LT8335 datasheet(9/20 Pages) AD

9 / 20 page

LT8338

Rev. 0

For more information www.analog.com

OPERATION

The LT8338 is a synchronous boost converter that uses a

fixed frequency, current mode control scheme to provide

excellent line and load regulation. Referring to the Block

Diagram, the Switching Logic and Charge Pump block

turns on the power switch MBOT through driver G1 at the

start of each oscillator cycle. The inductor current IL flows

through MBOT, whose current sensing signal is added to a

stabilizingslopecompensationrampandtheresultingsum

is fed into the positive terminal of the PWM comparator

A1. The level at the negative input of A1, labeled “VC”, is

set by the error amplifier EA and is an amplified version

of the difference between the feedback voltage (from the

internal voltage divider) and the reference voltage (from

CTRL pin). During the MBOT-on phase, IL increases.

When the signal at the positive input of A1 exceeds VC,

A1 sends a signal to turn off MBOT. When MBOT turns

off, the synchronous power switch MTOP turns on until

the next clock cycle begins or inductor current IL falls to

zero.Ifoverloadconditionsresultinexcesscurrentflowing

through the top switch, the next clock cycle will be delayed

until switch current returns to a safe level. Through this

repetitive action, the EA sets the correct IL peak current

level to keep the output voltage in regulation.

If the EN/UVLO pin is lower than 0.3V, the LT8338 is

shut down and draws < 0.2μA from the input. When the

EN/UVLO pin is above 1.05V, the switching regulator

becomes active.

The LT8338 can be configured to work in different modes

by setting SYNC/MODE pin. If the SYNC/MODE pin is tied

to ground directly, the LT8338 provides low output ripple

Burst Mode operation with ultralow quiescent current

at light loads. Connecting SYNC/MODE pin to ground

througha100kresistorenablesBurstModeoperationwith

frequency spread spectrum modulation (see Frequency

Spread Spectrum Modulation section). When Burst Mode

operation is selected, all circuitries associated with con-

trolling the output switches are shut down to reduce the

quiescent current between bursts. When SYNC/MODE pin

is floated, the LT8338 operates in pulse-skipping mode,

which reduces output ripple compared to Burst Mode

operationandincreasesthequiescentcurrenttohundreds

of microamps. Connecting the SYNC/MODE pin to INTVCC

selects pulse-skipping operation with spread spectrum

modulation enabled. Spread spectrum modulation varies

the switching frequency to reduce EMI. When the SYNC/

MODE pin is driven by an external clock, the converter

switching frequency is synchronized to that clock and

pulse-skipping mode is enabled.

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