Data Sheet

LT8615

analog.com

Rev 0

16 of 31

THEORY OF OPERATION

The LT8615 is a monolithic constant frequency current-mode step-down DC/DC converter. An oscillator with

frequency set using a resistor on the RT pin turns on the internal top power switch at the beginning of each clock

cycle. Current in the inductor then increases until the top switch current comparator trips and turns off the top power

switch. The peak inductor current at which the top switch turns off is controlled by the voltage on the internal VC

node. The error amplifier servos the VC node by comparing the FB pin voltage with an internal 0.787V reference.

When the load current increases, it causes a reduction in the feedback voltage relative to the reference leading 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 until the next clock cycle begins or the inductor

current falls to zero. If overload conditions result in excess current flowing through the bottom switch, the next clock

cycle is delayed until switch current returns to a safe level.

If the EN/UV pin is low, the LT8615 is shut down and draws 1µA from the input. When the EN/UV pin is above 1V, the

switching regulator becomes active.

To optimize efficiency at light loads, the LT8615 enters Burst Mode operation during light load situations. Between

bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to

1.7μA. In a typical application, 2.5μA is consumed from the input supply when regulating with no load. The SYNC pin

is tied low to use Burst Mode operation and can be floated to use pulse-skipping mode. If a clock is applied to the

SYNC pin, the part synchronizes to an external clock frequency and operates in pulse-skipping mode. While in pulse-

skipping mode, the oscillator operates continuously, and positive SW transitions are aligned to the clock. During light

loads, switch pulses are skipped to regulate the output, and the quiescent current is several mA. The SYNC pin may

be tied high for spread-spectrum modulation mode, and the LT8615 operates like pulse-skipping mode but varies

the clock frequency to reduce EMI.

Comparators on the LT8615 monitoring the FB pin voltage pull the PG pin low if the output voltage varies more than

±8% (typical) from the set point, or if a fault condition is present.

The oscillator reduces the LT8615’s operating frequency when the voltage at the FB pin is low, and the SYNC is setting

the part to Burst Mode. This frequency foldback helps to control the inductor current when the output voltage is

lower than the programmed value, which occurs during start-up. When a clock is applied to the SYNC pin, the SYNC

pin is floated, or held DC high, the LT8615 stays at the programmed frequency without foldback, and only slows

switching if the inductor current exceeds safe levels.