WHITE PAPER. Title What kind of NAND flash memory is used for each product? ~~~ Which is suitable SD card from reliability point of view?

Transcription

1 Panasonic SD memory card White Paper Number : 002 Issue Date : 25-March-2015 Rev : 1.00 Title What kind of NAND flash memory is used for each product? ~~~ Which is suitable SD card from reliability point of view? Overview Panasonic SD memory card uses four types of NAND flash memory device, such as SLC, SLC-Lite, MLC, and TLC. In this white paper, the difference of each device is explained, and tips for choosing appropriate device will be described Panasonic Page 1

2 1. NAND flash memory structure Panasonic SD memory card uses NAND flash memory inside. NAND flash memory is one of the key components of the card. NAND flash memory stores information as a variation of its threshold voltage of the memory cell. This threshold voltage change is controlled by how many electrons are stored in the floating gate. The threshold voltage becomes higher when the electron count increased. Figure 1 shows a conceptual diagram of NAND flash memory storage mechanism. Figure 1: Structure and threshold voltage of the memory cell 2. SLC, MLC, and TLC A memory cell type of NAND flash memory is defined by how many individual threshold voltage distribution exist in a memory cell. When there are two individual threshold voltage distribution, that is two states in a memory cell, it is called as SLC (single level cell) NAND which stores 1-bit binary information in a memory cell. When there are four individual threshold voltage distribution, that is four states in a memory cell, it is called as Panasonic Page 2

3 MLC (multi level cell) NAND which stores 2-bit binary information in a memory cell. Similarly defined, when there are eight individual threshold voltage distribution, that is eight states in a memory cell, it is called as TLC (triple level cell) NAND which stores 3-bit binary information in a cell. This classification is shown in Figure 2. Figure 2: Comparison of SLC, MLC, and TLC You can find that the voltage gap between each threshold voltage distribution becomes small with SLC > MLC > TLC. This threshold voltage gap affects reliability of the data stored in a memory cell. Generally, the SLC NAND flash is more robust and reliable compared with other devices. However, it is more expensive than others, because of die size for the same capacity chip. On the other hand, the TLC NAND flash memory is cheaper in its cost, but becomes poor reliable because of less gap margin between each threshold voltage distribution Panasonic Page 3

4 3. SLC-Lite Panasonic SD memory card has a special device type of SD memory card. It is called as SLC-Lite series. This SLC-Lite SD card uses an MLC NAND flash memory, but controls the NAND as equivalent to the SLC NAND. That is, by utilizing two distributions out of four threshold voltage distribution, the SD controller controls the MLC NAND flash memory as an SLC NAND. As a result, reliability and robustness of the SD card becomes improved like pure SLC NAND SD card. This SLC-Lite NAND provides cheaper cost than pure SLC NAND, and equivalent features such as fast write speed, power fail robustness, and improved endurance cycles comparing with MLC NAND. Figure 3 shows the concept of SLC-Lite NAND device. And Figure 4 summarizes the features of SLC-Lite NAND. Figure 5 shows a comparison of endurance characteristics for each devices. Figure 3: What is SLC-Lite Panasonic Page 4

5 Figure 4: Features of SLC-Lite Figure 5: Comparison of endurance characteristics Panasonic Page 5

6 4. Panasonic SD card The line-ups of Panasonic SD memory card for B-to-B industry are shown in Table 1. You can choose pure SLC NAND SD card from the industrial grade SD card products. It is suitable for mission-critical applications. The SLC-Lite SD cards are available in the Consumer Plus grade SD card products, which is a cost effective solution for write intensive applications. MLC NAND SD cards are available from both of the industrial grade and Consumer Plus grade SD cards. TLC NAND SD card is also available in Consumer Plus grade SD card products. Table 1: Line-ups of Panasonic SD memory card for B-to-B field 5. Conclusion There are three kind of device in NAND flash memory, such as SLC, MLC, and TLC in regarding to bit count stored in a memory cell. With more states is stored in a memory cell, data retention characteristics and tolerability against disturb phenomenon become worse. The SLC NAND is robust, but expensive. The TLC NAND is cheaper, but less reliable. The MLC NAND stands in between the two devices. Panasonic SD card utilizes MLC NAND as SLC-Lite NAND in order to enhance its reliability as pure SLC by design technology of the SD controller firmware. As a result, Panasonic add a new category grade of SLC-Lite NAND SD card. In order to winnow an appropriate SD memory card, Panasonic offers Use Case Log Analysis and Life time simulation service (which will be described in another white paper), and you can choose the appropriate product from a more objective perspective Panasonic Page 6

7 Document History Date Revision Details 25-March First release (by Harry Kawai) Panasonic Page 7