Datasheet

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   
   
SLVS317 − MAY 2001
20
POST OFFICE BOX 655303 DALLAS, TEXAS 75265
APPLICATION INFORMATION
overcurrent and overtemperature 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 extremely robust
systems could undergo rapid battery discharge into a damaged PC Card, resulting in the rather sudden and
unacceptable loss of system power. The reliability of fused systems is poor, in comparison, as blown fuses
require troubleshooting and repair, usually by the manufacturer.
The TPS2223, TPS2224 and TPS2226 take a two-pronged approach to overcurrent protection, which is
designed to activate if an output is shorted or when an overcurrent condition is present when switches are
powered up. First, instead of fuses, sense FETs monitor each of the xVCC and xVPP power outputs. Unlike
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. Excessive current
generates an error signal that limits the output current of only the affected output, preventing damage to the host.
Each xVCC output overcurrent limits from 1 A to 2.2 A, typically around 1.6 A; the xVPP outputs limit from 100
mA to 250 mA, typically around 200 mA.
Second, when an overcurrent condition is detected, the TPS2223, TPS2224 and TPS2226 assert an active low
OC
signal that can be monitored by the microprocessor or controller 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, which is ensured by a thermal shutdown hysteresis. Thermal limiting
prevents destruction of the IC from overheating beyond the package power-dissipation ratings.
During power up, the devices control the rise times of the xVCC and xVPP outputs and limit the inrush current
into a large load capacitance, faulty card, or connector.
12-V supply not required
A few PC Card switches use the externally supplied 12 V to power gate drive and other chip functions, which
requires that power be present at all times. The TPS2224 and TPS2226 offer considerable power savings by
using an internal charge pump to generate the required higher gate drive voltages from the VA input (3.3 V).
Therefore, the external 12-V supply can be disabled except when needed by the PC Card in the slot, thereby
extending battery lifetime. A special feature in the 12-V circuitry actually helps to reduce the supply current
demanded from the 3.3 V input. When 12 V is supplied and requested at the VPP output, a voltage selection
circuit will draw the charge-pump drive current for the 12-V FETs from the 12-V input. This selection is automatic
and effectively reduces demand fluctuations on the normal 3.3-V VCC rail. For proper operation of this feature,
a minimum 3.3-V input capacitance of 4.7 µF is recommended, and a minimum 12-V input ramp-up rate of 12
V/50 ms (240 V/s) is required. Additional power savings are realized by the TPS2226 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 TPS2223, TPS2224 and TPS2226 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
action ensures that sensitive 3.3-V circuitry is not subjected to any residual 5-V charge and functions as a power
reset. PC Card specification requires that V
CC
be discharged within 100 ms. PC Card resistance cannot 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. The devices include discharge transistors on all
xVCC and xVPP outputs to meet the specification requirement.