TPS2202AI

DUAL-SLOT PC CARD POWER-INTERFACE SWITCH

WITH RESET FOR SERIAL PCMCIA CONTROLLER

SLVS123A – SEPTEMBER 1995 – REVISED JUNE 1998

13

POST OFFICE BOX 655303

POST OFFICE BOX 1443

APPLICATION INFORMATION

overview

PC Cards were initially introduced as a means to add EEPROM (flash memory) to portable computers with

limited on-board memory. The idea of add-in cards quickly took hold; modems, wireless LANs, GPS systems,

multimedia, and hard-disk versions were soon available. As the number of PC Card applications grew, the

engineering community quickly recognized the need for a standard to ensure compatibility across platforms.

To this end, the PCMCIA (Personal Computer Memory Card International Association) was established,

comprised of members from leading computer, software, PC card, and semiconductor manufacturers. One key

goal was to realize the “plug and play” concept. Cards and hosts from different vendors should be compatible

and able to communicate with one another transparently.

PC Card power specification

System compatibility also means power compatibility. The most current set of specifications (PC Card Standard)

set forth by the PCMCIA committee states that power is to be transferred between the host and the card through

currents. Disk drives, for example, may need as much as 750-mA peak current to create the initial torque

requirements, typically under 120 mA.

future power trends

The 1-A physical-terminal current alluded to in the PC Card specification has caused some host-system

engineers to believe they are required to deliver 1 A within the voltage tolerance of the card. Future applications,

such as RF cards, could use the extra power for their radio transmitters. The 5 W required for these cards require

very robust power supplies and special cooling considerations. The limited number of host sockets that are able

to support cards makes the market for these high-powered PC Cards uncertain. The vast majority of the cards

require less than 600 mA continuous current, and the trend is towards even lower powered PC Cards that assure

compatibility with a greater number of host systems. Recognizing the need for power derating, an ad hoc

committee of the PCMCIA is currently working to limit the amount of steady-state dc current to the

PC Card to something less than the currently implied 1 A. When a system is designed to support 1 A, the switch

designing around 1-A delivery

Delivering 1 A means minimizing voltage and power losses across the PC Card power interface, which requires

transistors. The PC Card standard requires that 5 V

±5% or 3.3 V ±0.3 V be supplied to the card. The

conservative approach is to allow 2% for voltage-regulator tolerance and 1% for etch- and pin-resistance drops,

which leaves 2% (100 mV) for voltage drop at the 5-V switch and at least 6% (198 mV) for the 3.3-V switch.

the 5-V and 3.3-V switches would need to be 100 m

Ω and 198 mΩ respectively. One solution would be to pay

of the power supply–for example, to increase the 5 V supply 1.5% or to 5.075

±2%. Working through the

numbers once more, the 2% for the regulator plus 1 % for etch and terminal losses leaves 97% or 4.923 V. The

allowable