7

LTC4557

4557f

OPERATIO

The LTC4557 features two independent smart card chan-

nels. Only one of these channels may operate at a time.

Each channel is able to output two voltage levels: 1.8V

and 3V. The channel selection and voltage selection are

controlled by the ENABLE, M0 and M1 pins as shown in

Table 1.

Table 1. Channel and Voltage Truth Table

ENABLE

M1

M0

SELECTED CARD/VOLTAGE

1

0

0

Card A/1.8V

1

0

1

Card A/3V

1

1

0

Card B/1.8V

1

1

1

Card B/3V

0

X

X

A and B Disabled

Bidirectional Channels

The bidirectional channels are level shifted to the appropri-

transistor performs the level shifting. The gate of the

NMOS transistor is biased such that the transistor is

completely off when both sides have relinquished the

channel. If one side of the channel asserts a LOW, then the

transistor will convey the LOW to the other side. Note that

current passes from the receiving side of the channel to

the transmitting side. The low output voltage of the receiv-

ing side will be dependent upon the voltage at the transmit-

ting side plus the IR drop of the pass transistor.

When a card socket is selected, it becomes a candidate to

drive data on the DATA pin and likewise receive data from

the DATA pin. When a card socket is deselected, the

voltage on its I/OA,B pin will be disabled and set to LOW.

If both cards are deselected, a weak pull-up ensures that

the DATA pin is held HIGH.

Dynamic Pull-up Current Sources

The current sources on the bidirectional pins (DATA/

I/OA,B) are dynamically activated to achieve a fast rise

time with a relatively small static current. Once a bidirec-

tional pin is relinquished, a small start-up current begins

to charge the node. An edge rate detector determines if the

pin is released by comparing its slew rate with an internal

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 affecting a dynamic form of hysteresis.

Reset Channels

When a card is selected, the reset channel provides a level

shifted path from the RSTIN pin to the RSTA,B pin. When

a card is deselected its reset pin is pulled LOW.

4557 F01

BIDIRECTIONAL

PIN

LOCAL

SUPPLY

dv

dt

–

+

Figure 1. Dynamic Pull-Up Current Source