TPS2205

DUAL-SLOT PC CARD POWER-INTERFACE SWITCH

FOR PARALLEL PCMCIA CONTROLLERS

SLVS128D OCTOBER 1995 – REVISED JUNE 1998

19

POST OFFICE BOX 655303

APPLICATION INFORMATION

output ground switches

Several PCMCIA power-distribution switches on the market do not have an active-grounding FET switch. These

can not be relied on to provide a discharge path for voltages stored on PC Card capacitance because of possible

high-impedance isolation by power-management schemes. A method commonly shown to alleviate this

problem is to add to the switch output an external 100-k

Ω resistor in parallel with the PC Card. Considering that

this is the only discharge path to ground, a timing analysis shows that the RC time constant delays the required

discharge time to more than 2 seconds. The only way to ensure timing compatibility with PC Card standards

is to use a power-distribution switch that has an internal ground switch, like that of the TPS22xx family, or add

an external ground FET to each of the output lines with the control logic necessary to select it.

In summary, the TPS2205 is a complete single-chip dual-slot PC Card power interface. It meets all currently

defined PCMCIA specifications for power delivery in 5-V, 3.3-V, and mixed systems, and offers a serial control

interface. The TPS2205 offers functionality, power savings, overcurrent and thermal protection, and fault

reporting in one 30-pin SSOP surface-mount package, for maximum value added to new portable designs.

power supply considerations

outputs. Any individual pin can conduct the rated input or output current. Unless all pins are connected in

parallel, the series resistance is significantly higher than that specified, resulting in increased voltage drops and

output power pins be paralleled for optimum operation.

Although the TPS2205 is fairly immune to power input fluctuations and noise, it is generally considered good

design practice to bypass power supplies, typically with a 1-

µF electrolytic or tantalum capacitor paralleled by

a 0.047-

bypassed with a 0.1-

µF or larger capacitor; doing so improves the immunity of the TPS2205 to electrostatic

discharge (ESD). Care should be taken to minimize the inductance of PCB traces between the TPS2205 and

the load. High switching currents can produce large negative-voltage transients, which forward biases substrate

diodes, resulting in unpredictable performance. Similary, no pin should be taken below – 0.3 V.

overcurrent and thermal protection

sense resistors or polyfuses, these FETs do not add to the series resistance of the switch; therefore, voltage

and power losses are reduced. Overcurrent sensing is applied to each output separately. When an overcurrent

condition is detected, only the power output affected is limited; all other power outputs continue to function

normally. The OC indicator, normally a logic high, is a logic low when any overcurrent condition is detected,

providing for initiation of system diagnostics and/or sending a warning message to the user.

faulty card or connector. If a short circuit is applied after power is established (e.g., hot insertion of a bad card),

current is initially limited only by the impedance between the short and the power supply. In extreme cases, as

will reestablish normal operation.

The protection circuitry acts by linearly limiting the current passing through the switch rather than initiating a full

shutdown of the supply. Shutdown occurs only during thermal limiting.