LT8613 datasheet(12/26 Pages) LINER

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LT8619/LT8619-5

8619f

For more information www.linear.com/LT8619

OPERATION

The LT8619 is a monolithic, constant frequency current

mode step-down DC/DC converter. An oscillator, with fre-

quency 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 cur-

rent 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 volt-

age on the FB pin with an internal 0.8V 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 induc-

tor current matches the new load current. When the top

power switch turns off, the bottom power switch turns on

until the next clock cycle begins or inductor current falls

to zero (Burst Mode operation or pulse-skipping mode).

If overload conditions result in more than 1.8A flowing

through the bottom switch, the next clock cycle will be

delayed until the switch current returns to a safe level.

If the EN/UV pin is low, the LT8619 is shut down and

draws less than 0.6µA from the input. When the EN/UV

pin is above 1V, the switching regulator starts operation.

First, the internal LDO powers up, followed by the switch-

ing regulator 200μs soft-start ramp. During the soft-start

phase, the switcher operates in pulse-skipping mode and

gradually switches to forced continuous mode when VOUT

approaches the set point (if SYNC pin is forced high or

connected to an external clock). Typically, upon EN/UV

rising edge, it takes about 660μs for the switcher output

voltage to reach regulation and PG to be asserted.

To optimize efficiency at light loads, configure the LT8619

to operate in Burst Mode by grounding the SYNC pin.

At light load, in between bursts, all circuitry associated

with controlling the output switch is shut down, reducing

the input supply current. In a typical application, 6μA will

be consumed from the supply when regulating with no

load. Float the SYNC pin to enable pulse-skipping mode

operation. While in pulse-skipping mode, the oscillator

operates continuously and the bottom power switch turns

off when the inductor current falls to zero. During light

loads, switch pulses are skipped to regulate the output

and the quiescent current will be higher than Burst Mode

operation. Connecting the SYNC pin to INTVCC enables

forced continuous mode operation. In forced continuous

mode, the inductor current is allowed to reverse and the

switcher operates at a fixed frequency. If a clock is applied

to the SYNC pin, the part operates in forced continuous

mode and synchronizes to the external clock frequency;

with the rising SW signal synchronized to the external

clock positive edge.

To improve efficiency across all loads, supply current

to internal circuitry can be sourced from the BIAS pin

when biased above 3.1V. Else, the internal circuitry will

draw current from VIN. The BIAS pin should be connected

to VOUT if the LT8619 output is programmed to 3.3V or

above.

An overvoltage comparator, OV, guards against transient

overshoots. If VFB is higher than 0.83V, the OV compara-

tor trips, disables the top MOSFET and turns on the bot-

tom power switch until the next clock cycle begins or the

inductor reverse current reaches 0.55A. With high reverse

current, both top and bottom MOSFETs shut off till the

next cycle. Positive and negative power good compara-

tors pull the PG pin low if the FB voltage varies more than

±7.5% (typical) from the set point.

The oscillator reduces the LT8619’s operating frequency

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

foldback helps to control the inductor current when the

output voltage is lower than the programmed value which

occurs during overcurrent conditions.

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