Design Note 93: PCMCIA Socket Voltage Switching

Introduction

Most portable systems have built-in PCMCIA sockets as the sole means of expansion. The requirements of the PCMCIA specification have led to some confusion among system designers. This Design Note will attempt to lessen the confusion and highlight other practical system issues.

Host power delivery to the PC card socket flows through two paths: the main V_CC supply pins and the VPP programming pins. Both supplies are switchable to different voltages to accommodate a wide range of card types. The V_CC main card supply must be capable of delivering up to 1A at either 3.3V or 5V. The 1A rating is an absolute maximum derived from the contact rating of 500mA per pin for both V_CC pins and assumes that both pins are in good condition and current is shared equally. One of the most stringent actual current requirements is during hard drive spin-up. Present hard drives require 5V at 600mA to 800mA for a short duration during spin-up. Current draw drops to 300mA to 420mA during read and write operations. A low switch resistance on the 3.3V switch is critical to assure that the specified 3.0V minimum is maintained. The VPP supply must source 12V at up to 120mA and 3.3V or 5V at lesser currents. The VPP supply is intended solely for flash memory programming. The 120mA current requirement allows writing to flash devices and simultaneously erasing two other parts as required by many flash drives.

The host PCMCIA socket designer also has several other practical aspects of the design to consider. The exposed socket pins are vulnerable to being shorted by foreign objects such as paper clips. In addition, the users will attempt to install damaged cards. In short, once in the hands of the consumer, the designer and manufacturer have little control over use and abuse. To ensure a robust system and a satisfied customer, switch protection features such as current limiting and thermal shutdown are a necessity. The nature of the PC cards and portable systems requires the card being powered on and off as needed to conserve power. Many PC cards have large input capacitance and draw over 2W. The power up/down sequencing can put demanding transient requirements on your system power supply. To make the transient response of the system supply manageable, the PCMCIA switch should have break before-make switching, controlled rise and fall times and current limiting. The slowed rise time coupled with current limiting are critical in controlling the immense in-rush current difficulties seen when charging the large input capacitance of many cards.

LTC1472: Complete VCC and VPP PCMCIA Switch Matrix with Safe

The LTC1472 is a complete, fully integrated V_CC and VPP switch matrix that addresses all of the PCMCIA socket switching needs. Figure 1 shows a typical LTC1472 application used in conjunction with the LT1301 to supply 12V for flash memory programming. The LTC1472’s logic inputs allow direct interfacing with both logic high and logic low industry standard controllers without any external glue logic. The LTC1472 is available in the space saving narrow 16-pin SOIC package. The V_CC switch’s R_DS(ON) is 0.14Ω to support the 1A current requirement. The V_CC output is switched between 3.3V, 5V and high impedance. The VPP output pin is switched between 0V, V_CC, 12V and high impedance. Table 1 shows the V_CC and VPP truth tables.

The LTC1472 features SafeSlot protection. The built-in SafeSlot current limiting and thermal shutdown features are vital to ensuring a robust and reliable system. The V_CC current limit is above 1A to maintain compatibility with all existing cards yet provide protection. The VPP current limit is above 120mA to also maintain compatibility. All switches are break-before-make type with controlled, slowed rise and fall times for minimal system supply impact. In-rush current, from even the largest card input capacitance, is kept under control by the LTC1472’s slowed rise-time switching and current limiting.

The LTC1472 has on-chip charge pumps for switch driving. For this reason, the device does not require a continuous 12V source. Most of the time the LT1301 is in shutdown mode, consuming only 10μA. The LT1301 becomes operational only during flash memory programming. The LTC1472 itself conserves power by going to a low 1μA standby mode when VC_C and VPP outputs are switched off. The use of the LT1301 is optional. Any suitable 12V supply can be directly connected to the VPP_IN pin. Caution should be exercised when using a general purpose 12V supply; make certain that it does not have spikes or transients exceeding the fl ash memory 14V maximum voltage rating and that the regulation is within the 5% flash memory tolerance.

Conclusion

PCMCIA sockets are the preferred method of expansion in portable systems. As these devices proliferate to less sophisticated users, there will be greater opportunity for abuse. To counter this trend the portable system design must take safeguards to protect the system. The high level of integration, SafeSlot protection features and controlled rise and fall switching make the LTC1472 the ideal solution for portable systems.

Linear Technology has a family of PCMCIA socket voltage control products to suit a broad range of customer’s needs. Table 2 lists the present part offerings. For assistance on your specific design needs, call Linear Technology.

作者

Doug La Porte

Doug La Porte是ADI公司的设计经理，从事广泛的市场频率合成、硅振荡器和TimeBlox产品方面的工作。Doug拥有圣何塞州立大学电气工程学士学位和数学学士学位。他负责工业伺服器、通信系统和医学成像的硬件设计，并负责滤波器和振荡器领域的集成电路设计。Doug的爱好包括骑自行车、踢足球、看足球和谈论足球。