A Derived design - looking at SD Card LDO Voltage Reg


#1

Hi,
Am designing a minimalist board to run the WP8548 and am going to include the SD card functionality. My question relates to the 2V95 LDO voltage reg (U830 - TPS) that supplies the SD card, related resistors R805-R809 and the ESD protection D1202. Wondering why the LDO was implemented as it appears on some specs to me that the SD can work up to 3V6 (2V7 to 3V6), but typically 3V3, and the LDO is supplied with regulated 3V3 from U1301. Is it to isolate the SD card from all other 3V3 items? Additionally the 2V95 is run to pin 160 of WP8548 which is labelled “RESERVED” Any comment on that connection? Is the SD card operation expected to impact on the stability/reliability of the 3V3 ? Clues?


#2

WP8548 datasheet mentions a 3.0V IO voltage for SDIO. So better to be safe to go with that voltage.

You can ignore the 2V95 run to pin 160. It is not needed.

No the SD card operation does not in anyway impact stability/reliability of 3V3.

cheers,
ashish


#3

Ashish,
Thank you for your prompt reply.\

You have cleared up my understanding of the Elec Spec section of the WP-8548 Tech Spec document. Especially the Vdd_px values required for the different I/O functions. The ‘Digital I/O Characteristics’ section (3.5.2) and tables within are now telling me to make sure I have the SDIO Vdd at ~2V95 and that I should ensure a 1V8 supply not just for the GPIOs but also the UARTS etc!!

For interfacing, I see now the reference on page 20 - ‘Interfaces’, Section 3 items listed above and other references in Section 4 -
4.3 POWER_ON_N,
4.8 General Purpose Input/output (GPIO), and
4.11 Secure Digital IO (SDIO) interace

One thing I had also missed was that the tech Spec says in section 4.6 UART - that the interface complies with the RS-232 interface - but I see this relates to signal function rather than Voltage levels, so I have to ensure that I maintain the stated 1V8 Vdd_px for GPIO/UART.

My application doesn’t use devices powered by 1V8 so I assume that the VGPIO (pin 45) will allow me to provide the 1V8 that I need and I can use a level translator to interface with my associated on-board 3V3 Rx/Tx components. I will then use a solution similar to the mangOH U700 for external RS-232 comms

Thanks for your time.
Mark.


#4

Oh!

… and section 7.5 Serial link access!!!