ltc1555l-1.8 Linear Technology Corporation, ltc1555l-1.8 Datasheet - Page 6

• Manufacturer Part Number: ltc1555l-1.8
• Description: Sim Power Supply And Level Translator
• Manufacturer: Linear Technology Corporation

LTC1555L-1.8
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
provides level translators for communication between the
SIM and the controller.
V
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
powered directly from a battery (if desired) rather than
from a DC/DC converter output. When V
selected V
capacitor voltage doubler. When V
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
either 1.8V, 3V, 5V or direct connection to V
flexibility is useful in applications where multiple voltage
SIMs may be used. When the charge pump is put into
shutdown (M0, M1 = 0), V
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
Capacitor Selection
For best performance, it is recommended that low ESR
(< 0.5 ) capacitors be used for both C
noise and ripple. The C
either ceramic or tantalum and should be 1 F or greater
(ceramic capacitors will produce the smallest output ripple).
6
APPLICATIO S I FOR ATIO
CC
CC
Voltage Regulator
output. The wide V
CC
output can be programmed via the M0-M2 pins to
CC
voltage, the part operates as a switched
U
CC
IN
IN
to GND.
and C
U
CC
range enables the part to be
CC
supply for the SIM, and also
OUT
is pulled to GND via an
IN
capacitors should be
W
IN
is greater than V
and C
IN
is less than the
OUT
U
to reduce
IN
. This
CC
,
If the input source impedance is very low (< 0.5 ), C
not be needed. Increasing the size of C
will reduce output voltage ripple—particularly with high V
voltages (4V or greater). A ceramic capacitor is recom-
mended for the flying capacitor C1 with a value of 0.1 F or
0.22 F.
Output Ripple
Normal LTC1555L-1.8 operation produces voltage ripple
on the V
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
75mV peak-to-peak with a low ESR, 2.2 F output capaci-
tor (V
The magnitude of the ripple voltage depends on several
factors. High input voltages increase the output ripple
since more charge is delivered to C
A large C1 flying capacitor (> 0.22 F) also increases ripple
in step-up mode for the same reason. Large output current
load and/or a small output capacitor (< 1 F) results in
higher ripple due to higher output voltage dV/dt. High ESR
capacitors (ESR > 0.5 ) on the output pin cause high
frequency voltage spikes on V
A 2.2 F ceramic capacitor on the V
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
the SIM supply rails (V
SIM may be provided two different ways. The first method
is to use the DATA pin as a bidirectional level translator.
CC
CC
= 5V).
pin. Output voltage ripple is required for the
CC
IN
and GND). The data input to the
< 4V) under maximum load is
OUT
OUT
with every clock cycle.
CC
OUT
pin should produce
per charging cycle.
to 2.2 F or greater
CC
and GND) to
IN
may
IN