LTC1624CS datasheet(5/28 Pages) LINER

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LTC1624

TG (Pin 6): High Current Gate Drive for Top N-Channel

MOSFET. This is the output of a floating driver with a

voltage swing equal to INTVCC superimposed on the

switch node voltage SW.

BOOST (Pin 7): Supply to Topside Floating Driver. The

bootstrap capacitor CB is returned to this pin. Voltage

swing at this pin is from INTVCC to VIN + INTVCC in step-

down applications. In non step-down topologies the volt-

age at this pin is constant and equal to INTVCC if SW = 0V.

VIN (Pin 8): Main Supply Pin and the (+) Input to the

Current Comparator. Must be closely decoupled to ground.

PIN FUNCTIONS

(Refer to Functional Diagram)

OPERATIO

Main Control Loop

The LTC1624 uses a constant frequency, current mode

architecture. During normal operation, the top MOSFET is

turned on each cycle when the oscillator sets the RS latch

and turned off when the main current comparator I1 resets

the RS latch. The peak inductor current at which I1 resets

the RS latch is controlled by the voltage on the ITH/RUN

pin, which is the output of error amplifier EA. The VFB pin,

described in the pin functions, allows EA to receive an

output feedback voltage from an external resistive divider.

When the load current increases, it causes a slight

decrease in VFB relative to the 1.19V reference, which in

turn causes the ITH/RUN voltage to increase until the

average inductor current matches the new load current.

While the top MOSFET is off, the internal bottom MOSFET

is turned on for approximately 300ns to 400ns to recharge

the bootstrap capacitor CB.

The top MOSFET driver is biased from the floating boot-

strap capacitor CB that is recharged during each off cycle.

The dropout detector counts the number of oscillator

cycles that the top MOSFET remains on and periodically

forces a brief off period to allow CB to recharge.

The main control loop is shut down by pulling the ITH/RUN

pin below its 1.19V clamp voltage. Releasing ITH/RUN

allows an internal 2.5

µA current source to charge com-

pensation capacitor CC. When the ITH/RUN pin voltage

reaches 0.8V the main control loop is enabled with the ITH/

RUN voltage pulled up by the error amp. Soft start can be

implemented by ramping the voltage on the ITH/RUN pin

from 1.19V to its 2.4V maximum (see Applications Infor-

mation section).

Comparator OV guards against transient output over-

shoots >7.5% by turning off the top MOSFET and keeping

it off until the fault is removed.

Low Current Operation

The LTC1624 is capable of Burst Mode operation in which

the external MOSFET operates intermittently based on

load demand. The transition to low current operation

begins when comparator B detects when the ITH/RUN

voltage is below 1.5V. If the voltage across RSENSE does

not exceed the offset of I2 (approximately 20mV) for one

full cycle, then on following cycles the top and internal

bottom drives are disabled. This continues until the ITH

voltage exceeds 1.5V, which causes drive to be returned to

the TG pin on the next cycle.

INTVCC Power/Boost Supply

Power for the top and internal bottom MOSFET drivers is

derived from VIN. An internal regulator supplies INTVCC

power. To power the top driver in step-down applications

an internal high voltage diode recharges the bootstrap

capacitor CB during each off cycle from the INTVCC supply.

A small internal N-channel MOSFET pulls the switch node

(SW) to ground each cycle after the top MOSFET has

turned off ensuring the bootstrap capacitor is kept fully

charged.

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