I'm working on a sensor module that writes to miniSD powered by a 3v CR2032 Lithium Coin Cell and am having issues with maintaing the 3.3 voltage output from the IC with several loads. My current circuit runs a max 32.72mA simply writing values to a text file, but the voltage drops from a consistant 3.3v to around 1.6v and I get a card error. Just to double check I attempted to light a 20mA LED and had the same issue with voltage drops below 2v.
I'm working with a custom breakout board with suggested Taiyo Yuden 1uF and 2.2uF capacitors, LBI tied to ground, LBO tied to a 10k pull up resistor. I haven't checked this on a scope so I may be missing something, or I may not fully understand the capabilities of this IC , but based on the Output Current vs Output Voltage diagram for the 60212 I shouldn't be having this problem. Any suggestions would be greatly appreciated.
You'll need to double check with a scope, but you might be having either of these issues:
Large parasitics from breakout board are causing issues with this high frequency switching power supply. You need to evaluate this device on a PCB board (such as the EVM) not on those breadboards we used in school. (From your description, I'm not sure what type of board you have.) The extra parasitics interfere with the switching operation of most all switching power supplies.
The impedance of your coin cell is too high for the switching converter. You should do your first testing with a lab power supply, work out any bugs, and then switch over to your battery. If the impedance is too high, you would likely see the input voltage to the TPS60212 dropping and then rising back up. To fix this, you need to decrease your source impedance--usually adding more input capacitance does this.
Thanks Chris, indeed the battery impedance seems to be the culprit. The TPS60212 recommends 2.2uF or 4.7uF input capacitance, I'll assess the 4.7 but is there any reason in the circuitry not to use a higher input capacitance if necessary?
Nope, you can go as high as you want with the input C.
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