9

LTC1755

Self-Start Mode

By connecting the CARD pin to the CS and PWR pins the

LTC1755 can be made to start up automatically when a

Smart Card is detected (Figure 2). In this mode, the READY

pin becomes an interrupt signal indicating to the micro-

charge pump voltage, is at its final value. The Smart Card

remains powered as long as it is detected by the PRES pin.

When the Smart Card is removed the LTC1755 will auto-

matically be deactivated by the fault detection circuitry.

reference value. If a valid transition is detected, a large

pull-up current enhances the edge rate on the node. The

higher slew rate corroborates the decision to charge the

node thereby effecting a dynamic form of hysteresis. Once

the node has reached the power supply voltage the internal

comparator requires several hundred nanoseconds to

reset. Pulling down on the pin before the reset delay

expires will result in a momentary contention and a higher

current flow. Therefore, the comparator delay sets the

upper limit on the maximum data rate of the bidirectional

channels to about 500kHz.

Synchronous Card Applications

For multicard synchronous applications the LTC1755 has

separate control pins for the CLK and I/O lines. The PWR

pin enables the charge pump as well as the CLK and RST

channels while the CS pin enables only the bidirectional

channels. In multicard applications the CS pin can be used

to disable communication to one card while leaving its

clock line running. With this option the same I/O pin can

be used for multiple cards. For simpler applications the CS

and PWR pins can be connected together to enable or

disable the entire chip.

Low Power Operation

The LTC1755 is inherently a low power device. When there

is no Smart Card present the supply current is less than

10

Smart Card is present the LTC1755 operates with a quies-

cent current of only 60

µA, thus the majority of power is

consumed by charge pump losses and the card itself. If the

card can be made to consume less power during idle times

a significant power savings will be achieved. Whenever

possible Clock Stop Mode should be used (or alternatively

a very low “idling” clock speed). Furthermore, in the Active

state, the bidirectional pins should all be relinquished

whenever possible since there is some static current flow

when a bidirectional pin is pulled down.

Overtemperature Fault Protection

An overtemperature circuit disables the chip and activates

the ALARM pin if the IC’s junction temperature exceeds

150

°C.

Figure 2. Self-Start Mode

CARD

PWR

CS

1755 F02

Deactivation Sequence

For maximum flexibility the Smart Card can be deactivated

either manually or automatically. In manual mode the

deactivation is controlled by explicitly manipulating the

LTC1755 input and control pins (DATA, AUX1IN, AUX2IN,

RIN and CIN followed by PWR and CS). In automatic mode

the PWR pin is used to perform the built-in deactivation

sequence. Once PWR is brought high the built-in

deactivation sequence occurs as follows:

1755 F03

RST

CLK

I/O

AUX2

AUX1

DEACTIVATION DIRECTIVE

I/O = DATA

Figure 3. Deactivation Sequence

In the event of a fault, the LTC1755 automatically

implements the built-in deactivation sequence.