SPI isolator for MicroSD card

Hi Ruslan,

I suggest you to use digital isolators like ISO7242C or any other company say Analog Devices to isolate your SD Card from your internal circuit.

For short circuit protection just connect a forward biased diode between supply and ground in the SD Card Side. Make sure GND net used for SD Card must be different than the gnd used in the internal circuitry. Now left is your power supply for SD Card, if you can tell me the exact situation when SD Card will be inserted...I can think of it.

Please provide some more details...then it will be easy to suggest circuit for power supply for SD Card.

Thanks and Regards

-Anand Kumar Rai

Dear Mr. Anand Kumar Rai,

Thank you very much for your replay. 

I want to remind a problem. 

I have new task in our design. In our design I have STM32F105VCT6 CPU. It has 4-line SPI interface. I want to connect SPI flash (S25FL128P0XNFI001) and microSD card with to our CPU, through this 4-line SPI interface simultaneously. MicroSD card will insert to MicroSD connector. Our device is control device; therefore bad customers will attempt to put out of operation it. For example, it can insert aluminum foil instead MicroSD card to connector. Our device must to continue to operate, i.e. 4-line SPI interface between CPU and SPI flash (S25FL128P0XNFI001) must to continue to operate. MicroSD card connector contains GND, SPI interface (4- lines) and 3.3 V power supply. I use same 3.3 V power supply for CPU too. I have two tasks:

1) I must make 3.3V power supply short circuit protection for MicroSD card connector. And I use ST890DTR chip for this task 

2) I must make short circuit protection for SPI lines (MISO, MOSI, CLK, CS), because SPI interface between CPU and flash must to continue to operate. It may be buffer or other device. I must separate MicroSD connector signals and CPU SPI signals with the help of buffers or other devices. SPI clock is 10 - 20 MHz. (I have different CSs: CS0 for SPI flash, CS1 for microSD card. MISO, MOSI, CLK signals is common for CPU, SPI flash and microSD card). I use TXBN0304RUTR chip for this task. 

I have two questions:

1) Can I use TXBN0304RUTR in our design? Can you to check our scheme? Can I send our the scheme to you? Can you send to me your mail?

2) What chip is better TXBN0304RUTR or ISO7241 (TI) for our task? I want to consider and to compare cost, functional parameters and dimension of chips (TXBN0304RUTR - 0.55$| 1ku ISO7241 - 1.25 | 1ku). Our device is mobile equipment. The ground of VCC1 and VCC2 may be common.

Best regards,
Ruslan Chaban, Russian, Perm, Fort-Telecom Inc, rchaban@fort-telecom.ru, design engineer, 14.01.2013

Hi Ruslan,

Here is the answer of your question,

1) TXBN0304RURTR I will check and come back. I will check you schematic and you can mail me on anand.rai@eldaas.com or anandrai74@gmail.com

2) If you are using a TI chip look for TI engineers in your area, they have very nice technical support all around the world.

3) It is good to separate the grounds for VCC1 and VCC2, but I will confirm the use of common ground for both VCC1 and VCC2 after reviewing your schematic.

Thanks and Regards

-Anand Kumar Rai

Hi Ruslan,

As far as separating the power supply is concerned I suggest to use MAX253CSA+ with a transformer to isolate your SD Card power supply with Flash. But again that will add to your BOM cost of the product. So, I would like to review your design first the can recommend the MAX253CSA+ for your product.

  Thanks and Regards,

-Anand Kumar Rai

Hello folks, since you're already considering TI devices (which I greatly appreciate), let me please also suggest TI's smaller version of the MAX253, the SN6501. Its benefit is, it really works down to 3.0V and up to 5.5V without the external rectifier diodes the MAX253 needs for low-volt operation.

I'm attaching a short apps document describing some isolated power supply designs with various transformers.

Best regards, Thomas

Hi Thomas,

Thank you for the suggestion I will look into the datasheet for the details.

Ruslan, I got you scheme in my mailbox and have replied you back with block diagram to plan for your isolated scheme for SD-Card with short circuit protection.

Please acknowledge if you got my mail.

Best Regards

-Anand