TPS2205

DUAL-SLOT PC CARD POWER-INTERFACE SWITCH

FOR PARALLEL PCMCIA CONTROLLERS

SLVS128D OCTOBER 1995 – REVISED JUNE 1998

18

POST OFFICE BOX 655303

APPLICATION INFORMATION

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, as blown fuses require troubleshooting and repair, usually by the

manufacturer.

The TPS2205 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 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 TPS2205 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 TPS2205 offers considerable power savings

by using an internal charge pump to generate the required higher voltages from the 5-V or 3.3-V input; therefore,

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 the 12-V input is not used. Additional power savings are

realized by the TPS2205 during a software shutdown in which quiescent current drops to a maximum of 1

µA.

backward compatibility and 3.3-V low-voltage mode

TPS2201) is a no connect because bias current is derived from either the 3.3-V input pin or the 5-V input pin.

Also, the TPS2205 does not have the APWR_GOOD or BPWR_GOOD VPP reporting outputs. These are left

as no connects.

allows host and PC Cards to be operated in low-power 3.3-V-only modes such as sleep modes or pager modes.

Note that in this operation mode, the TPS2205 derives its bias current from the 3.3-V input pin and only 3.3 V

can be delivered to the PC Card. The 3.3-V switch resistance will be increased, but the added switch resistance

should not be critical, because only a small amount of current is delivered in this mode. If 6% (198 mV) is allowed

for the 3.3-V switch voltage drop, a 500 m

Ω switch could deliver over 350 mA to the PC Card.

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 TPS2205 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

capacitors on 3.3-V-compatible cards be discharged to below 0.8 V before applying 3.3-V power. This ensures

that sensitive 3.3-V circuitry is not subjected to any residual 5-V charge and functions as a power reset. The