Memory Card Interface Translates and Protects Signals
Figure 1. Logic-level translator circuit for memory cards. The pinout shown at J2 is for SD cards.
These devices use a proprietary architecture, which eliminates the need for a direction (DIR) or write/read (W/R) pins to indicate data direction. This approach has several benefits. First, the package size of the MAX13035E family of parts is reduced to only 2mm x 2mm in a 16-bump UCSP because fewer signals are needed. Second, the I/O on the baseband processor that would have been used to indicate the direction of the signals is now free to be used for something else. Finally, one fewer signal has to be routed in what is undoubtedly a tightly packed layout.
The proprietary architecture of the MAX13035E also uses internal current sources to guarantee a known state on the bus when the inputs are high impedance. This design eliminates the need for external pull-up or pull-down resistors. Each channel is interchangeable, and compatible with either CMOS push-pull signals of up to 50MHz (100Mbps) or open-drain signals at 200kHz (400kbps). This flexibility is especially useful when interfacing to memory cards like the MMC where the initialization mode can use open-drain signaling on one of the lines.
The MAX13035E and the other devices in the MAX13030E family of parts all shutdown when VCC < VL, that is when VCC is disconnected. The MAX13030E–MAX13034E include an EN pin that can be driven low to put the part in a lower power shutdown state. The MAX13035E, however, does not include an EN pin, but replaces it with a CLK_RET pin whose function is to feedback the clock signal to the host processor, as shown in Figure 2. This signal can be used by certain SD card controllers to improve performance.
Figure 2. The MAX13035E uses a CLK_RET pin to feed the clock signal back to the host processor.
All devices in the MAX13015E family include extended ESD protection of up to ±15kV (HBM) for all I/O lines on the VCC side (card side). To guarantee full ESD protection, a 1µF ceramic capacitor should be used to bypass VCC.
The circuit in Figure 1 also shows how to use the MAX3202E to provide ESD protection on the CARD_DETECT and WRITE_PROTECT signals. When using this circuit, it is strongly recommended that you use the CARD_DETECT provided by the SD card connector rather than the pull-up resistor on DAT3. This is because the MAX13035E is not compatible with the pull-up technique of card detection.