LTC4367 datasheet(13/32 Pages) AD

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LTC7862

Rev 0

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OPERATION

Main Control Loop

The LTC7862 uses a constant frequency, peak current

mode step-down architecture. During switching opera-

tion, the external top MOSFET is turned on when the

clock sets the RS latch, and is turned off when the main

current comparator, ICMP, resets the RS latch. The peak

inductor current at which ICMP trips and resets the latch

is controlled by the voltage on the ITH pin, which is the

output of the error amplifier, EA. The error amplifier com-

pares the output voltage feedback signal at the VFB pin

(which is generated with an external resistor divider con-

nected across the output voltage, VOUT, to ground) to the

internal 0.800V reference voltage. When the load current

increases, it causes a slight decrease in VFB relative to

the reference, which causes the EA to increase the ITH

voltage until the average inductor current matches the

new load current.

After the top MOSFET is turned off each cycle, the bot-

tom MOSFET is turned on until either the inductor current

starts to reverse, as indicated by the current comparator

IR, or the beginning of the next clock cycle.

DRVCC/EXTVCC/INTVCC Power

Power for the top and bottom MOSFET drivers is derived

from the DRVCC pin. The DRVCC supply voltage can be

programmed to 6V or 9V using the DRVUV pin. Two sepa-

rate LDOs (low dropout linear regulators) can provide

power from VIN to DRVCC. The internal VIN LDO uses

an internal P-channel pass device between the VIN and

DRVCC pins.

When the EXTVCC pin is tied to a voltage below its swi-

tchover voltage (4.7V or 7.7V depending on the DRVUV

pin), the VIN LDO is enabled and supplies power from

VIN to DRVCC.

If EXTVCC is taken above its switchover voltage, the VIN

LDO is turned off and an EXTVCC LDO is turned on. Once

enabled, the EXTVCC LDO supplies power from EXTVCC to

DRVCC. Using the EXTVCC pin allows the DRVCC power to

be derived from a high efficiency external source such as

one derived from the LTC7862 switching regulator output.

The INTVCC supply powers most of the other internal

circuits in the LTC7862. The INTVCC LDO regulates to a

fixed value of 5V and its power is derived from the DRVCC

supply.

Top MOSFET Driver, Charge Pump and Pass-Through

Mode

The top MOSFET driver is biased from the floating boot-

strap capacitor, CB, which recharges during each switch-

ing cycle through an external diode, DB, whenever SW

goes low.

If the input voltage is below the regulated output (clamp)

voltage, the loop enters dropout and turns on the top

MOSFET continuously. The LTC7862 includes an internal

charge pump that allows the top MOSFET to be turned on

continuously at 100% duty cycle. This charge pump deliv-

ers current to CB to keep it biased at approximately 8.5V.

Shutdown and Start-Up (RUN, SS Pins)

The LTC7862 can be shut down using the RUN pin.

Connecting the RUN pin below 1.12V shuts down the

main control loop. Connecting the RUN pin below 0.7V

disables the controller and most internal circuits, including

the DRVCC and INTVCC LDOs. In this state, the LTC7862

draws only 10μA of quiescent current.

The RUN pin has no internal pull-up current, so the pin

must be externally pulled up or driven directly by logic.

The RUN pin can tolerate up to 150V (absolute maxi-

mum), so it can be conveniently tied to VIN in always-on

applications where the controller is enabled continuously

and never shut down.

The start-up of the output voltage VOUT is controlled by

the voltage on the SS pin. When the voltage on the SS

pin is less than the 0.8V internal reference, the LTC7862

regulates the VFB voltage to the SS pin voltage instead of

the 0.8V reference. This allows the SS pin to be used to

program a soft-start by connecting an external capacitor

from the SS pin to GND. An internal 10μA pull-up current

charges this capacitor creating a voltage ramp on the SS

pin. As the SS voltage rises linearly from 0V to 0.8V (and

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