7

LTC1555/LTC1556

The LTC1555/LTC1556 perform the two primary func-

tions necessary for 3V controllers (e.g., GSM cellular

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

municate with 5V SIMs or smart cards. They produce a

translators for communication between the SIM and the

controller.

The regulator section of the LTC1555/LTC1556 (refer to

the Block Diagram) consists of a step-up/step-down charge

pump DC/DC converter. The charge pump can operate

over a wide input voltage range (2.7V to 10V) while

enables the parts to be powered directly from a battery (if

desired) rather than from a 3V DC/DC converter output.

converters. In either case, voltage conversion requires

only one small flying capacitor and output capacitor.

the M0 and M1 mode pins. This feature is useful in

applications where either a 5V or 3V SIM may be used. The

to aid in proper system supply sequencing.

The soft start feature limits inrush currents upon start-up

or coming out of shutdown mode. When the SS pin is tied

to GND, the soft start feature is enabled. This limits the ef-

when powering the LTC1555/LTC1556 from a 3V DC/DC

output since the unlimited inrush current may approach

200mA and cause voltage transients on the 3V supply. How-

ever, in cases where fast turn-on time is desired, the soft

Capacitor Selection

For best performance, it is recommended that low ESR

(< 0.5

either ceramic or tantalum and should be 10

µF or greater

(ceramic capacitors will produce the smallest output ripple).

If the input source impedance is very low (< 0.5

voltages (8V or greater). A ceramic capacitor is recom-

mended for the flying capacitor C1 with a value of 0.1

µF or

0.22

µF.

Output Ripple

Normal LTC1555/LTC1556 operation produces voltage

the parts to regulate. Low frequency ripple exists due to

the hysteresis 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.

75mV peak-to-peak with a low ESR, 10

µF output capaci-

mum ripple in the 75mV range.

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 (< 10

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 10

acceptable levels of output voltage ripple in nearly all

applications. However, there are several ways to further