LTC4558

8

4558f

OPERATION

The LTC4558 features two independent SIM/Smart Card

channels. Only one of these channels may be open for

communication at a time however both channels can be

enabled and made ready for communication using the

ENABLEA and ENABLEB pins. This allows faster transi-

tion from one channel to the other. Each channel is able

to produce two voltage levels, 1.8V and 3V. The channel

selection and voltage selection are controlled by the CSEL,

VSELA and VSELB pins as shown in the table below:

Bidirectional Channels

The bidirectional channels are level shifted to the appro-

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 receiving side will be

dependent upon the voltage at the transmitting 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, its

I/O pin will be pulled HIGH and communication with the

DATA pin will be disabled. If both channels are disabled,

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 bidirectional 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 refer-

ence 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 RST pin of the

selected card. When a card is deselected, the last state of

the RSTA,B pin is latched. This allows a deselected card to

remain active, and therefore eliminates delays associated

with card initialization.

Clock Run Mode

Various SIM/Smart Cards may have different requirements

for the state of the clock pin when the channel is not open

for communication. The CLKRUNA,B pins allow the user to

select whether the clock is brought LOW after the channel

is deselected or allowed to run. If a channel is enabled,

bringing its CLKRUN pin HIGH will transmit the clock to

the corresponding card socket, whether or not the channel

is selected using the CSEL.

Figure 1. Dynamic Pull-Up Current Source

Table 1. Channel and Voltage Truth Table

CSEL

VSELA

VSELB

SELECTED

CARD

VOLTAGES

A

B

0

0

0

A

1.8V

1.8V

0

0

1

A

1.8V

3V

0

1

0

A

3V

1.8V

011

A

3V

3V

1

0

0

B

1.8V

1.8V

1

0

1

B

1.8V

3V

1

1

0

B

3V

1.8V

111

B

3V

3V

4558 F01

BIDIRECTIONAL

PIN

LOCAL

SUPPLY

dv

dt

–

+