LT8607 datasheet(11/24 Pages) AD

11 / 24 page

LT8607/LT8607B

Rev. D

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OPERATION

The LT8607 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 internal

0.778V reference. The LT8607-5 fixed output part uses

the VOUT pin and an internal resistor divider to generate

an internal FB node. 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 volt-

age 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 overload

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 LT8607 is shut down and

draws 1µA from the input. When the EN/UV pin is above

1.05V, the switching regulator becomes active.

To optimize efficiency at light loads, the LT8607 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, 3.0µA will be 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 will synchronize to an external

clock frequency and operate in pulse-skipping mode. While

in pulse-skipping mode the oscillator operates continu-

ously 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 modu-

lation mode, and the LT8607 will operate similar to pulse-

skipping mode but vary the clock frequency to reduce EMI.

The LT8607 DFN has no SYNC pin and will always operate

in Burst Mode operation. The LT8607B has no SYNC pin

and will operate in pulse-skipping mode.

Comparators monitoring the FB pin voltage will pull the PG

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

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

In Burst Mode operation, the oscillator reduces the

LT8607's operating frequency when the voltage at the

FB pin is low, or the voltage at the VOUT pin is low on the

LT8607-5 fixed output option. This frequency foldback

helps to control the inductor current when the output volt-

age is lower than the programmed value which occurs

during start-up.

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