LTC7150S datasheet(10/22 Pages) LINER

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LTC7150S

7150sfa

For more information www.linear.com/LTC7150S

OPERATION

Main Control Loop

The LTC7150S is a current mode monolithic 20A step-

downregulator.Innormaloperation,theinternaltoppower

MOSFET is turned on for a fixed interval determined by a

one-shot timer, OST. When the top power MOSFET turns

off, the bottom power MOSFET turns on until the current

comparator, ICMP, trips, restarting the one-shot timer and

initiating the next cycle. Inductor current is determined

by sensing the voltage drop across the bottom power

MOSFET when it is on. The voltage on the ITH pin sets the

comparatorthresholdcorrespondingtotheinductorvalley

current. The error amplifier, EA, adjusts this ITH voltage by

comparing the feedback signal, VFB, with an internal 0.6V

reference. If the load current increases, it causes a drop in

the feedback voltage relative to the internal reference, the

ITH voltage then rises until the average inductor current

matches that of the load current.

At low load currents, the inductor current can drop to

zero and become negative. In discontinuous mode (DCM),

this is detected by the current reversal comparator, IREV,

which then shuts off the bottom power MOSFET. Both

power MOSFETs will remain off with the output capacitor

supplying the load current until the ITH voltage rises above

zero current level to initiate the next cycle. If continuous

modeofoperationisdesired,simplyfloattheMODE/SYNC

pin or tie it to INTVCC.

The operating frequency is determined by the value of the

RT resistor, which programs the current for the internal

oscillator. An internal phase-lock loop servos the oscillator

frequency to an external clock signal if one is present on

the MODE/SYNC pin. Another internal phase-lock loop

servostheswitchingregulatoron-timetotracktheinternal

oscillator to force a constant switching frequency.

Overvoltage and undervoltage comparators OV and UV

pull the PGOOD output low if the output feedback volt-

age, VFB, exits a ±7.5% window around the regulation

point. Continuous operation is forced during OV and UV

conditions except during start-up when the TRACK pin is

ramping up to 0.6V.

The “S” in LTC7150S refers to the second generation

Silent Switcher technology. The IC has integrated ceramic

capacitors for VIN and BOOST to keep all the fast AC cur-

rent loops small, thus improving the EMI performance.

Furthermore, it allows for faster switching edges which

greatly improves efficiency at high switching frequencies.

RUN Threshold

Pulling the RUN pin to ground forces the LTC7150S into

its shutdown state. Bringing the RUN pin to above 0.6V

will turn on the internal reference only, while keeping the

power MOSFETs off. Further increasing the RUN voltage

above the RUN rising threshold (nominally 1.2V) turns on

theentirechip.Theaccurate1.2VRUNthresholdallowsthe

user to program the SVIN under voltage lockout threshold

by placing a resistor divider from SVIN.

INTVCC Regulator

Aninternallowdropout(LDO)regulatorproducesthe3.3V

supply that powers the drivers and internal bias circuitry.

The INTVCC must be bypassed to ground with a minimum

of a 4.7µF ceramic capacitor. Good bypassing is neces-

sary to supply the high transient currents required by the

power MOSFET gate drivers. Applications with high input

voltage and high switching frequency will experience an

increase in die temperature due to the higher power dis-

sipation across the LDO. In such cases, if there’s another

5V or 3.3V supply rail available, consider using that to

drive the SVIN pin to lower the power dissipation across

the internal LDO.

VIN Overvoltage Protection

In order to protect the internal power MOSFET devices

against transient voltage spikes, the LTC7150S constantly

monitors the PVIN pin for an overvoltage condition. When

the PVIN rises above 24.5V, the regulator suspends opera-

tionbyshuttingoffbothpowerMOSFETs.OncePVINdrops

below 21.5V, the regulator immediately resumes normal

operation. During an overvoltage event, the internal soft-

start voltage is clamped to a voltage slightly higher than

the feedback voltage, thus the soft-start feature will be

present upon exiting an overvoltage condition.

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