LTC3370 datasheet(11/26 Pages) AD

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LTC3307A

Rev. A

For more information www.analog.com

OPERATION

Voltage Regulation

The LTC3307A is a 5V, 3A monolithic, constant frequency,

peak current mode control, step-down DC/DC converter.

The synchronous buck switching regulators are internally

compensated and require only external feedback resistors

to set the output voltage. An internal oscillator, with the

frequency set using a resistor on the RT pin or synchro-

nized to an external clock, turns on the internal top power

switch at the beginning of each clock cycle. Current in the

inductor ramps up until the top switch current comparator

trips and turns off the top power switch. The peak induc-

tor current at which the top switch turns off is controlled

by an internal VC voltage. The error amplifier regulates VC

by comparing the voltage on the FB pin with an internal

500mV reference. An increase in the load current causes a

reduction in the feedback voltage relative to the reference,

causing the error amplifier to raise the VC voltage until the

average inductor current matches the new load current.

When the top power switch turns off, the synchronous

power switch turns on and ramps down the inductor cur-

rent for the remainder of the clock cycle or, if in pulse skip

or Burst mode, until the inductor current falls to zero. If

an overload condition results in excessive current flowing

through the bottom switch, the next clock cycle will be

skipped until switch current returns to a safe level.

The enable pin has a precision 400mV threshold to pro-

vide event-based power-up sequencing by connecting the

EN pin to the output of another buck through a resistor

divider. If the EN pin is low, the device is shut down and

in a low quiescent current state. When the EN pin is above

its threshold, the switching regulator will be enabled.

The LTC3307A has forward and reverse inductor current

limiting, short-circuit protection, output over-voltage pro-

tection, and soft-start to limit inrush current during start-

up or recovery from a short-circuit.

Mode Selection

The LTC3307A operates in three different modes set by

the MODE/SYNC pin: pulse skip mode (when the MODE/

SYNC pin is set low), forced continuous mode (when the

MODE/SYNC pin is floating) and Burst Mode operation

(when the MODE/SYNC pin is set high).

In pulse skip mode, the oscillator operates continuously

and positive SW transitions are aligned to the clock.

Negative inductor current is disallowed and, during light

loads, switch pulses are skipped to regulate the output

voltage.

In forced continuous mode, the oscillator operates con-

tinuously. The top switch turns on every cycle and regu-

lation is maintained by allowing the inductor current to

reverse at light load. This mode allows the buck to run

at a fixed frequency with minimal output ripple. In forced

continuous mode, if the inductor current reaches IREVMAX

(into the SW pin), the bottom switch will turn off for the

remainder of the cycle to limit the current.

In Burst Mode operation at light loads, the output capaci-

tor is charged to a voltage slightly higher than its regu-

lation 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 voltage drops 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 will burst at light

loads whereas at higher loads it will operate in constant

frequency PWM mode.

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