LT8607 datasheet(11/24 Pages) AD

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LT8609S

Rev. D

For more information www.analog.com

OPERATION

The LT8609S 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 voltage on the VFB pin with an inter-

nal 0.774V 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 inductor current falls to zero. If over-

load conditions result in excess current flowing through

the bottom switch, the next clock cycle will be delayed

until switch current returns to a safe level.

If the EN/UV pin is low, the LT8609S 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 LT8609S 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 will be

consumed from the input supply when regulating with no

load. The SYNC pin is tied low to use Burst Mode opera-

tion and can be floated to use pulse-skipping mode. If a

clock is applied to the SYNC pin the part will synchronize

to an external clock frequency and operate in pulse-skip-

ping 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

will be several mA. The SYNC pin may be tied high for

spread spectrum modulation mode, and the LT8609S will

operate similar to pulse-skipping mode but vary the clock

frequency to reduce EMI.

Comparators monitoring the FB pin voltage will pull the PG

pin low if the output voltage varies more than ±8.5% (typ-

ical) from the set point, or if a fault condition is present.

The oscillator reduces the LT8609S’s operating frequency

when the voltage at the FB pin is low. This frequency fold-

back 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 frequency foldback is disabled. Frequency foldback is

only enabled when the SYNC pin is tied to ground.

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