LTC3370_15 Datasheet, PDF (15/24 Pages) Linear Technology – 4-Channel 8A Configurable Buck DC/DCs

LTC3370
Operation
Buck Switching Regulators
The LTC3370 contains eight monolithic 1A synchronous
buck switching channels. These are controlled by up to
four current mode regulator controllers. All of the switch-
ing regulators are internally compensated and need only
external feedback resistors to set the output voltage. The
switching regulators offer two operating modes: Burst
Mode operation (PLL/MODE = LOW) for higher efficiency
at light loads and forced continuous PWM mode (PLL/
MODE = HIGH or switching) for lower noise at light loads.
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 a sleep state, during
which time the output capacitor provides the load current.
In sleep most of the regulator’s circuitry is powered down,
helping conserve input power. When the output capaci-
tor droops below its programmed value, the circuitry is
powered on and another burst cycle begins. The sleep
time decreases as load current increases. In Burst Mode
operation, the regulator bursts at light loads whereas at
higher loads it operates at constant frequency PWM mode
operation. In forced continuous mode, the oscillator runs
continuously and the buck switch currents are allowed
to reverse under very light load conditions to maintain
regulation. This mode allows the buck to run at a fixed
frequency with minimal output ripple.
Each buck switching regulator can operate at an indepen-
dent V_IN voltage and has its own FB and EN pin to maxi-
mize flexibility. The enable pins have two different enable
threshold voltages that depend on the operating state of
the LTC3370. With all regulators disabled, the enable pin
threshold is set to 730mV (typical). Once any regulator
is enabled, the enable pin thresholds of the remaining
regulators are set to a bandgap-based 400mV and the EN
pins are each monitored by a precision comparator. This
precision EN threshold may be used to provide event-
based sequencing via feedback from other previously
enabled regulators. All buck regulators have forward and
reverse-current limiting, soft-start to limit inrush current
during start-up, and short-circuit protection.
The buck switching regulators are phased in 90° steps to
reduce noise and input ripple. The phase step determines
the fixed edge of the switching sequence, which is when
the PMOS turns on. The PMOS off (NMOS on) phase is
subject to the duty cycle demanded by the regulator. Buck 1
is set to 0°, Buck 2 is set to 90°, Buck 3 is set to 270°, and
Buck 4 is set to 180°. In shutdown all SW nodes are high
impedance. The buck regulator enable pins may be tied
to V_OUT voltages through a resistor divider, to program
power-up sequencing.
The buck switching regulators feature a controlled shut-
down scheme where the inductor current ramps down to
zero through the NMOS switch. If any event causes the
buck regulator to shut down (EN = LOW, OT, V_INA-H or V_CC
UVLO) the NMOS switch turns on until the inductor current
reaches 0mA (typical). Then, the switch pin becomes Hi-Z.
Buck Regulators with Combined Power Stages
Up to four adjacent buck regulators may be combined in
a master-slave configuration by setting the configuration
via the C1, C2, and C3 pins. These pins should either be
tied to ground or pin strapped to V_CC in accordance with
the desired configuration code (Table 1). Any combined
SW pins must be tied together, as must any of the com-
bined V_IN pins. EN1 and FB1 are utilized by Buck 1, EN2
and FB2 by Buck 2, EN3 and FB3 by Buck 3, and EN4 and
FB4 by Buck 4. If any buck is not used or is not available
in the desired configuration, then the associated FB and
EN pins must be tied to ground.
Any available combination of 2, 3, or 4 adjacent Buck
regulators serve to provide up to either 2A, 3A, or 4A
of average output load current. For example, code 110
(C3C2C1) configures Buck 1 to operate as a 4A regula-
tor through V_IN/SW pairs A, B, C, and D, while Buck 2
is disabled, Buck 3 operates as a 1A regulator through
V_IN/SW pair E, and Buck 4 operates as a 3A regulator
through V_IN/SW pairs F, G, and H.
For more information www.linear.com/LTC3370
3370f
15