8

LTC1756

Low Power Operation

The LTC1756 is inherently a low power device. When there

is no Smart Card present the supply current is less than

2.5

µA. When a Smart Card is present the LTC1756 oper-

ates with a quiescent current of only 75

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 places

the LTC1765 in the Alarm state if the IC’s junction tem-

perature exceeds 150

°C.

on the input supply due to ESR may cause problems with

the internal circuitry of the LTC1756. A good choice for the

input bypass capacitor is a 10

µF Y5V style ceramic

Dynamic Pull-Up Current Sources

The current sources on the bidirectional pins (DATA and

I/O) are dynamically activated to achieve a fast rise time

with a relatively small static current (Figure 1). Once a

bidirectional pin is relinquished, a small start-up current

begins to charge the node. An edge rate detector deter-

mines 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 effecting a dynamic form of hysteresis.

Once the node has reached the power supply voltage the

internal comparator requires several hundred nanosec-

onds 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 bidirec-

tional channels to about 500kHz.

Figure 1. Dynamic Pull-Up Current Sources

–

+

δV

δt

1756 F01

BIDIRECTIONAL PIN