TPS2202AI

DUAL-SLOT PC CARD POWER-INTERFACE SWITCH

WITH RESET FOR SERIAL PCMCIA CONTROLLER

SLVS123A – SEPTEMBER 1995 – REVISED JUNE 1998

14

POST OFFICE BOX 655303

POST OFFICE BOX 1443

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. This practice 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 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. Sixty degrees centigrade 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. 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 TPS2202AI 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 an added

advantage in 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 TPS2202AI 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 activates, shutting down all power outputs until the device cools to within a safe

operating region.

12-V supply not required

functions, which requires that power be present at all times. The TPS2202AI offers considerable power savings

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

battery lifetime. Do not ground the 12-V inputs when 12-V supply is not in use. Additional power savings are

realized by the TPS2202AI during a software shutdown in which quiescent current drops to a maximum of

1

µA.