LT8613 datasheet(11/24 Pages) LINER

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LT8613

8613f

operaTion

The LT8613 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

0.97Vreference.Whentheloadcurrentincreasesitcauses

areductioninthefeedbackvoltagerelativetothereference

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 overload conditions result

inmorethan10Aflowingthroughthebottomswitch(valley

current), the next clock cycle will be delayed until switch

current returns to a safe level.

The LT8613 includes a current control and monitoring

loop using the ISN, ISP, IMON and ICTRL pins. The ISP/

ISN pins monitor the voltage across an external sense

resistor such that the VISP-VISN does not exceed 50mV

by limiting the peak inductor current controlled by the VC

node.Thecurrentsenseamplifierinputs(ISP/ISN)arerail-

to-rail such that input, output, or other system currents

may be monitored and regulated. The IMON pin outputs

a ground-referenced voltage equal to 20 times the voltage

between the ISP-ISN pins for monitoring system currents.

The ICTRL pin can be used to override the internal 50mV

limit between the ISP, ISN pin to a lower set point for the

current control loop.

If the EN/UV pin is low, the LT8613 is shut down and

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

1V, the switching regulator will become active.

To optimize efficiency at light loads, the LT8613 operates

in Burst Mode operation in 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, 3μA 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

tied to a logic high 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 oper-

ates 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 hundred µA.

To improve efficiency across all loads, supply current to

internal circuitry can be sourced from the BIAS pin when

biasedat3.3Vorabove.Else,theinternalcircuitrywilldraw

current from VIN. The BIAS pin should be connected to

VOUTiftheLT8613outputisprogrammedat3.3Vorabove.

Comparators monitoring the FB pin voltage will pull the

PG pin low if the output voltage varies more than ±9%

(typical)fromthesetpoint,orifafaultconditionispresent.

The oscillator reduces the LT8613’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 start-up or overcurrent conditions. When

a clock is applied to the SYNC pin or the SYNC pin is

held DC high, the frequency foldback is disabled and the

switching frequency will slow down only during overcur-

rent conditions.

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