.
I am curious what the project is. The ISSI Chip (IS31FL3236A) looks pretty interesting.
jremington:
Voltage tolerance is not the issue. It is the waste of coin cell power. I would just use two Schottky diodes.Do you really expect to light up to 36 LEDs from a CR2477 coin cell, as suggested by your schematic?
It was a concern that had me stumped for quite a while, but then I found some really good low-current LEDs that only draw 2mA at full intensity. Plus there will seldom be more than 25% of them lit at a time. It's still going to burn up the CR2477s fast, but it was the best that I could find that fit the size constraints.
SteveMann:
I am curious what the project is. The ISSI Chip (IS31FL3236A) looks pretty interesting.
It's for a friend's cosplay costume. He has a big anime character wig that he wanted to add lighting effects to and didn't want any wires hanging down. The ISSI chip drives enough LEDs, is constant current, and has a simple I2C interface.
The CR2477 is rated at 1000 mAh assuming 0.2 mA continuous current draw. You might get 15-30 mA for a few minutes.
I remember with such a coin cell you can light up an LED without current limiting resistor - instead using the internal resistance of the cell. That's how limited their current supply really is.
You sure you can't hide a pair of AAA (or even AA) batteries inside a big wig? Those would power the LEDs easily.
jremington:
The CR2477 is rated at 1000 mAh assuming 0.2 mA continuous current draw. You might get 15-30 mA for a few minutes.
Yeah, I'll see what happens in prototyping. Worst that can happen is that it doesn't work or doesn't work well enough. I looked at using a couple of Quad-A batteries, but it would still require an outside battery holder.
