TPS2201, TPS2201Y

DUAL-SLOT PC CARD POWER-INTERFACE SWITCHES

FOR PARALLEL PCMCIA CONTROLLERS

SLVS094B – AUGUST 1994 – REVISED AUGUST 1995

6–15

POST OFFICE BOX 655303

APPLICATION INFORMATION

designing around 1-A delivery (continued)

voltage loss across the power distribution switch is now 4.923 V minus 4.750 V or 173 mV. Therefore, a switch

with 173 m

Ω or less could deliver 1 A or greater. Setting the power supply high is a common practice for

delivering voltages to allow for system switch, connector, and etch losses and has a minimal effect on overall

battery life. In the example above, setting the power supply 1.5% high would only decrease a 3-hour battery life

by approximately 2.7 minutes, trivial when compared with the decrease in battery life when running a 5-W PC

Card.

heat dissipation

A greater concern in delivering 1 A or 5 W is the ability of the host to dissipate the heat generated by the PC

Card. For desktop computers the solution is simpler: locate the PC Card cage such that it receives convection

cooling from the forced air of the fan. Notebooks and other handheld equipment will not be able to rely on

convection, but must rely on conduction of heat away from the PC Card through the rails into the card cage. This

is difficult because PC Card/card cage heat transfer is very poor. A typical design scenario would require the

PC Card to be held at 60

°C maximum with the host platform operating as high as 50°C. Preliminary testing

reveals that a PC Card can have a 20

°C rise, exceeding the 10°C differential in the example, when dissipating

less than 2 W of continuous power. The 60

°C temperature was chosen because it is the maximum operating

temperature allowable by PC Card specification. Power handling requirements and temperature rises are topics

of concern and are currently being addressed by the PCMCIA committee.

overcurrent and over-temperature protection

PC Cards are inherently subject to damage that can result from mishandling. Host systems require protection

against short-circuited cards that could lead to power supply or PCB-trace damage. Even systems sufficiently

robust to withstand a short circuit would still undergo rapid battery discharge into the damaged PC Card,

resulting in the rather sudden and unacceptable loss of system power. This can be particularly frustrating to the

consumer who has already experienced problems with shortened battery life due to improper Nicad conditioning

or memory effect. Most hosts include fuses for protection. The reliability of fused systems is poor, though, as

blown fuses require troubleshooting and repair, usually by the manufacturer. The TPS2201 takes a two-pronged

approach to overcurrent protection. First, instead of fuses, sense FETs monitor each of the power outputs.

Excessive current generates an error signal that linearly limits the output current, preventing host damage or

failure. Sense FETs, unlike sense resistors or polyfuses, have the added advantage that they do not add to the

series resistance of the switch and thus produce no additional voltage losses. Second, when an overcurrent

condition is detected, the TPS2201 asserts a signal at OC that can be monitored by the microprocessor to initiate

diagnostics and/or send the user a warning message. In the event that an overcurrent condition persists,

causing the IC to exceed its maximum junction temperature, thermal-protection circuitry engages, shutting

down all power outputs until the device cools to within a safe operating region.

12-V supply not required

functions, requiring that it be present at all times. The TPS2201 offers considerable power savings by using an

12-V supply can be disabled except when needed for flash-memory functions, thereby extending battery

lifetime. Additional power savings are realized by the TPS2201 during a software shutdown, in which quiescent

current drops to a maximum of 1

µA.

voltage transitioning requirement

PC Cards, like portables, are migrating from 5 V to 3.3 V to minimize power consumption, optimize board space,

and increase logic speeds. The TPS2201 is designed to meet all combinations of power delivery as currently

defined in the PCMCIA standard. The latest protocol accommodates mixed 3.3-V/5-V systems by first powering

the card with 5 V, then polling it to determine its 3.3-V compatibility. The PCMCIA specification requires that the