TPS2201, TPS2201Y

DUAL-SLOT PC CARD POWER-INTERFACE SWITCHES

FOR PARALLEL PCMCIA CONTROLLERS

SLVS094B – AUGUST 1994 – REVISED AUGUST 1995

6–14

POST OFFICE BOX 655303

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 and was

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

goal was to realize the concept of plug and play – 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

were originally specified as separate signals but are commonly tied together in the host to form a single node

supply up to 1 A, assuming equal pin resistance and no pin failure. A conservative design would limit current

other hand, are defined by flash-memory programming requirements, typically under 120 mA.

future power trends

The 1-A physical-pin 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 will require

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

able to support them 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

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

adhoc 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. If a system is designed to support 1 A, then the switch

designing around 1-A delivery

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

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 terminal-resistance

drops, which leaves 2% (100 mV) voltage drop for 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