Datasheet

LTC1555L-1.8
6
APPLICATIO S I FOR ATIO
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The LTC1555L-1.8 performs the two primary functions
necessary 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
a regulated 1.8V, 3V or 5V V
CC
supply for the SIM, and also
provides level translators for communication between the
SIM and the controller.
V
CC
Voltage Regulator
The regulator section of the LTC1555L-1.8 (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
V
CC
output. The wide V
IN
range enables the part to be
powered directly from a battery (if desired) rather than
from a DC/DC converter output. When V
IN
is less than the
selected V
CC
voltage, the part operates as a switched
capacitor voltage doubler. When V
IN
is greater than V
CC
,
the part operates as gated switch step-down converter. In
either case, voltage conversion requires only one small
flying capacitor and output capacitor.
The V
CC
output can be programmed via the M0-M2 pins to
either 1.8V, 3V, 5V or direct connection to V
IN
. This
flexibility is useful in applications where multiple voltage
SIMs may be used. When the charge pump is put into
shutdown (M0, M1 = 0), V
CC
is pulled to GND via an
internal switch to aid 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-1.8 from a DC/DC output since the unlim-
ited 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
indefinite short from V
CC
to GND.
Capacitor Selection
For best performance, it is recommended that low ESR
(<0.5Ω) capacitors be used for both C
IN
and C
OUT
to reduce
noise and ripple. The C
IN
and C
OUT
capacitors should be
either ceramic or tantalum and should be 1µF or greater
(ceramic capacitors will produce the smallest output ripple).
If the input source impedance is very low (< 0.5Ω), C
IN
may
not be needed. Increasing the size of C
OUT
to 2.2µF or greater
will reduce output voltage ripple—particularly with high V
IN
voltages (4V or greater). A ceramic capacitor is recom-
mended for the flying capacitor C1 with a value of 0.1µF or
0.22µF.
Output Ripple
Normal LTC1555L-1.8 operation produces voltage ripple
on the V
CC
pin. Output voltage ripple is required for the
parts to regulate. Low frequency ripple exists due to the
hysteresis in the sense comparator and propagation de-
lays 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 (V
IN
< 4V) under maximum load is
75mV peak-to-peak with a low ESR, 2.2µF output capaci-
tor (V
CC
= 5V).
The magnitude of the ripple voltage depends on several
factors. High input voltages increase the output ripple
since more charge is delivered to C
OUT
per charging cycle.
A large C1 flying capacitor (> 0.22µF) also increases ripple
in step-up mode for the same reason. Large output current
load and/or a small output capacitor (< 1µF) results in
higher ripple due to higher output voltage dV/dt. High ESR
capacitors (ESR > 0.5Ω) on the output pin cause high
frequency voltage spikes on V
OUT
with every clock cycle.
A 2.2µF ceramic capacitor on the V
CC
pin should produce
acceptable levels of output voltage ripple in nearly all
applications. Also, in order to keep noise down all capaci-
tors should be placed close to LTC1555L-1.8.
Level Translators
All SIMs and smart cards contain a clock input, a reset
input, and a bidirectional data input/output. The
LTC1555L-1.8 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 controller supply rails (DV
CC
and GND) to
the SIM supply rails (V
CC
and GND). The data input to the
SIM may be provided two different ways. The first method
is to use the DATA pin as a bidirectional level translator.