LTC1555L

6

APPLICATIO S I FOR ATIO

The LTC1555L performs the two primary functions nec-

essary for low voltage controllers (e.g., GSM cellular

telephone controllers, smart card readers, etc.) to com-

municate with 5V SIMs or smart cards. The part produces

provides level translators for communication between the

SIM and the controller.

The regulator section of the LTC1555L (refer to Block

Diagram) consists of a buck/boost charge pump DC/DC

converter. The charge pump can operate over a wide input

voltage range (2.6V to 6V) while maintaining a regulated

powered directly from a battery (if desired) rather than

the part operates as gated switch step-down converter. In

either case, voltage conversion requires only one small

flying capacitor and output capacitor.

useful in applications where multiple voltage SIMs may be

used. When the charge pump is put into shutdown (M0,

in proper system supply sequencing.

An internal soft-start feature helps to limit inrush currents

upon start-up or when coming out of shutdown mode.

Inrush current limiting is especially useful when powering

the LTC1555L from a DC/DC output since the unlimited

inrush current may approach 300mA and cause voltage

transients on the 3V supply. The part is fully short-circuit

and over temperature protected, and can survive an indefi-

Capacitor Selection

For best performance, it is recommended that low ESR

(< 0.5

either ceramic or tantalum and should be 1

µF or greater

(ceramic capacitors will produce the smallest output ripple).

If the input source impedance is very low (< 0.5

voltages (4V or greater). A ceramic X5R or X7R type

capacitor is recommended for the flying capacitor C1 with

a value of 0.1

µF or 0.22µF.

Output Ripple

Normal LTC1555L operation produces voltage ripple on

to regulate. Low frequency ripple exists due to the hyster-

esis in the sense comparator and propagation delays in the

charge pump enable/disable circuits. High frequency ripple

is also present mainly from the ESR (equivalent series

resistance) in the output capacitor. Typical output ripple

with a low ESR, 2.2

The magnitude of the ripple voltage depends on several

factors. High input voltages increase the output ripple

A large C1 flying capacitor (> 0.22

µF) also increases ripple

in step-up mode for the same reason. Large output current

load and/or a small output capacitor (< 1

µF) results in

higher ripple due to higher output voltage dV/dt. High ESR

capacitors (ESR > 0.5

Ω) on the output pin cause high

A 2.2

acceptable levels of output voltage ripple in nearly all

applications. Also, in order to keep noise down to a

minimum all capacitors should be placed close to

LTC1555L.

Level Translators

All SIMs and smart cards contain a clock input, a reset

input, and a bidirectional data input/output. The LTC1555L

provides level translators to allow controllers to

communicate with the SIM. (See Figure 1a and 1b). The

CLK and RST inputs to the SIM are level shifted from the

provided two different ways. The first method is to use the

DATA pin as a bidirectional level translator. This configu-

ration is only allowed if the controller data output pin is