LTC3555/LTC3555-X
22
3555fe
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the LTC3555 family will remember the last input of valid
data that it received. Once all chips on the bus have been
addressed and sent valid data, a global STOP condition can
be sent and the LTC3555 family will update its command
latch with the data that it had received.
In certain circumstances the data on the I
2
C bus may
become corrupted. In these cases the LTC3555 family
responds appropriately by preserving only the last set of
complete data that it has received. For example, assume
the LTC3555 family has been successfully addressed and is
receiving data when a STOP condition mistakenly occurs.
The LTC3555 family will ignore this STOP condition and
will not respond until a new START condition, correct ad-
dress, new set of data and STOP condition are transmitted.
Likewise, with only one exception, if the LTC3555 family was
previously addressed and sent valid data but not updated
with a STOP, it will respond to any STOP that appears on
the bus, independent of the number of REPEAT-STARTS
that have occurred. If a REPEAT-START is given and the
LTC3555 family successfully acknowledges its address and
first byte, it will not respond to a STOP until both bytes
of the new data have been received and acknowledged.
Disabling the I
2
C Port
The I
2
C serial port can be disabled by grounding the DV
CC
pin. In this mode, control automatically passes to the in-
dividual logic input pins EN1, EN2, EN3, I
LIM0
, I
LIM1
, SDA
and SCL. Some functionality is not available in this mode
such as the programmability of switching regulators 2
and 3’s output voltage and the battery charger disable
feature. In this mode, both of the programmable switching
regulators have a fixed servo voltage of 0.8V.
Because the SDA and SCL pins have no other context when
DV
CC
is grounded, these pins are re-mapped to control
the switching regulator mode bits B5 and B6. SCL maps
to B5 and SDA maps to B6.
RST3 Pin
The RST3 pin is an open-drain output used to indicate that
switching regulator 3 has reached its final voltage. RST3
remains low impedance until regulator 3 reaches 92% of
its regulation value. A 230ms delay is included to allow a
system microcontroller ample time to reset itself. RST3
may be used as a power-on reset to the microprocessor
powered by regulator 3 or may be used to enable regulators
1 and/or 2 for supply sequencing. RST3 is an open-drain
output and requires a pull-up resistor to the output volt-
age of regulator 3 or another appropriate power source.
General Purpose Step-Down Switching Regulators
The LTC3555 family contains three general purpose
2.25MHz step-down constant-frequency current mode
switching regulators. Two regulators provide up to 400mA
and a third switching regulator can produce up to 1A.
All three switching regulators can be programmed for a
minimum output voltage of 0.8V and can be used to power
a microcontroller core, microcontroller I/O, memory, disk
drive or other logic circuitry. Two of the switching regulators
have I
2
C programmable set-points for on-the-fly power
savings. All three converters support 100% duty cycle
operation (low dropout mode) when their input voltage
drops very close to their output voltage. To suit a variety
of applications, selectable mode functions can be used
to trade-off noise for efficiency. Four modes are available
to control the operation of the LTC3555 family’s general
purpose switching regulators. At moderate to heavy loads,
the pulse skip mode provides the least noise switching
solution. At lighter loads, either Burst Mode operation,
forced Burst Mode operation or LDO mode may be selected.
The switching regulators include soft-start to limit inrush
current when powering on, short-circuit current protection
and switch node slew limiting circuitry to reduce radiated
EMI. No external compensation components are required.
The operating mode of the regulators may be set by either
I
2
C control or by manual control of the SDA and SCL pins
if the I
2
C port is not used. Each converter may be individu-
ally enabled by either their external control pins EN1, EN2,
EN3 or by the I
2
C port. Switching regulators 2 and 3 have
individual programmable feedback servo voltages via I
2
C
control. The switching regulator input supplies V
IN1
, V
IN2
and V
IN3
will generally be connected to the system load
pin V
OUT
.
OPERATION
