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
regulated 5V V
CC
supply for the SIM and provide level
translators for communication between the SIM and the
controller.
V
CC
Voltage Regulator
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
maintaining a regulated V
CC
output. The wide V
IN
range
enables the parts to be powered directly from a battery (if
desired) rather than from a 3V DC/DC converter output.
When V
IN
is less than the desired V
CC
the parts operate as
switched capacitor voltage doublers. When V
IN
is greater
than V
CC
the parts operate as gated switch step-down
converters. In either case, voltage conversion requires
only one small flying capacitor and output capacitor.
The V
CC
output can be programmed to either 5V or 3V via
the M0 and M1 mode pins. This feature is useful in
applications where either a 5V or 3V SIM may be used. The
charge pump V
CC
output may also be connected directly to
V
IN
if desired. 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.
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-
fective inrush current out of V
IN
to approximately 25mA
(C
OUT
= 10µF). Inrush current limiting is especially useful
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
start feature may be overridden by tying the SS pin to DV
CC
.
APPLICATIONS INFORMATION
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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 10µF 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 22µF or greater
will reduce output voltage ripple—particularly with high V
IN
voltages (8V 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 LTC1555/LTC1556 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
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 (V
IN
< 8V) under maximum load is
75mV peak-to-peak with a low ESR, 10µF output capaci-
tor. For applications requiring V
IN
to exceed 8V, a 22µF or
larger C
OUT
capacitor is recommended to maintain maxi-
mum ripple in the 75mV range.
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 (< 10µF) 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 10µF ceramic capacitor on the V
CC
pin should produce
acceptable levels of output voltage ripple in nearly all
applications. However, there are several ways to further
