LTC3675_12 Datasheet, PDF (17/38 Pages) Linear Technology – 7-Channel Confi gurable High Power PMIC

LTC3675
OPERATION
The LTC3675 has six monolithic synchronous switching
regulators and a dual string boost LED driver and is designed
to operate from a single Li-Ion battery. All of the switching
regulators and the LED driver are internally compensated
and need only external feedback resistors for regulation.
The switching regulators also offer two operating modes:
Burst Mode operation for higher efficiency at light loads
and pulse-skipping/PWM mode. In Burst Mode operation
at light loads, the output capacitor is charged to a voltage
slightly higher than its regulation point. The regulator then
goes into sleep, during which the output capacitor provides
the load current. In sleep most of the regulator’s circuitry
is powered down, helping conserve battery power. When
the output capacitor droops below its programmed value,
the circuitry is powered on and another burst cycle begins.
The sleep time decreases as load current increases.
All switching regulators and LED driver may be configured
via I²C, providing the user with the flexibility to operate the
LTC3675 in the most efficient manner. I²C commands can
also be read back via the I²C port, to ensure a command
was not corrupted during a transmission.
All the regulators can be enabled via I²C commands. The
buck regulators and the buck-boost regulator may also be
enabled via enable pins. The enable pins have two different
enable threshold voltages that depend on the operating state
of the LTC3675. With all regulators disabled, the enable pin
threshold is at 650mV. If any regulator is enabled either
by its enable pin or an I²C command, then the enable pin
thresholds are at 400mV. A precision comparator detects
a voltage greater than 400mV on the enable pin and turns
that regulator on. This precision threshold may be used to
sequentially enable regulators. If all regulators are disabled,
all the command registers are set in their default state.
There are also 2 bytes of data that report any fault condi-
tions on the LTC3675 via I²C read back.
The buck regulators can operate in either of two modes. In
pulse-skipping mode, the regulator will skip pulses at light
loads but will operate at a constant frequency of 2.25MHz
at higher loads. In Burst Mode operation, the regulator will
burst at light loads whereas at higher loads it will operate
at constant frequency PWM mode of operation, much the
same as pulse-skipping mode at high load. In shutdown,
an I²C control bit provides the flexibility to either keep the
SW node in a high impedance state or pull the SW node
to GND through a 10k resistor.
The buck regulators have forward and reverse current
limiting, soft-start to limit inrush current during start-up,
short-circuit protection and slew rate control for lower
radiated EMI.
Each buck regulator may be enabled via its enable pin or
I²C. The mode of operation, the feedback regulation volt-
age and switch slew rate can all be controlled via I²C. For
applications that require higher power, buck regulators
may be combined together.
BUCK REGULATORS WITH COMBINED POWER STAGES
Two adjacent buck regulators may be combined in a
master-slave configuration by connecting their SW pins
together and connecting the higher numbered buck’s FB
pin to the input supply. The lower numbered buck is always
the master. In Figure 1, buck regulator 1 is the master. The
feedback network connected to the FB1 pin programs the
V_IN
SW1
BUCK REGULATOR 1
(MASTER)
EN1
FB1
V_IN
L1
400k
800k
C_OUT
1.2V
2A
V_OUT
BUCK SWITCHING REGULATOR
The LTC3675 contains four buck regulators. Two of the
buck regulators are designed to deliver up to 1A load
current each while the other two regulators can deliver
up to 500mA each.
SW2
BUCK REGULATOR 2 ^VIN
(SLAVE)
EN2
FB2
3675 F01
Figure 1. Buck Regulators Configured as Master-Slave
3675fa
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