Data Sheet

LT8342

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Rev. 0

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THEORY OF OPERATION

The LT8342 is a monolithic, synchronous step-up DC/DC converter that uses fixed frequency and peak current

mode control to provide excellent line and load regulation. It contains a Silent Switcher architecture that enables a

significant reduction in EMI and can be configured to control an optional external input-disconnect N-channel FET

(MEXT) for output short-circuit protection and reduction of inrush current.

See the Block Diagram for the following explanation of the LT8342’s operation. An optional resistor divider from the

converter input to the EN/UVLO pin, with a 1.08V rising threshold and an accurate 1.02V falling threshold, allows

the user to program turn on/off levels for the converter input. Alternatively, the EN/UVLO pin can be connected to

the converter input or driven from an external logic-level signal. EN/UVLO pin voltage below 0.15V shuts down all

internal circuitry and draws less than 1μA from the VIN pin.

With the EN/UVLO pin voltage high enough and the VIN pin voltage at least 2.8V, the internal circuitry is activated.

The LT8342 will check whether internal feedback (FBINT) or external feedback (VSET2/FB pin) will be used as the

feedback node (FB0). If internal feedback is used, FB_SEL will automatically determine the correct internal

feedback ratio accordingly. 10μA from the SS pin will gradually charge up the external soft-start capacitor (CSS),

which slowly ramps up the regulation reference voltage. An oscillator (A1), with frequency set using a resistor from

the RT pin to GND, turns on the internal bottom power switch (M1) at the beginning of each clock cycle.

Current in the inductor then increases until the bottom switch current comparator (A2) trips and turns off the

bottom power switch. The peak inductor current at which the bottom switch turns off is controlled by the voltage

on the VC pin. The error amplifier (A3) servos the VC pin by comparing the voltage on the feedback node (FB0) with

the internal 1V reference. The error amplifier drives the VC pin voltage to set the correct peak switch current level

that keeps the output voltage in regulation. When the load current increases, it causes a reduction in the feedback

node (FB0) voltage relative to the internal 1V reference. This causes the error amplifier to raise the VC pin voltage, to

increase the peak inductor current until enough power is delivered to the output to maintain the output voltage in

regulation. When the bottom power switch (M1) turns off, the synchronous power switch (M2) turns on until the

next clock cycle begins or the inductor current falls to zero.

For applications requiring a PreBoost regulator to maintain output voltage, for example, during the automotive

cold crank or start-stop, the VIN and VOUT pins are constantly monitored to determine when VIN exceeds VOUT. If VIN

exceeds VOUT and VOUT is above its programmed regulation voltage, the LT8342 enters PassThru Mode and turns on

the top side synchronous switch to maximize efficiency.

Optional output short-circuit protection, and inrush current reduction are available by adding an external sense

resistor (RSNS) and an N-channel FET (MEXT) in series with the converter input. If MEXT is not used, connect the GATE

pin to GND. The LT8342 can automatically detect whether MEXT is configured.

At startup, an internal 9μA current source charges up the GATE pin (connected to the gate of MEXT) to 6.5V above VIN.

During an output short circuit, when the converter input current rises high enough in the sense resistor RSNS to trip

the 45mV threshold of comparator A4, the LT8342 pulls the GATE Pin to GND, turning off MEXT and limits the

maximum short circuit inductor current. After MEXT is turned off, the LT8342 waits 40ms before charging up the

GATE pin to turn on MEXT again.

The LT8342's SYNC/MODE pin allows synchronization to an external clock. It can also be used to select between

burst or pulse-skipping modes of operation to optimize the converter performance based on the application

requirements. SSFM can be activated with either of the two modes to reduce the EMI further.