I'm trying to find a reliable way to connect my nRF52840 BLE-module to a 3.3V SD-card. The application is going to run on a lithium-ion battery and power is extremely budgeted.
The nRF52840 module can be powered in high-voltage mode by a supply between 2.5V and 5V and internally down-regulate that, BUT there is a 0.3V voltage drop. This means that the 3.3V will start drooping when the lithium-ion battery reaches 3.6 volts.
There is a solar-charging IC in my circuit (bq25570) which has a built-in configurable buck-regulator but that too has a voltage-drop of 0.2V, so the 3.3V will only be maintained until 3.5V.
I've been looking into buck/boost converters to regulate the voltage of BLE-module and SD-card to 3.3V, but seeing the poor efficiency at low currents (the nRF52840 consumes about 10uA when sleeping) I'm afraid battery-life will take a huge hit. I could only connect the buck-converter to the SD-card but when the battery is almost empty, the nRF52's SPI-lines will only be able to deliver up to 2.7V. What would be the most straightforward way around this?
Best Answer
Nope. That regulator has 25mA maximum external output, SD card needs 100mA minimum in SPI mode.
The sd card operating voltage can down to about 2.7V - I recommend using an LDO with enable pin just for the card alone. Thus it can be switched off completely when not in use. I recommend 3.0V as output voltage.
The SD LDO output voltage and the NRF chip VDD voltage must match - the default 1V8 VDD level (when using REG0) would be waaay too low. See REGOUT0 in UICR.