LT8646SEVPBF datasheet(15/30 Pages) AD

15 / 30 page

LT8645S/LT8646S

Rev. B

For more information www.analog.com

OPERATION

TheLT8645S/LT8646Sisamonolithic,constantfrequency,

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 0.97V 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

voltageuntiltheaverageinductorcurrentmatchesthenew

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 more than 11A flowing through the

bottom switch, the next clock cycle will be delayed until

switch current returns to a safe level.

The “S” in LT8645S/LT8646S refers to the second genera-

tion Silent Switcher technology. This technology allows

fast switching edges for high efficiency at high switching

frequencies, while simultaneously achieving good EMI

performance. This includes the integration of ceramic

capacitors into the package for VIN, BST, and INTVCC (see

Block Diagram). These caps keep all the fast AC current

loops small, which improves EMI performance.

If the EN/UV pin is low, the LT8645S/LT8646S is shut

down and draws approximately 1µA from the input. When

the EN/UV pin is above 1.01V, the switching regulator will

become active.

Tooptimizeefficiencyatlightloads,theLT8645S/LT8646S

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 sup-

ply current to 1.7μA (LT8645S) or 230μA (LT8646S with

BIAS = 0). In a typical application, 2.5μA (LT8645S) or

120μA (LT8646S with BIAS = 5VOUT) will be consumed

from the input supply when regulating with no load. The

SYNC/MODE pin is tied low to use Burst Mode operation

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

appliedtotheSYNC/MODEpin,thepartwillsynchronizeto

an external clock frequency and operate in pulse-skipping

mode.Whileinpulse-skippingmodetheoscillatoroperates

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 EMI, the LT8645S/LT8646S can operate in

spread spectrum mode. This feature varies the clock with

a triangular frequency modulation of +20%. For example,

if the LT8645S/LT8646S’s frequency is programmed to

switch at 2MHz, spread spectrum mode will modulate the

oscillator between 2MHz and 2.4MHz. The SYNC/MODE

pin should be tied high to INTVCC (~3.4V) or an external

supply of 3V to 4V to enable spread spectrum modulation

with pulse-skipping mode.

To improve efficiency across all loads, supply current to

internal circuitry can be sourced from the BIAS pin when

biased at 3.3V or above. Else, the internal circuitry will

draw current from VIN. The BIAS pin should be connected

to VOUT if the LT8645S/LT8646S output is programmed

at 3.3V to 25V.

The VC pin optimizes the loop compensation of the

switching regulator based on the programmed switching

frequency, allowing for a fast transient response. The VC

pinalsoenablescurrentsharingandaCLKOUTpinenables

synchronizing other regulators to the LT8646S.

Comparators monitoring the FB pin voltage will pull the

PG pin low if the output voltage varies more than ±8%

(typical)fromthesetpoint,orifafaultconditionispresent.

The oscillator reduces the LT8645S/LT8646S’s operat-

ing 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 condi-

tions. When a clock is applied to the SYNC/MODE pin, the

SYNC/MODE pin is floated, or held DC high, the frequency

foldback is disabled and the switching frequency will slow

down only during overcurrent conditions.

LT8646SEVPBF Datasheet(PDF) 15 Page - Analog Devices

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