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LTC3370

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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.

InBurstModeoperationatlightloads,theoutputcapacitor

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 VIN 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

subjecttothedutycycledemandedbytheregulator.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 VOUT 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, VINA-H or VCC

UVLO)theNMOSswitchturnsonuntiltheinductorcurrent

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 VCC in accordance with

the desired configuration code (Table 1). Any combined

SW pins must be tied together, as must any of the com-

bined VIN 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 VIN/SW pairs A, B, C, and D, while Buck 2

is disabled, Buck 3 operates as a 1A regulator through

VIN/SW pair E, and Buck 4 operates as a 3A regulator

through VIN/SW pairs F, G, and H.

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